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LORY

APR 12 1960

COMPARATIVE BREEDING BEHAVIOR
OF FOUR SPECIES OF
NORTH AMERICAN HERONS

PUBLICATIONS OF THE NUTTALL ORNITHOLOGICAL CLUB

*

NO. 2

COMPARATIVE BREEDING BEHAVIOR

OF FOUR SPECIES OF
NORTH AMERICAN HERONS

by

ANDREW J. MEYERRIECKS

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Cambridge, Massachusetts
Published by the Club
1960

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. ^ , Publications of the Nuttall Ornithological Club

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1. 77if Comparative Biology of the Meadowlarks ( Sturnella ) in Wisconsin.
Wesley E. Lanyon 1957. [viii] + 67 pp., 31 pi.

2. Comparative Breeding Behavior of Four Species of North American Herons.
Andrew J. Meyerriecks 1960. viii + 158 pp., 15 pi.

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Printed in the United States of America
Allen Press • Lawrence, Kansas

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APR 12 1360

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Publications of the Nuttall Ornithological Club may be obtained from the
Massachusetts Audubon Society, Drumlin Farm, South Lincoln, Massachusetts.

ACKNOWLEDGMENTS

I am grateful to Professor Ernst Mayr, my graduate sponsor, for
his assistance in every phase of this study. For help in the field, I
should like to thank Robert Porter Allen, Robert Meyerriecks, Herbert
Johnson, Charles M. Brookfield, and Miguel Alvarez del Toro. For
the loan of photographs, I am grateful to Andreas Feininger (Plates
1, 3, 4, and 6), Robert Meyerriecks (Plates 2, 5, and 7), and Samuel A.
Grimes (Plate 15). The remaining photographs were taken by the
author. Charts and maps were prepared by Robert and George
Meyerriecks.

Field work was supported by grants from the Society of Fellows,
Harvard University, and by Grants-in-Aid, Sigma Xi-RESA.

Mrs. Nancy Buffler, of the Museum of Comparative Zoology, Har¬
vard College, deserves special thanks for her excellent line drawings
of herons.

I wish to thank the following persons for their suggestions and critical
reading of the manuscript: Professor Ernst Mayr, Dr. Oliver L. Austin,
Jr., Dr. William H. Drury, Jr., Dr. P. Marler, and Oscar M. Root.

Finally, I am particularly indebted to my wife, Norma, for help
in many aspects of this work.

Andrew J. Meyerriecks
Assistant Director
Hatheway School of
Conservation Education
South Lincoln, Massachusetts

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CONTENTS

INTRODUCTION _ 1

PART I. THE BREEDING BEHAVIOR OF THE

GREEN HERON _ 3

DISTRIBUTION AND DESCRIPTION _ ... _ 5

MAINTENANCE ACTIVITIES _ 5

MIGRATION AND BEHAVIOR ON ARRIVAL _ 20

HOSTILE AND SEXUAL BEHAVIOR _ 24

HOSTILE BEHAVIOR _ 25

SEXUAL BEHAVIOR _ 43

EARLY PAIRING BEHAVIOR _ 53

LATE PAIRING BEHAVIOR _ 76

PART II. COMPARATIVE BEHAVIOR OF OTHER

NORTH AMERICAN HERONS _ 84

GREAT BLUE AND GREAT WHITE HERONS _ 86

DISTRIBUTION AND DESCRIPTION _ 86

MAINTENANCE ACTIVITIES _ 87

MIGRATION AND BEHAVIOR ON ARRIVAL _ 92

HOSTILE BEHAVIOR _ 93

SEXUAL BEHAVIOR _ 102

EARLY PAIRING BEHAVIOR _ 104

REDDISH EGRET _ 106

DISTRIBUTION AND DESCRIPTION _ 106

MAINTENANCE ACTIVITIES _ 107

MIGRATION AND BEHAVIOR ON ARRIVAL _ 110

HOSTILE BEHAVIOR _ 110

SEXUAL BEHAVIOR _ 117

EARLY PAIRING BEHAVIOR _ 123

SNOWY EGRET _ 125

DISTRIBUTION AND DESCRIPTION _ 125

MAINTENANCE ACTIVITIES _ 125

HOSTILE BEHAVIOR _ 127

SEXUAL BEHAVIOR _ 133

EARLY PAIRING BEHAVIOR _ 138

vii

CONTENTS

PART III. SOME EVOLUTIONARY TRENDS IN

HERON REHAVIOR _ 141

SOCIALITY _ 141

POLYMORPHISM _ 143

SEXUAL DIMORPHISM AND PAIR FORMATION _ 145

COLORATION OF SOFT-PARTS _ 146

SIZE AND ACTIVITY _ 149

COMPARATIVE REHAVIOR CHART _ 151

LITERATURE CITED _ 153

PLATES _ following 158

Vlll

INTRODUCTION

The herons, egrets, and bitterns of the family Ardeidae are, for
various reasons, particularly suitable subjects for a comparative etho-
logical study. The family includes approximately sixty-three species,
grouped in fifteen genera by the latest reviser (Bock, 1956). Study
of geographical variation of individual behavior traits, a neglected
aspect of ethology, is facilitated by the broad distributions of many
species in the temperate and tropical regions of both hemispheres
(for example, the Black-crowned Night Heron, Nycticorax nycticorax
( Linnaeus ) ) . The local abundance of many species affords an oppor¬
tunity for the repetition so necessary in observations of behavior
patterns.

Herons and their allies possess a variety of modified plume types,
and the existence of such structures readily leads to a study of their
use as isolating mechanisms during pair formation in mixed colonies.
Herons perform highly ritualized ceremonies during pair formation
and nest relief, thus affording an opportunity for comparisons of species
and genera. Within the family, breeding behavior ranges from solitary
nesting ( American Bittern, Botaurus lentiginosus ( Rackett ) ) through
loose aggregations ( Green Heron, Butorides virescens ( Linnaeus ) ) to
highly social nesting (Snowy Egret, Leucophoyx thula (Molina) ), thus
facilitating the study of the origin and evolution of social nesting habits.

Polymorphism, in the form of color phases, is common among ardeid
birds (e.g., Reddish Egret, Dichromanassa rufescens (Gmelin), and
Reef Heron, Demigretta sacra ( Gmelin ) ) , and the occurrence of such
phases readily leads to a study of their selective and potential values
as isolating mechanisms during pair formation.

Bock based his generic revision of the family primarily on morpho¬
logical characters; a comparative behavior study might lead to a more
satisfactory classification using both behavioral and structural char¬
acters. In addition, Heinroth (1929), O. von Frisch (1957), and
others have shown how easily herons may be raised in captivity, a
welcome adjunct to field observations.

Unfortunately, the student of heron behavior lacks the rich literature
available to the student of gull or stickleback behavior. With the
exception of the valuable papers of Verwey (1930), Lorenz (1938),
Noble et al., (1938, 1940, 1942), and a few others, the available litera¬
ture on heron behavior is scattered and cursory. Hence, the conclusions
presented here are to be considered first approximations.

Prior to any comparative study, however, an attempt must be made
to study intensively one species in order to obtain a comparative stand¬
ard for the group. The Green Heron was selected for this purpose,

1

2 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

not only because it is a widespread and locally abundant species, but
also because little was known about the general reproductive biology
of the bird. Such information would naturally accumulate during the
course of the general ethological investigation. Accordingly, I began
a study of the Green Heron in the spring of 1954 and continued research
during the breeding seasons of 1955 and 1957. However, this research
did not result in the accumulation of large amounts of quantitative data.
Although Green Herons are locally common and may breed in small
to large colonies, the actual number of breeding pairs is not great
when compared with other more highly social species of herons such
as the Snowy Egret. In addition, the reed-choked nesting areas pre¬
vented observation of more than a few birds at any one time; hence,
some of the conclusions presented here are based on relatively few
repeat observations. Study of a more highly social species such as the
Snowy Egret, which breeds in large numbers along the Gulf Coast
of North America, might lead to quantitative results rivaling those
gathered in certain European gulleries.

Part I of this paper presents the Green Heron study. Part II deals
with the Great White-Great Blue Heron, Reddish Egret, and Snowy
Egret. Some evolutionary trends in heron behavior are discussed in
Part III. A comparative behavior chart follows Part III; it is designed
to show at a glance the existing state of knowledge of the behavior of
most North American herons.

This paper is part of a Doctoral Dissertation presented to the Faculty
of the Biological Laboratories, Harvard University, in partial fulfillment
of the requirements for the degree of Doctor of Philosophy, May, 1958.

I have used, with few exceptions, the scientific and common names
listed in Check-List of North American Birds , American Ornithologists’
Union, 1957.

Part I

THE BREEDING BEHAVIOR OF THE GREEN HERON

Butorides virescens (Linnaeus)

ORGANIZATION

Part I presents a preliminary description and analysis of the repro¬
ductive behavior of the Green Heron. I have arbitrarily limited the
description to that portion of the breeding cycle which extends from
the time the birds arrive on the breeding grounds until the first egg
is laid. I have tried to describe the behavior of the Green Heron in
a chronological order, but so much of the reproductive behavior,
especially the early phases, involves hostile (attack and/or escape)
behavior, that a special section is devoted to describing it before the
description of pair formation and related activities. For comparative
purposes, a brief introductory section describes flight, preening, and
other maintenance activities.

METHODS

I made field observations of the Green Heron during one partial
season (1954) at breeding sites in eastern Massachusetts (Wayland,
Burlington, and Newburyport) and western Long Island, New York,
and during two complete seasons (1955, 1957) in colonies on Rulers
Bar Hassock, an island in Jamaica Bay, Long Island, New York
(Figure 1). I made additional observations in Florida Bay in the
winter of 1955-56 and a few near Puerto Arista, Chiapas, Mexico, in
the summer of 1956. Most of the observations I made from blinds with
binoculars (7x) and a telescope (20x). The drawings illustrating
the text were made from my own field sketches and from photographs.
A conservative estimate of the time I spent in the field observing
the Green Heron is 3,000 hours.

STUDY AREA

Most of the field work was concentrated on Rulers Bar Hassock,
a salt marsh island about three miles long and one-half mile wide in
the middle of Jamaica Bay. The town of Broad Channel occupies the
lower third of the island, and a main thoroughfare, Cross Bay Boulevard,
runs down the central axis. I found four separate heron breeding sites,
two east of the boulevard and two west ot it (areas 1, 2, 3, and 4,
Figure 1). All four sites contained reeds, Phragmites communis ;
bayberry, Myrica carolinensis ; poison ivy, Rhus radicans ; wing-rib
sumach, R. copallina ; staghorn sumach, R. typhina ; cottonwood poplar.

3

4

COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

RULERS BAR
HASSOCK

-JAMAICA BAY

Figure 1. Map of Rulers Bar Hassock, an island in Jamaica Bay, western Long
Island, New York. “FA” stands for feeding area.

BREEDING BEHAVIOR OF THE GREEN HERON

5

Populus deltoides ; and willow, Salix discolor. The herons fed in the
extensive marshes surrounding the breeding areas. In 1955 the largest
number of Green Herons bred in area 1 (70 pairs), somewhat fewer
in area 2 (41 pairs), and scattered pairs in areas 3 (17 pairs) and 4
(8 pairs). I made observations in all four areas throughout the 1955
and 1957 breeding seasons, but I recorded most of the data in area 1.

DISTRIBUTION AND DESCRIPTION

The Green Heron, a highly migratory species, is widely distributed
as a breeding bird in North and Central America, ranging from
western Washington, southern Ontario, and southern New Brunswick
in the north, south through Mexico, Central America, and the West
Indies to the Pearl Islands in the Gulf of Panama. It breeds in a few
localities in northern South America, where it meets the closely related
Striated Heron, Butorides striatus (Linnaeus), of South America,
Africa, and Austral- Asia. Another close relative is the Galapagos
Heron, Butorides sundevalli Reichenow, restricted to the Galapagos
Archipelago.

These small herons may nest solitarily, in loose aggregations, or rarely
in colonies. They are locally common breeders in the United States
near fresh and salt water.

The Green Heron, about eighteen inches long, is among the smaller
members of the family Ardeidae, with an adult weight of approximately
200 grams (Slipp, 1942; Meyerriecks, unpub. MS). Haverschmidt
( 1948b ) records the weights of two adult Striated Herons from Surinam
as 173 and 187 grams.

In the field, Green Herons appear as small, dark, short-legged
herons with rather shaggy crests. The back plumage often appears
to be more blue than green. The plumage of the two sexes is similar,
with dark, greenish-black crests, dark green nape and back, chestnut-
maroon neck and brownish underparts striped with white. The green,
lanceolate plumes of the scapular region reach the tail. The nuptial
coloration of the male is full, intense, and lustrous, whereas that of
the female is noticeably less brilliant. Hindwood (1933) states, for
striatus during the breeding season, “although the female is said to be
similar to the male, it always seemed to me when I had both birds
in view at the same time that she was duller in the colouration of her
plumage.” I agree emphatically with respect to virescens.

MAINTENANCE ACTIVITIES

Certain movements and behavior patterns that occur throughout the
life of the bird must be known to understand the specialized displays,
especially of the reproductive period, that are derived from them.

6

COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Locomotion

Flight. The flight is similar to that of the Common Crow, Corvus
brachyrhynchos Brehm, but it is more direct and generally unhurried.
The wing beats appear to be more rapid, the wings are arched in
flight, and the birds body seems to rise and fall with every flap.
Blake (1948) records 2.8 flaps per second in normal flight, the rate
increasing to 3.8 when the bird hurried. McLean (1930) gives the
speed of a Green Heron in “steady, easy flight” as 34 mph. The bird
was flying at an altitude of twenty-five feet, and McLean timed the
bird with an automobile. Wood ( 1933 ) gives a flight speed of 22 mph.
I timed a Green Heron flying north on Cross Bay Boulevard on April
20, 1955. The wind was about 10 mph ( Beaufort 3 ) from the northeast.
For three-fourths of a mile the bird kept pace with my automobile at
a ground speed of 25 mph.

On long flights the neck is retracted, and the legs and feet are held
to the rear in line with the body. However, on short flights the bird
keeps its head and neck extended, the head slightly above the level
of the back. During the breeding season, this latter type of flight is
very common on short and long flights. In Pursuit Flight (p. 39)
the bird being chased flies with its head and neck held well above
the level of its back, and watches its pursuer intently.

Two variants of normal flight are conspicuous during the breeding
season: (1) the Flap Flight, a sexual display with hostile overtones
(see p. 41), and (2) a direct, rapid flight for attacking another bird
either perched nearby or passing in flight.

Green Herons occasionally glide just before landing; they almost
never glide during a sustained flight. Only once have I seen this
species perform the spectacular spiral-descent type of flight so often
displayed by other herons. A Green Heron flying over the Waters River
near Danvers, Massachusetts, on June 6, 1954, suddenly started to
drop toward the river in a whirling, twisting fashion, with its feathers,
especially the scapular plumes, streaming in the wind. The bird
abruptly checked its spiral descent when it was a few feet from the
shore of the river, where it landed and proceeded to feed.

Taking off. The actions of a bird about to take flight are of signifi¬
cance as intention movements. As long ago as 1910 Oskar Heinroth
showed that certain bird movements give the observer a clue to what
the bird intends to do. Heinroth called these incomplete or prepara¬
tory movements, especially those involved in locomotion, Intentions-
bewegungen (intention movements). (See Daanje (1950) for a
thorough review of the function and evolution ( “ritualization” ) of
intention movements in birds.) Many displays appear to be derived
from the basic movements of walking and intention movements such
as tail-flipping.

An accurate and complete description of all the movements per-

BREEDING BEHAVIOR OF THE GREEN HERON

7

formed by a Green Heron while taking off requires high-speed photog¬
raphy. The following actions are based only on field observations and
ordinary still photographs:

1. all feathers are sleeked

2. legs are bent

3. body is brought to a horizontal posture, wings slightly extended

4. head and neck are partially retracted

5. tail appears to be raised and fanned laterally, or it may be flipped

6. body is thrust forward and upward by extension of the legs

7. wings are extended upward

8. tail is depressed

9. head and neck are extended, crest may or may not be erected

10. legs and feet are brought together and extended horizontally rearward

11. head and neck are withdrawn to the body.

Green Herons perched on the ground in a wooded area characteris¬
tically walk to a nearby shrub and then make very short jumps to the
top of the shrub before taking off. If they are perched in the lower
branches of an open tree, they usually climb to the crown branches
before flying.

Numerous variations occur in this typical sequence. For example,
a bird perched in a vegetation tangle jumps from a semivertical rather
than a horizontal stance. Retraction of the head and neck may begin
from an alert posture. Still photographs show that some Green Herons
extend the legs rearward at once, while others delay this movement
until after the head and neck are drawn back to the body.

Landing. As with taking off, an exact description of landing is
difficult without the aid of high-speed photographs. However, the
Green Heron appears to execute the following movements:

1. short glide, legs and feet trailing, toes flexed, head and neck retracted
(glide may be omitted)

2. head and neck extended and elevated, crest erected (almost without
exception)1

3. legs dangled, then thrust forward and downward

4. tail spread

5. wings beat as if hovering

6. perch is grasped, head and neck retracted, crest lowered, body in up¬
right position; or, head and neck extended, crest lowered, body upright.

Walking. “Sneaking” is a more descriptive term than “walking” for
the Green Heron. Before starting, the bird follows steps 2, 3, and 4
of the preparation for flight: it bends the legs slightly, inclines the
body forward, and retracts the head and neck. Then the bird slowly
brings one leg forward, toes relaxed; gradually it brings the body
forward as the foot with toes widespread touches the ground. The
bill and head keep in line with the crouching body. The bird’s every
action is deliberate and reminiscent of a cat hunting.

1Glenister (1951) and Smythies (1953), among others, remark that the Striated
Heron, B. striatus, usually erects its crest just before landing.

8

COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Feeding Behavior

Feeding behavior is primarily of two types, Stand and Wait and
Wade or Walk Slowly. I shall also describe several other interesting
but relatively unimportant types.

Stand and Wait. The Green Heron stands motionless, head and neck
retracted, body held either low and horizontally (Figure 2) or semi-
vertically. When striking from the horizontal position the bird darts
its head and neck forward and down; if striking from the semivertical
position it shoots the head and neck out and down, at the same time
lowering its body to the horizontal position. Green Herons feed this
way most commonly in shallow water on the shore of a lake or stream.
A common variant is waiting while perched on a post ( Stone, 1937 ) ,
a limb, stub, or log (Saunders, 1926), or the transom of a rowboat.
Warburton ( 1948 ) watched a Green Heron perched on a log catching
flying dragonflies. When using such a perch, the hunting bird may
either crouch with head and neck retracted, or may wait with its head
and neck extended and down, peering into the water. The short legs
prevent wading in deep water; it fishes in deep water by perching
on logs and prop roots of Red Mangrove, Rhizophora mangle , and by
diving.

Wade or Walk Slowly. This is very similar to walking. As the bird
stalks closer to its prey, its steps become slower and longer; each foot
is brought forward, placed, and then lifted so slowly that the move¬
ments are barely perceptible. The entire tarsus may almost touch

BREEDING BEHAVIOR OF THE GREEN HERON

9

the ground. The bird may retract its head and neck or hold them
extended over the water or ground; or rarely the head and neck are
held momentarily in an extended “peering over” attitude. The strike
is very swift. I have never seen a Green Heron seize its prey with any
but a scissor-like grip; that is, the prey is not pierced by the bill.

Diving. Catching prey by diving into the water from a plank
(Barker, 1901), the stone edge of a park pool (Brooks, 1923), or from
the shore of a pond or stream (Kalter, 1932; Hawbecker, 1949) are
fairly common variants of normal feeding behavior. Brown (1949)
observed B. striatus diving for food.

Once caught, the prey is held crosswise between the tips of the
mandibles, and then it is tossed head first into the gape. Character¬
istically, Green Herons dip their bills in the water and shake their
heads after swallowing aquatic prey. The bird lowers the forward
part of its body toward the water and dips its bill vertically into the
surface, raises its head and bill clear of the water and then shakes
the bill vigorously. Kalter (1932) has also observed bill-dipping by
the Green Heron after swallowing prey. On many occasions, I have
seen Green Herons dip their bills after they had missed a strike.

Care of the Body Surface

Preening. Green Herons can reach their entire body surface with
their bill except the top of the head and the upper part of the neck.
The sequence in which the feather tracts are preened varies with the
individual, but there is almost no variation in the movements used.
Some individuals preen first the entire right side and then the left,
others rapidly alternate right-left-right. Most birds begin by preening
the breast feathers and end by oiling the bill on the oil gland.

The following preening sequence is fairly typical: the bird first
nibbled the breast powder down patch and then the breast feathers.
It grasped each feather at the base in the tips of the mandibles and
pulled up and out with short, smooth movements. Next the heron
nibbled the breast powder down patch, pulled additional breast feathers
out, nibbled several, and then partly opened one wing and put its
head and bill between the wing and body. Now the bird shook and
rubbed its head vigorously against the body. It then preened its abdom¬
inal feathers with rapid, flicking movements of the head, each feather
being grasped momentarily, then released with a short, outward flick.
Abdominal feathers were nibbled for several minutes. Flank feathers
were barely touched with the bill, but occasionally one was flicked out.
The breast powder down patch was nibbled again, and by this time
the bill and fore part of the head were covered with down, which
gave these parts, especially the bill, a bluish-white cast.

The bird next lifted the long, lanceolate scapular plumes singly,
and either pulled each one entirely through the tip of the bill, or

10 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

merely flicked one feather upward and released it with a rapid toss
of the head. To preen its neck feathers, the bird assumed a rather
grotesque attitude (Plate 1). After erecting all the neck feathers, it
nibbled individual ones as it drew them out rather than drawing them
through the tips of the bill in one smooth stroke.

The bird nibbled its primaries over their entire length or drew them
out in one smooth stroke, and it usually preened the primaries while
the wing was closed or only partly opened. It flicked primary coverts
upward and then released them. Underwing preening was accom¬
plished by fanning one wing, then inverting the head and lightly
touching the underside of certain primaries and secondaries. Plate 2
shows a Green Heron preening its underwing. The exact function of
this is obscure. The delicate manner in which the tip of the bill
barely touches certain feathers suggests the picking off of parasites,
but I have never seen anything swallowed at this time.

The last act of preening usually is the rubbing of the bill over the
oil gland. The bill is black during the courtship phases of the breed¬
ing cycle, and the oiling makes it glisten.

One complete preening bout recorded on 16mm film is as follows:
bird nibbled breast powder down patch — pulled out breast feathers —
nibbled breast feathers — nibbled breast powder down — placed head
under left wing — pulled out and nibbled abdominal feathers — nibbled
breast powder down — pulled out breast feathers — placed head under
right wing — nibbled breast — flicked scapulars — pulled out abdominals
— flicked scapulars — nibbled upper neck feathers — nibbled scapulars —
flicked abdominals — upper neck — scapulars — lower neck — scapulars —
lower neck — scapulars — lower neck — abdominals — upper neck — flanks
— abdominals — head under left wing — upper neck — scapulars — pri¬
mary coverts — primaries — under wings — abdominals — nibbled flanks
— abdominals — bird flew.

Hindwood (1933) gives a good description of powder down patch
nibbling and preening in Australian B. striatus.

Scratching. The Green Heron scratches by bringing the foot directly
up to the head; the wing is not drooped (“vornherum” of Heinroth,
1930; “directly” of Simmons, 1957). Scratching, normally prolonged
and vigorous, appears to be done mainly by the extended middle toe,
that is, the one bearing the pectinated claw (field observations sup¬
ported by 16mm motion pictures). During all my observations of
scratching, the bird’s crest was erected (see Plate 3). Hindwood
(op. cit.) noted that B. striatus in Australia characteristically erects
its crest when head-scratching. With the crest fully erect, a Green
Heron slowly rotates its head while the middle toe works the entire
surface. Head-scratching may occur at any time during a preening
bout; normally the bird scratches its head at least once during a
prolonged bout.

BREEDING BEHAVIOR OF THE GREEN HERON

11

Shaking. Occasionally during a preening bout, especially near its
end, the bird leans forward, extends its wings about half way, and
shakes out all its plumage (Plate 4). It erects all feathers and moves
the wings in and out vigorously. When the bird has finished, it sleeks
all of its feathers and then rapidly preens a few primaries with swift,
smooth strokes. Shaking out is at times a prelude to flight; shaking
out in flight after take-off is rare in the Green Heron.

Bill-cleaning. After preening, the bird’s bill may have one or more
tiny feathers stuck to the tip. The bird removes these with the
pectinated claw of the middle toe by lifting its foot upward and
forward, extending the middle toe, and flicking the fluff from the bill.
The powder down that covers the bill occasionally during prolonged
preening is usually brushed off before the end of the bout; if it is not,
the oiling of the bill removes any remainder.

Bill-wiping. Green Herons wipe their bills by stropping them on
a branch. The bird lowers its head, bill vertical, and rubs the bill on
the branch, alternating left-right-left with swift and smooth strokes.

Pecking the feet. At any time during a preening bout the bird may
reach down and peck its toes with the tip of the bill. As far as I can
tell, the bird does not touch the tarsi.

Stretching. In their usual stretch motion, Green Herons bend their
body forward, extend their head and neck, shift their weight to one
leg, and then stretch the wing and leg of the opposite side. They
then shift their weight to the other side and stretch the other wing and
leg. They may stretch their wings and legs at any time, but usually
during a preening bout.

In another type of stretching, seen more often in nestlings than in
adults, the bird stretches its head and neck fully forward, then raises
both wings over the body so that they meet.

Gagging and bubbling. After feeding, Green Herons often raise
their head, open their bill and make one or more swallowing movements.
Unlike the normal movements of swallowing, the bird appears to be
gagging as if something were caught in its throat, and the violent
movements apparently remove the obstruction. Adults feeding a brood
characteristically bubble their gular region by rapidly moving the
gular area in and out. This seems to facilitate disgorgement of the
food. Adults may follow gagging with bubbling movements.

Sun-bathing. Hauser ( 1957 ) lists the Green Heron as a “voluntary”
sun-bather, but she does not illustrate or describe the position. When
sun-bathing, these herons open their wings about halfway and let them
droop, raise their heads and necks slightly, and usually face the sun
(Figure 3). They may open their bills and elevate various feather
tracts, but they show much individual variation. Sun-bathing birds
may close their eyes, and they are then easily approached.

Defecation. The defecating bird lowers the back end of its body.

12 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 3. Green Heron sun-bathing.

elevates its tail, slightly opens and droops its wings, and then raises its
head and neck slightly. The female makes the same movements during
egg-laying. Green Herons frequently defecate when they are frightened,
usually right after take-off. This usually occurs in high-intensity escape
situations when the bird has little or no warning of the approach of
the intruder, and is part of a general fright response. When they have
ample warning, Green Herons seldom defecate on take-off.

Sleeping. A sleeping individual usually has the neck fully retracted,
wings slightly drooped, body feathers slightly erected, and eyes closed.
Birds sleeping on the ground frequently raise one leg so that only
the foot is visible protruding from the fluffed feathers. They frequently
sleep while incubating and brooding (Plate 5) and the head may rest
on the rim of the nest. In this position, the bird usually closes its

BREEDING BEHAVIOR OF THE GREEN HERON

13

eyes, but sometimes only the nictitating membrane. The lids are
closed for varying periods.

Yawning. This movement occurs most frequently after long periods
of incubation, and may indicate an oxygen need. Yawning seems to
be one of the general comfort activities ( stretching, walking, preening )
following long periods of inactivity while sitting on the eggs.

Relaxation. Often Green Herons adopt the normal perch position
(Plate 6) and spend long periods without moving. The bird appears
to be relaxed, no special movements occurring other than an occasional
glance at a passing bird. When perched low in protective cover, the
relaxing bird may cover its eyes with the nictitating membranes for
several seconds at a time.

Alarm, "‘Freezing,” and Escape Responses

Responses to danger depend on the type and distance of the stimulus
source. Responses may be grouped into three categories: (1) flight;
(2) alert responses; and (3) Bittern Stance. The Bittern Stance is
never accompanied by a call, but the others may be associated with
one or more calls.

Flight. If a Green Heron is fully aware of the approach of a human,
it usually flies away unhurriedly. Such a flight is accompanied by a
single, rather high-pitched “skyow” or “skeow,” the typical call of the
species. The flight of a startled bird is usually hurried, with one or
more “skyow” calls. Precipitous flights are invariably accompanied by
one or more high-pitched “skyows.” If a Marsh Hawk comes too close,
the heron flies off unhurriedly, sometimes with several “skyow” calls.

Incubating or brooding birds usually rise from the nest and walk
rapidly to the top of the nest tree when a human approaches slowly.
They may then fly to a nearby tree, or rarely out of sight. Flight is
unhurried but usually accompanied by “skyows.” If the intruder
remains quiet or hides, the bird will fly back to the vicinity of the
nest, but as soon as it sees the intruder, it explodes into flight, giving
a rapid burst of very high-pitched “skyows,” low, raspy “skuk-skuk”
or “kuk-kuk” calls, or high-pitched, rasping “raaah-raaah” calls. These
calls, especially the last, are repeated until the intruder goes away.
The “raaah” call expresses stronger hostile tendencies than does the
“skuk-skuk” call.

If an intruder surprises the bird on the nest, the bird flies off
explosively giving “skuk-skuk” or “raaah-raaah” calls, lands nearby,
and keeps calling. Green Herons do not hover over the nest or the
intruder as Black-crowned Night Herons do. If the visitor stays quiet,
the heron starts to look for him.

Alert postures. When they have ample warning of approach, Green
Herons take one of several alert postures (Figure 4), varying in the
amount of stretching of the neck, flipping of the tail, and/or raising
of the crest.

14 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 4. Alarm and alert postures of the Green Heron. From the normal
perch position (“A” — and see Plate 6), the bird may assume the Forward (“B”) or
Full Forward (“C”) Display with an increase in the attack tendency, or the bird
may take flight, assume the Bittern Stance (“J”), or any of the alert postures
(“D” — “I”), depending upon the strength of the escape tendency.

BREEDING BEHAVIOR OF THE GREEN HERON

15

Plate 6 shows the normal perch position; alert postures are modifica¬
tions of this. E in Figure 4 shows a bird in the normal alert posture.
I believe that the extension of the neck reflects the strength of the
escape tendency — the greater the escape tendency, the greater the
extension of the neck. Alert postures show components of preparation
for flight (see list, p. 7), specifically feather-sleeking, leg-bending,
and tail-flipping. From any alert posture, forward inclination of the
body and slight neck retraction prepare the bird for flight. I, Figure 4,
is an extreme alert posture adopted by birds attempting to locate a
predator hidden in a tangle some distance away. It is an extended
alert posture.

When the predator is close to a perched bird, the latter may adopt
a horizontal alert posture (F, Figure 4). This posture has the elements
of flight preparation and adds rapid extension and withdrawal of the
neck as the bird attempts to locate the hidden predator. A bird in
these two inquisitive alert postures appears very tense, and it raises
its crest and flips its tail conspicuously. Crest-raising in some alert
postures indicates a conflict between the tendency to escape and the
tendency to remain. I doubt that crest-raising in alert postures is a
hostile component because the crest remains fully erect in hostile
displays, while in alert postures, the bird rapidly raises and lowers
its crest — up-down-up-down. Crest-raising is the most common of all
the feather movements of the Green Heron with the exception of wing
movements during flight.

Bittern Stance. Green Herons show the Bittern Stance ( Pfahlstellung
of Portielje, 1926) in response to: (1) the sudden appearance of a
Marsh Hawk; (2) the alarm calls of several species of birds given
suddenly and very close to the Green Heron; and (3) the sudden
appearance of a human who surprises an incubating or brooding bird.
Three excerpts from my notes demonstrate these three situations :

May 1, 1954 — 0937 — Green Heron, with brilliant soft-part coloration,
perched in sumach ten yards from blind — preening slowly.

0938 — A male Marsh Hawk suddenly swept over the tree, and the
heron immediately assumed a full “bittern stance” — when the hawk flew on,
the heron relaxed and resumed its preening.

May 5, 1955 — 1138 — Green Heron preening about 15 yards south of
blind — male Redwinged Blackbird, perched about 10 yards from the heron
and not visible to the latter, suddenly gave one loud “check,” and the heron
assumed the “bittern stance” at once — the heron relaxed a few moments
later and resumed preening.

August 4, 1955 — 1630 — checked nest 1-66 (nest number 66 in breeding
area 1) — no adult present — three newly hatched young in nest — I took
position in tangle of vines near nest —

1635 — adult returned to nest 1-66 — started to brood young at
once — I waited for bird to get settled, then suddenly stood up — brooding
bird immediately assumed the “bittern stance” still brooding the young! —
held this position for one full minute — flushed wildly when I made sudden
move (Figure 5).

16 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

4

Figure 5. Green Heron in the Bittern Stance while brooding young.

BREEDING BEHAVIOR OF THE GREEN HERON

17

Several heron species show variations of the Bittern Stance, but
none, to my knowledge, is so strikingly similar to that of the Botaurus-
Ixobrychus bitterns as Butorides. Cowles (1930) observed a Bittern
Stance at the nest of a Striated Heron, B. striatus, in Africa.

The Green Heron’s Bittern Stance (Figure 6) involves:

1. immobility — except for occasional head-turning (see number 6), the
bird remains motionless until the predator leaves or forces the bird to fly

2. body held vertical — the bird holds its head and neck in line with the
body; the tail is depressed and the neck is stretched to its maximum
and pointed straight upward

3. legs straight — instead of flexing the legs as in flight intention movements
(p. 7), the legs are extended

4. feathers sleeked — the bird sleeks its feathers to the extreme

5. eyes bulged — the bird’s eyes appear to bulge from its head, and the
iris becomes a deep orange

6. head-turning — if the predator is distant, the bird may turn its head
very slightly in order to follow the predator’s path.

The Bittern Stance components eliminate, reverse, or highly modify
a number of the flight preparation movements (p. 7). The straight
legs and the vertical body, neck, and head, together with immobility
and extreme feather-sleeking, assist in concealment. I agree with
Daanje (1950), who feels that the Bittern Stance has evolved (rit¬
ualized) from the second phase of the jump, and that the “freezing”
in this phase “has the greatest selective value in relation to the given
environment and its own colouration.”

The stripes on the neck of a Green Heron holding the Bittern Stance
blend with the overall vertical aspect of a background of reeds, Phrag-
mites communis. The similarity is heightened by the extreme vertical
thinness of the bird. Cowles ( 1930 ) remarks on the striking similarity
of a Striated Heron in the Bittern Stance to a bed of reeds. Butorides
is practically cosmopolitan; Phragmites is cosmopolitan. It is quite
possible that the Bittern Stance of Butorides originally evolved as an
adaptation to a reed or reed-like environment, even though today the
different species of Butorides frequently occur in habitats free of reed
vegetation.

The bulging eyes and the slight head movements help keep the
predator in constant view. Rarely, a Green Heron in the Bittern Stance
will turn very slowly on its perch in order to follow a passing predator,
but only when the predator is some distance away. Cowles (1930)
noted this same slow turning in an African striatus holding the Bittern
Stance.

Interspecific warning. Green Herons react to the warning calls of
many species of birds and assume one of the alert postures or Bittern
Stance, or fly. The heron flies only when the call is given suddenly,
loudly, and very close, especially if the heron is previously unaware
of the presence of the calling bird. Table 1 lists species to whose alarm
notes Green Herons react.

18 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 6. Green Heron in the Bittern Stance.

BREEDING BEHAVIOR OF THE GREEN HERON

19

Table 1

Birds and their calls which release alert postures and/or flight

in the Green Heron.

Species

Call

Great White and Great Blue Herons, Ardea herodias
Little Blue Heron, Florida caerulea
Reddish Egret, Dichromanassa rufescens
Common Egret, Casmerodius albus
Snowy Egret, Leucophoyx thula
Louisiana Heron, Hydranassa tricolor
Black-crowned Night Heron, Nycticorax nycticorax
Yellow-crowned Night Heron, Nyctanassa violacea
American Bittern, Botaurus lentiginosus
Tiger Bittern, Tigrisoma mexicanwn
Fish Crow, Corvus ossifragus
Redwinged Blackbird, Agelaius phoeniceus
Common Grackle, Quiscalus quiscula

"frawnk”

« i »

aaah

“crog”

"frawnk”

44 99

aarg

it i y>

raah

44 1 >y

squok

44 i yy

squak

"wok-wok”

"wok-wok”

"caw-caw”

"check”

“chuck”

I observed no responses to the alarm notes of a number of warbler
species (Parulidae), sparrows, or the many Herring Gulls, Lams argen-
tatus Pontoppidan, that flew over the breeding areas on Rulers Bar
Hassock. Surprisingly, the loud, high-pitched alarm calls of the Killdeer,
Charadrius vocifems Linnaeus, rarely evoked even a mild alert posture
from a Green Heron. Many of the species listed in Table 1 bred in the
same areas as the Green Herons, while the Killdeer on Rulers Bar
Hassock nested around the heron breeding sites. Gulls bred in the
marshes far from the heronry, so the inter-specific responses of the
Green Heron may be conditioned by learning the warning calls of
nearby breeding associates.

Responses to other stimuli. When they first arrive on the breeding
grounds, Green Herons react, sometimes violently, to a number of
sounds with no biological meaning (roar of fire engines, police sirens,
and aircraft). In most cases, habituation (in Thorpe’s sense, 1956)
rapidly leads the birds to ignore such extraneous sounds. But one pair
may become habituated while another never learns. Male and female
1-13 were under daily observation from May 9 to June 15, 1955, on
Rulers Bar Hassock. A small abandoned settlement, “The Raunt,”
about 200 yards from nest 1-13, was set on fire by local juveniles almost
every weekend during this period. I was in my blind near nest 1-13
on May 14, 1955, when the fires were set for the first time after the
arrival of the Green Herons. When the fire engines came roaring
north on Cross Bay Boulevard, male 1-13, which was incubating one
egg at this time, fairly exploded from the nest, almost knocking the
egg out. The bird flew out of sight, but returned shortly after the
fire engines left. The male’s flight on the following weekend was
not quite as precipitous, but the bird did leave the nest hurriedly.
The female flew violently from the nest later the same day after the

20 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

engines were recalled to the burning “Raunt.” The following weekend,
male 1-13 only stood over the eggs when the fire trucks roared by, but
the bird was obviously tense. Female 1-13 merely turned her head
in the direction of the sounds. The male continued to rise from the
eggs or young throughout the observational period, but the female
finally ignored the sirens completely.

The opposite of these sexual differences was found at nest 1-12.
Here, the male became habituated very rapidly, but the female never
did, and her precipitous flight from the nest lasted throughout the
breeding season.

Responses to predators . Green Herons respond in many ways to
the approach of predators. An incubating bird invariably threatens
and/or attacks an approaching Common Grackle, Quiscalus quiscula
( Linnaeus ) . I have never seen a Green Heron intimidated by grackles,
which only take Green Heron eggs from unguarded nests. Grackles
are smaller than Green Herons and are usually intimidated by them.

These herons, however, respond to the egg-hunting of Fish Crows,
Corvus ossifragus Wilson, in various ways. Some threaten the crows
briefly and then leave, some leave their eggs as soon as the crow
advances toward the nest, while a few threaten, attack, and drive
away the predator. Green Herons with young will usually remain and
act hostile, but action varies with the individual. The Fish Crow has
a more formidable threat display than the Grackle. Even the large
Black-crowned Night Herons and Yellow-crowned Night Herons,
Nyctarmssa violacea (Linnaeus), are quick to leave their eggs and
young. Thus the intensity of the displays and the persistence of the
crows, not their size and aspect, must be of chief importance.

Green Herons are more prone to flee and land and then scold after
they are frightened by a person if young are in the nest, and they are
more likely to fly far away if the nest contains eggs. Presumably the
calling has survival value in that it stimulates young nestlings to crouch,
and older ones to give the Bittern Stance. The “raaah” calls appear
to have three functions : ( 1 ) to intimidate a potential predator; ( 2 ) to
warn the young; and (3) to warn nearby herons. Sometimes adults
flushed from nests containing eggs give one or more “skyow” calls as
they fly away. Nearby Green Herons become alerted, actions which
suggest a social warning function for the “skyow” call.

Only those Green Herons whose nests are on or very close to the
ground react to the presence of a dog. They rise to a nearby branch
without trying to peck at the dog and give the “skuk-skuk” or “raaah”
call. The dog usually looks briefly at the heron and then trots off.

MIGRATION AND BEHAVIOR ON ARRIVAL

The Green Heron moves northward in the late winter and early
spring from wintering grounds in northern South America, Central

BREEDING BEHAVIOR OF THE GREEN HERON

21

America, Mexico, and the southern portions of the Gulf states. Large
migratory flocks are mentioned in the literature (Scott, 1890); and
many authors speak of nocturnal migrations (for example, Weston,
cited in Howell, 1932; Cruickshank, 1942). Griscom (1923) states,
"especially in August Green Herons are often heard migrating at
night in extraordinary [sic] numbers.”

Sexing on the basis of observed behavior and soft-part color suggests
to me that both sexes arrive on the breeding grounds at the same time.
Allen’s ( 1929 ) statement that the females arrive first does not hold for
Rulers Bar Hassock, where I made observations during the years 1954,
1955, and 1957. Table 2 lists significant events during the breeding
seasons of 1955 and 1957.

Table 2

Arrival behavior, significant nesting events, and departure of Green
Herons, Rulers Bar Hassock, seasons of 1955, 1957.

Date of Occurrence

Calendar of Events -

1955 1957

First Green Heron arrives

April

16

April

20

First Flying Around behavior

n

20

//

21

First Pursuit Flight

n

20

//

21

First “Skowing”

n

22

//

21

First Stretch Display

n

23

n

27

First Flap Flight Display

n

23

n

22

First prolonged nest repairs

n

28

9t

27

First egg of season laid

n

30

n

29

First young hatched

May

22

May

21

First brood fledged

June

13

June

11

First egg of second nesting

//

22

unknown

Last egg of season laid

August

4

unknown

Last Green Heron observed on area

October

6

unknown

Migration. In each of the three years the first Green Heron arrived
during the third week in April. I agree with Cruickshank’s (1942)
statement, “the first widespread wave generally arrives by the end
of the third week in April.”

Table 3 lists the time of arrival, numbers, and flight paths of Green
Herons believed to be migrating, observed on Rulers Bar Hassock
from April 16 to April 23, 1955. Many of these records are of two
birds arriving together. Some of the migrants landed in the breeding
areas and seemed to stay. Others did not land but continued north¬
ward, while a few settled for a few minutes and then resumed their
northward flights. In 1955, at my request, Dr. William H. Drury, Jr.,
then of Cambridge, Massachusetts, made observations of the arrival
of Green Herons in eastern Massachusetts. He commented that the
species arrived in 1955 most commonly in twos. Figure 7 shows the

22 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 7. Map of Rulers Bar Hassock showing main migration routes of
Green Herons.

BREEDING BEHAVIOR OF THE GREEN HERON

23

principal lines of flight to and beyond Rulers Bar Hassock during the
same period indicated in Table 3. Most herons flew at a height of
about 25-50 feet. At no time did they call in flight.

Time

OF

ARRIVAL,

Table 3

NUMBERS, AND FLIGHT PATHS OF MIGRATING GREEN HERONS

on Rulers Bar Hassock, April, 1955.*

Date

Time

Number

Flight Path

April

16

1130

1

Army FA, N to 1 BA, lands, flushes N out of sight

ft

ft

1201

1

Raunt FA, N out of sight

April

19

1655

1

Raunt FA, over CBB, lands in 1 BA

tt

tt

1701

1

Army FA, N to 1 BA, lands

tt

ft

1748

2

Army FA, N to 1 BA, land 100 yards apart, roost

ft

tt

1801

3

Army FA, N to 1 BA, land

tt

ft

1807

2

Raunt FA, N to 4 BA, land and roost

April

20

0802

1

Army FA, N to 1 BA, lands and stays

ft

ft

0803

1

W along dike to 1 FA, lands and feeds

tt

tt

0805

1

Army FA, N to 1 BA, lands

tt

tt

0941

1

Raunt FA, N to 4 BA, lands

tt

ft

1213

1

N on CBB, speed was timed-lost sight of near NCB

ft

tt

1748

2

Raunt FA, N to 4 BA, land

tt

ft

1817

1

Army FA, N to 1 BA, lands

tt

tt

1819

2

Army FA, N to 1 BA, land apart, roost

April

21

1615

1

Army FA, N to 1 BA, lands

tt

ft

1625

3

W along dike to 1 FA, then to 1 BA, land

tt

tt

1805

2

Army FA, N to 1 BA, land

April

22

1700

2

Army FA, N to 1 BA, land

ft

tt

1815

3

Army FA, N to 1 BA, land

tt

tt

1817

2

Army FA, N to 1 BA, land

tt

tt

1818

1

Army FA, N to 1 BA, lands

tt

tt

1819

1

Army FA, N to 1 BA, lands

tt

tt

1829

1

Army FA, N to 1 BA, lands

April

23

1444

1

Raunt FA, N to 4 BA, lands

ft

ft

1733

1

Raunt FA, N to 4 BA, lands

tt

tt

1736

2

Raunt FA, N to 4 BA, land apart

tt

tt

1737

1

Army FA, N to 1 BA, lands

tt

ft

1802

1

Raunt FA, N to 4 BA, lands

° Abbreviations as in Figure 1. All times are EST.

I have never seen “flocks of from 20 to 50 individuals” as recorded
by Audubon (1840) and quoted by Townsend (in Bent, 1926); it is
possible, however, that larger flocks may have dispersed before they
reached Long Island. Furthermore, during my three years of observa¬
tion on Rulers Bar Hassock, I found no traces of any migratory move¬
ments at night. Migration appeared to end shortly after sunset; the
birds landed and roosted in reeds and bushes. I spent several nights
on Rulers Bar Hassock during the migration period but heard no
migrating Green Herons. The diurnal migrants did not call; if silent,
nocturnal migrants would be missed. Audubon (op. cit.) states that

24 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

the large flocks he saw landed for the day just after sunrise; I saw no
such movements at any time, and I was on the observation platform
at least one hour before sunrise and remained there for at least one
hour after sunset.

Many of the early migrants on Rulers Bar Hassock had brilliant
soft-part colors, but because of the generally poor visibility, no general
statement can be made for all migrants. Later migrants varied from
brilliantly colored to dull colored individuals.

Behavior. Early migrants rarely shifted from place to place after
settling in the roost. Aggressive behavior during the early arrival stage
was limited to an occasional “skow” or Forward Display. Many of
the early migrants appeared to be tired.

In their close attachment to their territorial site after arrival, Green
Herons differ from the other herons on Rulers Bar Hassock. The former
move in at once and start reproductive activities. I saw no dancing
ground behavior by Green Herons before occupying the nesting sites.

HOSTILE AND SEXUAL BEHAVIOR

I use “hostile behavior” for activities that appear to express attack
motivation and “sexual behavior” for displays that have few or no
apparent hostile or other non-sexual components. However, the de¬
scriptions that follow show that the designation “hostile” or “sexual”
is frequently artificial. It is difficult to interpret the range of expression
resulting from interactions of varying drive strengths as attack, escape,
sexual, or otherwise. Recent ethological publications emphasize the
roles of the attack and escape drives in the causation of threat displays
(see, for example, the review by Moynihan, 1955a), but the relation of
these two drives with others, principally the sex drive, must be con¬
sidered (Hinde, 1953a).

For example, crest-raising is a component of many Green Heron
displays. In the Full Forward Display it is a hostile component, but
in the extended alert posture it may express conflict (p. 15). Also,
crest-raising in some displays seems to express general excitedness,
separate from hostility or sexuality.

I should like to offer an example as an argument against pigeon¬
holing displays as sexual or hostile. Moynihan (1955b) states, “they
[i.e., appeasement displays] are produced, of course, by the usual type
of hostile motivation, but the escape drive of an appeasing bird is
almost always stronger, usually much stronger, than the attack drive.”
The Stretch Display of the male Green Heron is an appeasement display
in certain circumstances. Although male Green Herons usually orient
their Stretch Displays toward a female, they frequently perform Stretch
Displays when no other Green Herons are in sight. I cannot imagine
the activation of a strong escape drive in such males. Moynihan
(op. cit.), however, does add, “the immediate causation of many ap-

BREEDING BEHAVIOR OF THE GREEN HERON

25

peasement displays, apparently including three or more distinctly dif¬
ferent drives, is far more complicated than that of the majority of
threat displays.”

Moynihan assigns a subsidiary role in pair formation to advertising
songs, their primary role being intimidatory in nature. My work on
the Green Heron shows that at an early stage, the male’s "skow” func¬
tions not only as a long distance threat, but also as a "sex recognition
marker” for unmated females. Later, the male’s advertising calls stimu¬
late one particular female to return to the vicinity of the male’s display
post and engage in mutual sexual activities. One function of the
female’s "skeow” appears to be to stimulate the male to perform
sexual displays. These problems will be discussed later, but the assign¬
ment of only a threat or intimidatory function to the advertising calls
of the Green Heron would be misleading.

Many displays, at least those in the birds with which I am most
familiar, grade from purely hostile to purely sexual. The causation
of the extreme hostile and sexual displays seems clear, but for many
displays of lower intensity I cannot decide whether a certain com¬
ponent appears to express hostile, sexual, or other motivation; further¬
more, I find it difficult to apply the concept of a balance or conflict
of drives in some low intensity displays.

Even though I consider them artificial categories, I shall use the
terms "hostile behavior” and "sexual behavior,” but for purposes of
presentation only.

HOSTILE BEHAVIOR

All observations involving hostile behavior were of encounters among
free-living birds during the breeding season. Green Herons are solitary
once they have arrived on their wintering grounds, which may be
more than a thousand miles from their breeding areas. At no time
during the winter of 1955-56 did I see more than one Green Heron
in any one area in Florida Bay. Lack of experimental data and the
rarity of encounters among non-breeding birds make the evaluation
of breeding season fighting difficult because of the influence of sexual
and other drives.

The terms "drive” and “motivation” have been used variously in
ethological and psychological literature, and in some recent ethological
papers (Moynihan, 1955c; Dilger, 1956), the terms are used inter¬
changeably. I shall use the term “tendency,” following Hinde ( 1956a),
Marler (1956), and Andrew (1956-57). For example, when it is my
impression that a given individual is behaving in a sexual or hostile
fashion, I shall use "sexual tendency” or "attack tendency.” For a
critical review of the use of the term "drive” in ethological literature,
see Hinde ( 1956a ) .

Because no experiments were made, the estimates of relative and

26 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

actual strengths of the tendencies I have assigned to some displays
must be considered strictly descriptive.

I recognize the following four overlapping types of hostile behavior:
( 1 ) non-aerial, non-vocal; ( 2 ) non-aerial, vocal; ( 3 ) aerial, non-vocal;
and (4) aerial, vocal. This division is for presentation only, and does
not imply that this is the only method of classification.

Non- aerial. Non- vocal Displays

Crest-raising. The crest of the Green Heron is the most mobile of
its feather units, and there is great variation in crest-elevation ( Plate 7 ) .
Green Herons show crest-raising in numerous situations. Moderate
crest-raising, together with alternating erection and depression, is char¬
acteristic of conflict situations, while extreme erection is always an
expression of high intensity attack tendencies. Full erection always
accompanies the Full Forward Display, and some degree of erection
is shown by a bird assuming the Forward Display. A male on its
territory, surprised by the sudden appearance of another Green Heron,
shows crest up-down behavior, but if the intruder alights on or close to
the territory, the owner assumes the forward posture with crest erect.
The alternating crest up-down may indicate a conflict of escape-attack
tendencies.

Green Herons landing anywhere on the breeding area, including
their own territory, always erect their crest on landing. It seems as
if territorial stimuli trigger such crest-raising; a bird landing in its
own territory is presumably prepared to defend the territory, while an
intruder is prepared to defend itself against the attacks of the owner,
even if the owner is not present when the intruder alights. Green
Herons landing out in the vast feeding areas surrounding their breeding
sites at Rulers Bar Hassock rarely erected their crest on landing.
Exceptions occurred in the proximity of other feeding Green Herons.
At Newburyport Harbor, Massachusetts, where Green Herons defended
feeding areas, crest-erection on landing in the feeding areas was usual.

Habituation and recognition of neighbors may account for peculiar
changes in hostile behavior exhibited at later stages in the breeding
season. As the male's behavior shifts from hostile to sexual displays,
the territory shrinks until only the nest and a few feet around it are
defended. A bird defending such a small territory rarely shows any
crest-erection at the approach of a neighbor. Recent arrivals, however,
that have just been able to carve out a territory, apparently because of
the shift from hostile to sexual behavior by the earlier arrivals, show
vigorous hostile behavior toward these neighbors, but only for a brief
period. As soon as the late arrivals direct their displays toward one
female, the flights of nesting neighbors are almost completely ignored.
Crest-raising is almost completely eliminated among neighbors at the
same stage of the breeding cycle. The decrease of hostile responses,

BREEDING BEHAVIOR OF THE GREEN HERON

27

however, may be either the result of over-riding parental tendencies or
of learning the characteristics of neighbors.

When adults are disturbed at the nest, crest-raising from the Forward
Display posture is typical if young are present. If eggs are in the nest,
the flushed bird rapidly raises and lowers its crest.

At times, slight crest-raising appears to be a low intensity intention
movement of attack. The full erection accompanying the Full Forward
Display makes the displaying bird appear much larger. The crest is
dark, glossy, greenish-black, sharply set off from the maroon of the
neck. This contrast emphasizes crest-raising. The color and striking
movements of the crest suggest a true releasing function. Presumably
the primary function is to increase the apparent size of the bird, thus
helping to intimidate an opponent; that is, to release avoidance in the
latter. However, the objects of the display may react otherwise. Some
respond by displaying hostility in return; some react with a direct
attack; while some ignore the display. Crest-raising may also have a
secondary function as a social warning. Birds flushed are made much
more conspicuous by alternately raising and lowering the crest, thereby
alerting their neighbors.

I doubt that crest-raising in any alert posture indicates an activated
attack tendency ( p. 15 ) . Moynihan ( 1955b ) , however, feels that both
attack and escape drives are activated simultaneously; that is, when¬
ever one or the other of these two drives is activated, the other is also
activated. Dilgers (1956) work on the North American thrushes
reveals that crest-raising in these birds is usually associated with an
increased attack motivation, but Marler ( 1956 ) associates crest-raising
in the Chaffinch, Fringilla coelebs Linnaeus, with escape motivation.

A Green Heron fleeing from the direct attack of another Green Heron
does not erect its crest during the escape flight. During a Pursuit
Flight the pursued often erects its crest as the pursuer draws near.
Shortly after alighting, the pursued individual assumes the normal
perch posture ( Plate 6 ) with no crest-erection if the attacker does not
continue or renew the attack.

The crest is never raised in pure escape movements such as the
Bittern Stance or the Withdrawn Crouch (p. 36). On one occasion
I saw a Green Heron in the Bittern Stance erect its crest as it very
slowly turned its head to watch a Marsh Hawk, but only after the
hawk had flown some distance from the heron.

The causal analysis of crest-raising is further complicated by its
occurrence in the Crooked Neck Flight Display (p. 41 ). In this display
a raised crest emphasizes what is primarily a sexual display. I have
dwelt at some length on crest-raising because it is a component of
so many diverse displays, and its meaning varies with each display.

Tail- flipping. The degree and intensity of tail-flipping vary with
the intensity of the tendencies involved. A bird attacking an opponent

28 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 8. Green Heron in the Forward Display. Dotted lines indicate tail¬
flipping.

from the Full Forward Display posture always shows exaggerated tail¬
flipping, both in intensity and frequency. The tail is moved, as far
as I was able to tell, only in a vertical plane. A bird in one of the
alert postures frequently shows slight tail-flipping, a flight prepara¬
tion movement; in hostile displays flipping becomes greatly exaggerated.

Feeding Green Herons frequently flip their tails just before striking,
but the causal basis here seems obscure. Tail-flipping, like crest-raising,
may also serve as a social warning signal because alarmed birds are
thus made more conspicuous. Birds in the Bittern Stance or the With¬
drawn Crouch never flip their tails.

Forward Display . A bird assuming this display posture (Figure 8)
from the normal perch posture (Plate 6) moves as follows:

1. lowers the head and body to the horizontal

2. withdraws the head and neck so as to appear neckless

3. bends its legs at the heel joint, thus assuming a crouched position

4. erects its crest and keeps the feathers erect throughout the display

5. erects neck, breast, back, and flank feathers moderately, and erects back
(scapular) plumes slightly

6. if it flips its tail, the motion is slow

7. holds its bill in a horizontal position and keeps it closed

8. orients its body to face the opponent.

The Forward Display almost always precedes the Full Forward Dis¬
play. In one variant of the Forward that I have called “stalking,” the
displaying bird assumes the Forward Display, then walks slowly and

BREEDING BEHAVIOR OF THE GREEN HERON

29

Figure 9. Green Heron in the Full Forward Display. Dotted lines indicate
extreme tail-flipping.

deliberately toward the opponent. Males displaying on the nest plat¬
form frequently alternate Forwards with stalking around the nest rim.

Full Forward Display. Except for actual attack, this display expresses
the highest level of the attack tendency ( Figure 9 ) . From the Forward
posture, the bird moves as follows:

1. extends the head and neck fully forward, body still horizontal

2. erects crest, neck, back, breast, and flank feathers to an extreme, and
keeps them erected throughout the display

3. erects scapular plumes in a fan-like manner

4. flips tail vigorously

5. bulges eyes (iris may change from yellow to deep orange)

6. opens its bill, showing the rich, red lining of its mouth

7. lunges toward the opponent, extends its wings slightly and waves them

8. at the height of the lunge, utters rasping “raaah” calls (p. 38).

The bulging, yellow-orange eyes of the displaying bird are accen¬
tuated by the dark, blue-black lores, and the effect of the gaping bill
is heightened by the rich, red lining of the mouth. Of all the threat
displays of the Green Heron, the Full Forward is the most effective
because the opponent is almost always intimidated, retreating im¬
mediately. If, however, the opponent displays in return, a vicious
fight ensues.

30 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

An aerial variant of the Full Forward occurs when the attacker
flies a short distance toward a displaying opponent.

Fighting. Two Green Herons engaged in fighting are so preoccupied
that I was able on several occasions to approach and almost touch
fighting birds. During contact both combatants peck viciously at each
other, mostly at each other’s head. They flail away with extended wings
trying to buffet each other. It is characteristic of fighting Green Herons
that they stab from a horizontal stance, rather than from the upright
position of many other herons.

Fights are rare and usually last less than a minute or two. I have
never seen a fight that appeared to result in real bodily harm. The
fight usually ends when one bird flees giving several high-pitched
“skeow” calls.

Stiff-necked Upright Display. This is the rarest of the threat displays,
and it is difficult to analyze. It is my impression that in some situations
the strengths of three tendencies — attack, escape, and sexual — are all
high and in conflict; in other situations only attack and escape ten¬
dencies are intense and in conflict; while occasionally attack and sexual
tendencies are involved.

I have seen the Stiff-necked Upright Display in males just before
and immediately following an attempted copulation with a non-
receptive female. These males had just been vigorously displaying
the Stretch and/or Snap Displays (p. 33). As the female approached
the male on his territory, the latter showed the Stiff-necked Upright
Display instead of the normally expected Forward or Full Forward
Display. The male suddenly stretched his head and neck upward and
slightly forward, erected the neck feathers, giving the neck a very thick,
stiff look, and erected the crest slightly. Crest feather movements
were variable. The aspect of the male was tense and rigid, and he
moved in a very stiff, awkward manner toward the female (Figure 10).

If the female suddenly moved toward the male, the latter pecked
down at her, but the pecks were short, and the male immediately
retracted his head and neck, as if the thrusts were being delivered in
a half-hearted manner. The appearance of the male was that of a bird
that wanted to attack, flee, and copulate all at the same time. If the
female turned from the male, the latter attempted to mount her, but
never successfully as far as I could see.

Always after the thwarted copulatory attempt, the male showed
what appeared to be displacement preening from the stiff-necked
position. In normal preening of the neck region the bird stretches its
head and neck up and slightly forward, but there is little or no erection
of crest feathers, and the preening is accomplished with deliberate,
smooth strokes. The neck-preening shown from the stiff-necked posi¬
tion is rapid and performed with jerky, uneven strokes; it is never
complete. This display is never accompanied by a call.

BREEDING BEHAVIOR OF THE GREEN HERON

31

Figure 10. Green Heron in the Stiff-necked Upright Display.

32 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Possibly the Stiff-necked Upright Display has been ritualized from
displacement neck preening with hostile elements superimposed. The
appearance of preening movements after an unsuccessful copulatory
attempt may indicate thwarting of the sexual tendency. The actual
bodily contact resulting from the abortive copulatory attempt may
serve to increase the sexual tendency. An alternative explanation for
the incomplete neck-preening is that the bird is simply smoothing
feathers that have been disarranged during the display. The continued
presence of the female may account for the brevity of the male’s
preening movements.

The Stiff-necked Upright Display is also shown under conditions that
may express antagonistic activation of attack and escape tendencies.
When “skowing,” typically a hostile display (p. 36), a territorial male
normally adopts the Forward Display if another individual lands on its
defended area. Under certain conditions, however, the defending male
will assume the Stiff-necked Upright Display. If the intruder is a male
and “skows” upon landing, the territory holder assumes the Stiff¬
necked Upright. The “skowing” and the sudden appearance of the
intruder might momentarily activate a strong escape tendency in the
territory holder. This display is shown only for a few moments, and
in almost every case the intruder fled when the owner displayed. In
exceptional cases the intruder did not flee until the owner assumed
the Full Forward Display.

On one occasion I saw the Stiff-necked Upright Display when it
seemed to involve both attack and sexual tendencies. A recently
formed pair were showing mutual feather-nibbling, normally a prelude
to copulation, when suddenly a neighboring male landed in the same
tree and “skowed” once. The male of the pair immediately assumed
the Stiff-necked Upright Display and lashed out and down at the
intruder, which immediately fled. It seems likely that in this situation,
the displaying male’s sexual tendency had been quite intense, and that
the sudden appearance of the intruding male activated a strong attack
tendency, resulting in the Stiff-necked Upright Display.

Bill-snapping. This action is shown before copulation, especially
before the first few copulations. The activity involves an interaction
of hostile and sexual elements. Both sexes show this behavior, but the
male shows more hostile elements in bill-snapping than the female,
although they are of low intensity. As the male approaches the female,
he adopts the forward position with very slight erection of crest and
neck feathers; he extends his head and neck forward, then rapidly
clicks his mandibles together close to the head of the female. She
reacts by adopting the forward position, extending her head and neck
toward the male’s head, and then she bill-snaps. The crest and neck
feathers of the female are barely erected.

Feather-nibbling. Both birds usually engage in mutual feather-

BREEDING BEHAVIOR OF THE GREEN HERON

33

Figure 11. The Snap Display or Schnappbewegung of the Green Heron.

nibbling before copulation. The head, neck, and flank feathers of the
partner are gently grasped in the tips of the mandibles, and then the
feathers are rapidly released and grasped again repeatedly. Both
sexes may flutter their wings during feather-nibbling activities.

A sudden movement on the part of either bird causes its partner to
retreat at once, showing a low intensity Forward Display. A few
moments later the bird that has retreated again approaches its partner,
showing bill-snapping, then feather-nibbling behavior. Bill-snapping
and feather-nibbling are reduced to a minimum after the first few
copulations.

Snap Display. A male when snapping assumes the forward position,
extends his head and neck fully forward and down, and then snaps
his mandibles together to produce an audible click. The crest and neck
feathers are slightly erected (Figure 11). I believe that these feather
movements are indicative of low intensity hostile tendencies, because
a male approached by a female during or immediately after the per¬
formance of a Snap Display may show more intense hostile behavior
toward the female.

This display is the Schnappbewegung of the Common Heron, Ardea
cinerea Linnaeus (Verwey, 1930). The movements of B. virescens are
very similar to those of A. cinerea , but it is my impression that the
snap of cinerea has been more ritualized than the snap of the Green

34 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Heron, because additional, highly exaggerated movements are involved
in the Snap Display of virescens. The movements of cinerea are few and
smoothly performed, indicating, according to the “rules,” more ad¬
vanced ritualization. Daanje (1950) and Tinbergen (1954) have
indicated that the course of ritualization involves exaggeration and
simplification of movements.

I have never seen female Green Herons perform the Snap Display,
and neither has Verwey in cinerea ; de Waard (1937)1 and Baerends
and Baerends (1950:152) claim otherwise for cinerea.

I regard the Snap Display of the Green Heron primarily as a sexual
display, but it appears to contain some hostile elements.

A sexually active male Green Heron will show a rapid alternation
of Stretch and Snap Displays, and the longer the male’s sexual ten¬
dencies remain unsatisfied, the more frequent and the more intense
his Snap Displays will be. After prolonged and intense sexual dis¬
playing, the male frequently shows modified versions of the Snap
Display. For example, the bird may rarely erect its feathers, and it
adds bobbing and bowing movements. With increased sexual activity,
the whole body of the displaying male vibrates, and the branch on
which the male is perched shakes violently with the intense bobbing
motions. A male may begin a bout of Snap Displays by first extending
the head and neck out horizontally, then bowing low very slowly;
the initial bow may then be followed by a fantastic series of intense
bobs, then a few slow bows, and then a repetition of the entire sequence.
Males showing such intense Snap Displays usually shift readily to
bill-snapping and feather-nibbling at the approach of a female. This
shift probably indicates a sharp reduction in hostility on the part
of the performing male. Sometimes a male showing such intense Snap
Displays over a long period of time will attempt to force copulation on
an approaching female, and under these conditions, the Stiff-necked
Upright Display described previously may be shown.

Verwey (op. cit.) and Tinbergen (1952) conclude that the Snap
Display of cinerea has been derived from the fish-catching movements,
but I feel that the display is more likely ritualized twig-grasping, a
component of nest-building behavior. Verwey saw the movement in
great detail only in cinerea , where from his description and fine drawing
(adapted from Holstein, 1927), the action certainly resembles fish-
catching movements. My comparison of snap movements in virescens
and other North American heron species, including the Great White
and Great Blue Herons, points to a derivation from nest-building
movements. In typical nest-building, the male Green Heron secures

1 1 have not seen de Waard’s paper. Baerends and Baerends state that their own
unpublished data reveal only occasional snap movements in female cinerea. My
own data include no observations of a female showing the snap; unmarked females,
however, might be mistaken for males.

BREEDING BEHAVIOR OF THE GREEN HERON

35

Figure 12. The Withdrawn Crouch escape posture of the Green Heron.

a twig and passes it to the female standing on the nest platform; the
latter inserts the twig into the nest with the highly characteristic
"tremble-shoving” movements ( Lorenz, 1955 ) . When grasping a twig,
the male does so by reaching out and down, head and neck fully
extended forward, and grasping the twig between the tips of the
mandibles. During functional twig-grasping, the crest, neck and flank
feathers are never erected; in addition, the movements are slow and
smooth, not formalized as in the Snap Display. Males showing the
Snap Display occasionally seize, then release, a twig during the per¬
formance. Rarely, a twig will be actually broken off, but it is always
dropped at once. Verwey stated that cinerea never seized anything
during the Snap Display, but my observations of herodias show that
twigs may be grasped momentarily during the snap performance.

It might be argued that low intensity nest-building tendencies are
shown by a male performing the Snap Display, but in the derived
twig-grasping movements, as described above, such movements only
partly resemble typical or functional twig-grasping. The incomplete
twig-grasping seems to favor the derivation of such movements in
the Snap Display from nest-building activities, because Green Herons
showing true, but low intensity, twig-grasping normally seize and
wrestle with the twig for some time before dropping it. They may
movq about the tree, inspecting and grasping a number of twigs. If
functional twig-grasping behavior was being shown in the Snap Dis¬
play, we would expect an occasional male to break off a twig and
carry it to the nest, but I have never seen such behavior prior to

36 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

pair formation. After the pair is formed, the male no longer engages
in Snap Displays, and his twig-grasping at this time is definitely
functional.

Withdrawn Crouch. This posture indicates high escape tendencies,
and it is assumed by a Green Heron when surprised by the sudden
appearance of another bird, typically another Green Heron, from above.
The head is completely withdrawn, the plumage is flattened, and the
bird crouches low (Figure 12). A bird may change from the With¬
drawn Crouch to a number of other postures depending upon the
nature of the disturbance. For example, if a passing Redwinged
Blackbird, Agelaius phoeniceus (Linnaeus), or Common Grackle has
evoked this response, the posturing heron normally reverts to the
normal perch posture in a matter of seconds, but if the intruder is
another Green Heron, the bird in the Withdrawn Crouch may flee
or assume the Forward Display, depending on circumstances.

Non- aerial. Vocal Displays

Advertising calls. Males and females advertise their presence in the
breeding areas with distinct calls. During the first few days after
arrival on the breeding area, both sexes may call anywhere, but after
a few days, the males especially restrict their calling to more and
more limited areas. The male takes a position in the top of a tree
or other exposed perch, raises his head and bill to about a 45 degree
angle and gives a single, low-pitched “skow” (Figure 13). The male
frequently stands on the floor of an old nest and “skows” from this
position. If it has been selected as the future nest site, the male con¬
centrates his “skowing” on this nest or its immediate vicinity, and
defends the nest vigorously.

The first adequate description of the “skow” call of male Green
Herons is that of Simmons (1925), near Austin, Texas. Simmons de¬
scribes the call as “skah-ooow (long o)” and states that it may be
heard from a distance of about seventy feet. Townsend (1928) de¬
scribed the “song” in much greater detail, stating that the calls sounded
like deep groans, “ow-er,” the o as in cow, er as in the German ch [sic];
hence, “ow-ch.” The typical call of the males breeding on Rulers
Bar Hassock was of the one note variety, that is, a single, guttural
“skow.” A few individual males gave calls of much lower pitch that
can best be described as “skow-ch,” resembling Townsend’s “ow-ch.”

My observations of Green Herons in widely separated localities
supply interesting data on geographical variation in the pitch of the
“skow” call. In Rulers Bar Hassock, most of the males uttered a
moderately low-pitched “skow,” some gave a very low-pitched “skow-
ch,” and a few others gave a high-pitched “skow.” In eastern Mass¬
achusetts (Wayland, Burlington), all males gave the moderately
low-pitched “skow.” In Florida Bay, all males gave the highest pitched

BREEDING BEHAVIOR OF THE GREEN HERON

37

“skow” of any Green Herons I have observed. On the other hand,
males around the Laguna de la Joyas near Puerto Arista, Chiapas,
Mexico, uttered the lowest-pitched “skow” calls I have ever heard;
these males occasionally uttered the two-parted “skow-ch,” still very
low-pitched. Simmons’ description, that is, a long, two-parted “skah-
ooow,” may indicate another variation of the basic call. Fisher’s
(1933) rendition of the call given by a startled Green Heron, “scow-
scow,” is probably his translation of the typical “skeow” or “skyow”
call described on p. 13, and is not to be considered the advertising call.

The “skow” of Green Herons is certainly analogous, if not homo¬
logous, to the “rwo” call of A. cinerea ( Verwey). Both the “skow” and
“rwo” calls closely resemble calls given outside the breeding season,
and both seem to function as a long distance threat and as an attraction
call for females. Verwey, however, calls the “rwo” of cinerea a mating
call, because the calling male stops if a female is attracted, whereas
the male continues to call if the reacting individual turns out to be
a male. Verwey feels that the biological significance of the “rwo”
call is attraction of the female from some distance. However, the
fact that male Common Herons renew their “rwo” calling if another

38 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

male calls points to a threat function. This is certainly the case with
Green Herons.

The advertising call of female Green Herons resembles the alarm
call of both sexes ( p. 13 ) . The female may or may not raise her head
and bill when calling, while the male always erects his head to some
degree. The advertising call of the female is a single, rather high-
pitched “skeow” or “skyow.” A female attracted by the “skowing” of
a male usually perches in an exposed position near the male, and
a “skowing-skeowing” bout normally follows. I could find no evidence
of any geographical variation in the “skeow” call of female Green
Herons. Verwey states that the female Common Heron does not
have the mating call.

Attack calls. Males or females attacking an opponent from the
Full Forward Display almost always give a series of low, harsh “raaah”
calls. Both sexes use the same call when threatening a potential egg
predator such as a Common Grackle or Fish Crow. I have heard
Green Herons on their territories use the same “raaah” call from the
Full Forward position in threatening Black-crowned Night Herons.
The display of the Green Heron intimidated the Black-crowns in
every case. Normally, the much larger, bulkier Black-crowns force
Green Herons to leave a perch.

Escape and Alarm calls. These calls have been described on
p. 13.

Aerial, Non- vocal Displays

Flying Around. Lone birds were observed flying back and forth
over one breeding area or from one breeding area to another early
in the breeding season. The flights were interspersed with preening
on some exposed perch. Often the birds ended the displays by flying
directly to one of the surrounding feeding areas. Many times the
herons did not feed. They landed for a minute or two and then
flew back. There is no tie at this point to one particular area. The
details of flight during this behavior are identical with those of
normal flight, but the orientation is limited to the breeding areas.
Crest-raising is exceedingly common. This behavior appears to be
hostile because a bird Flying Around rapidly shifts into higher inten¬
sity hostile patterns such as Pursuit Flights and Supplanting Attacks.
When the breeding areas are first occupied, Flying Around is followed
occasionally by pursuit of another Green Heron on the breeding area,
but shortly, within a day or two, birds Flying Around readily pursue
and make Supplanting Attacks throughout the day. Birds that perch
after a bout of Flying Around respond readily to another Green Heron,
and they will interrupt their activities to fly toward the intruder, even
if several hundred yards distant. Perched individuals will assume a
low intensity alert posture as the first indication of their awareness

BREEDING BEHAVIOR OF THE GREEN HERON

39

of an approaching Green Heron, and they frequently show crest¬
raising. “Skeows” ( warning call of both sexes ) anywhere within hear¬
ing range release “alerts” immediately. The function(s) of this
behavior seems to be to familiarize the newly arrived birds with the
habitat, with the available old nests (exposed early in the breeding
season), and with other Green Herons.

Pursuit Flight. Aerial hostilities are conspicuous during the early
part of the breeding period. A Pursuit Flight typically involves two
birds, but I have observed on many occasions three to five birds
involved. The flights are normally brief (50-75 yards), but long
flights (over 100 yards) are not rare. Although no birds were indi¬
vidually marked, I am convinced that the pursuer was usually a male,
and the pursued either a male or female. Occasionally, pursuing males
uttered the “skow” call in flight, and at times the pursued gave the
“skeow” call in flight (p. 43). “Skowing” males were more aggressive
and persistent in their Pursuit Flights than non-calling males; that
is, a calling male often changed to more intense hostile behavior (for
example, Supplanting Attacks or Forward Displays on landing),
whereas non-calling males usually broke off the pursuit and returned
to another part of the breeding area. I rarely observed a pursuer over¬
take and attempt to attack a bird in flight. When this happened, the
attack consisted of several attempts to nip the tail of the pursued. The
pursuer erected its crest every time it made a nip. If the pursuer drew
very close, it also erected its neck feathers. The pursued frequently
erected its crest feathers briefly, or very rapidly raised and lowered
them. The pursued characteristically flies with its head and neck
raised, which enables it to watch the pursuer closely. Occasionally
the pursued turned its head slightly to keep the pursuer in view.

I believe that most Pursuit Flights are expressions of rather low
attack (pursuer) and escape (pursued) tendencies. Additional evi¬
dence exists in the unhurried nature and shortness of the majority of
Pursuit Flights. Furthermore, the pursuer continued flying in most
Pursuit Flights even if it had overtaken the lead bird, or if the pursued
had landed.

Supplanting Attack. In this behavior the attacking bird flies toward
a perched individual which vacates its perch. While landing, the
attacking bird erects its crest and neck feathers and frequently at¬
tempts to peck at the departing bird. Supplanting Attacks are most
common during the early part of the reproductive cycle, and I am
convinced that the supplanting bird may be a sexually active male,
because in some attacks the supplanting bird made a crude copulatory
attempt on landing. In addition, a supplanted female would frequently
return after a brief flight, especially if the male gave the Stretch
Display. The return of the supplanted female usually triggered a
bout of Supplanting Attacks and Pursuit Flights, revealing the com-

40 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

plicated interaction of hostile and sexual tendencies. This interaction
is also shown by males that gave a Flap Flight Display just before a
Supplanting Attack.

Soon after the birds arrive and start Flying Around and Pursuit
Flights, the first Supplanting Attacks are seen. At first, such attacks
may occur anywhere on the breeding area, but in a few days these
attacks are restricted to the territory defended by the male. As the
territory decreases in size, the male restricts Supplanting Attacks to
one female, that is, the one that continues to return even though
repeatedly driven from the male's territory. A territorial male will
always supplant any Green Heron it finds on its territory upon return
from a feeding area, Pursuit Flight, or other trip.

Circle Flight Display. The displaying bird of either sex launches
into normal flight that carries it in a rough circle back to the starting
place. The diameter of the flight circle rarely exceeds fifty yards, and
shorter flights of 20-25 yards are most common. I found no differences
in the form of the flight from normal flight, but a displaying bird
flies in a rough circle, calls frequently in flight ( “skow” or “skeow”),
and is usually a territorial male, or a female associating with a par¬
ticular male. A male in Circle Flight Displays returns to his territory
unless he has been attacked during the flight. Late in the season the
female usually returns to the vicinity of the male she is interested in,
but early in the breeding season she may be interrupted and wander
off the Circle Flight to the territories of neighboring males. After they
have made several mutual Circle Flights, however, the pair tend
to restrict their displays to each other.

After a male and a female have engaged for a few days in a number
of displays (Pursuit Flights, Supplanting Attacks), they may perform
the Circle Flight together, and these displaying pairs become con¬
spicuous just before the appearance of the next higher intensity form,
the Crooked Neck Flight Display.

The Circle Flight differs from Flying Around in that the flight is
in a rough circle, has the same starting and ending points, is rarely
interrupted, and is oriented toward one individual. Pursuit Flights
differ in that they may be protracted, wandering, and contain more
hostile elements. Supplanting Attacks are normally performed by one
bird (usually a male) directed at another individual (perched male
or female). A bird giving the Circle Flight (male or female) flies
around or even directly over the “object” of the flight (perched male
or female), and attacks are rare.

The Circle Flight Display has more sexual than hostile components.
The bird rarely raises its crest or other feathers, it rarely ends the display
with a Supplanting Attack, and it orients toward a familiar individual.
In addition, a female that displays the Circle Flight returns again and
again to the territory of the same male, even though she is attacked

BREEDING BEHAVIOR OF THE GREEN HERON

41

repeatedly. The Circle Flight of the female definitely stimulates a
territory holding male to perform a Circle Flight in return, to engage
in mutual Circle Flights, or even to perform sexual displays such as
the Snap Display and Flap Flight Display. The female is stimulated by
these performances of the male to approach the nest tree more closely
and to engage in mutual Circle Flights.

However, “skowing” and “skeowing” and occasional Supplanting
Attacks and/or Pursuit Flights, point to some hostility in the Circle
Flight Display. It is, however, best considered a low intensity sexual
display whose function is mutual sexual stimulation to synchronize
the behavior of the forming pair.

Crooked Neck Flight Display. This display is intermediate in form
between the Circle Flight Display and the Flap Flight Display, and
it is shown by both sexes. The flight is generally in a circle, although
considerably shorter than the Circle Flight Display. Occasionally the
display may involve a short, straight flight, then a return display flight;
the bird may perch after the direct flight. In this display, flaps are
slower and much deeper than in normal flight; this results in an
undulating flight. The bird does not retract its neck fully, and it
carries its neck crooked or kinked throughout the brief flight. The crest
is slightly raised, and the legs may be dangled occasionally. “Skowing”
and “skeowing” are common. This display appears after the pair
have mutually performed the Circle Flight Display, and have been
associated for some time on the male's territory.

Sexual components are more pronounced than hostile ones in this
display. The Supplanting Attack that rarely ends a Crooked Neck
Flight Display is half-hearted, and the supplanted bird, usually the
female, shifts its perch only a few yards, then resumes its former
position almost at once. This display stimulates the prospective partner
to closer association and synchronization of behavior which leads to
the first copulation.

Flap Flight Display. This bizzare aerial performance is highly ritual¬
ized, and it is shown more frequently by the males. The Flap Flight
Display is the highest intensity form of the sequence: Circle Flight
Display — » Crooked Neck Flight Display — » Flap Flight Display.
The last display amplifies components found in the other two.

The displaying male leave its perch, often the nest platform, and
makes an exaggerated flight toward a female. The wing beats are very
slow and deep, and the tips appear to touch beneath the body. Lurching
through the air is the best description of the slow flight. The wings
produce a hollow “whoom-whoom-whoom.” The bird holds its head
and neck even more crooked than in the Crooked Neck Flight Display,
and it usually dangles its legs throughout the flight. Males frequently
“skow” and females “skeow” in flight. The flight is usually short,
direct, and only occasionally in a complete circle. The male erects his

42 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 14. Male Green Heron showing the Flap Flight Display.

crest and neck feathers and frequently his scapular plumes to a full fan.
Females erect these three feather units less, scapular plume erection
especially being quite rare. On several occasions the eyes of the
males bulged at the end of this display flight. Only those birds of
both sexes that showed brilliant soft-part coloration engaged in this
bizarre display, and the dark lores, bright orange, bulging irises,
lustrous plumage, and brilliant, coral-orange legs, together with the
exaggerated movements, made the whole performance striking
(Figure 14).

One variant includes stalking around just before flight, and this
is shown only by those males whose territories have been considerably
constricted as a result of territorial encroachment by newly arrived
males (p. 62). Stalking around in these cases is definitely hostile
because it appears only when the female actually lands in the nest
tree. Invasion of his “must defend” area stimulates the male momen¬
tarily to stalk around the nest rim, but sexual tendencies prevail, and
the male then launches into a Flap Flight.

Another variant involves a series of soft “roo-roo-roo” calls just before
landing after a Flap Flight. These sound like the end of the “aaroo”
call in the Stretch Display. The Flap Flight with “roo-roo” calls is

BREEDING BEHAVIOR OF THE GREEN HERON

43

typically followed by a series of Stretch Displays from the nest, and
I believe that such males are showing a very strong sexual tendency.

The “roo-roo” variant of the Green Heron’s Flap Flight may be
the first indication or the last remnant of the evolution of an aerial
Stretch Display in virescens ; such aerial Stretch Displays are common
in the Reddish and Snowy Egrets.

The calls, the feather erection, and the bulging eyes may be minor
hostile components, but in this flight display they emphasize and
exaggerate the primarily sexual performance. The Flap Flight is
preceded and/or followed by intense bouts of sexual displays such
as the Stretch Display even more than is the Crooked Neck Flight
Display. The supplanting “attack” that rarely ends this display is
of very low intensity and suggests not a true Supplanting Attack but
actually the end of the Flap Flight. The attack components of the
female’s display are even less; the crest, neck, and scapular feathers
are barely erected, and she does not end with a Supplanting Attack.

This display, especially in the male, apparently occupies all the
bird’s attention. On several occasions displaying males have flown
directly toward me even though I was not concealed. They landed
within a few feet of me, peered for several seconds, slowly moved to
an alert posture, then exploded into flight and uttered high-pitched
“skeows” as they left. The displaying birds appeared ecstatic during
and immediately after the display flight, and on many occasions they
crashed after seeming to lose their ability to coordinate. The frequency
of poor landings suggest that they are a part of the Flap Flight Display.

Only males and females that have associated closely at the male’s
nest tree perform this display. The function is probably to stimulate
the partner sexually; that is, the start of a Flap Flight usually stimulates
the perched partner to launch into the same display. Evidence for
this appears in the section on Late Pairing Behavior.

Aerial, Vocal Displays

“Showing” and “ skeowing .” These calls in flight have been inter¬
preted as hostile in nature, at least for some displays (p. 39). Such
calls may stimulate the partner to perform either sexual or hostile
displays. The same individual may react to the same vocal stimulus
in a hostile or sexual way at different stages in the breeding cycle.

SEXUAL BEHAVIOR

Only those displays preponderantly sexual, having only minor hostile
components, will be described in this section.

Stretch Display. The Stretch Display of the Green Heron is the
Reckbewegung of the Common Heron (Verwey). Because Verwey’s
terminology is so descriptive, I shall use his terms for describing similar
displays by the Green Heron.

A male showing a typical Stretch Display performs the following

44 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 15. The Stretch Display or Reckbewegtmg of the male Green Heron
seen from the side.

BREEDING BEHAVIOR OF THE GREEN HERON

45

movements from any perched position, but usually while standing on
the floor of an old nest:

1. the bird stretches its head and neck upwards so that the bill, head, and
neck are in a straight line pointing to the zenith, then lowers its head
so that the occipital region almost touches the back, the upward and
backward movements being performed in one smooth, uninterrupted
motion

2. the bird erects its interscapular plumes to their fullest, forming a fan
behind the head, during the downward movement

3. the bird now smoothly sways pendulum-like from side to side, but
only the bill, head, and neck move to any appreciable extent

4. the male emits one or more very low, soft, somewhat gurgling calls
best transcribed as “aaroo,” “aaroo,” during the swaying movements

5. the legs and feet are stationary, or moved slightly by the swaying
movements

6. the crest, breast, and flank feathers are depressed

7. rarely the wings are drooped very slightly outward and downward

8. occasionally the eyes may bulge slightly, and very rarely, the iris may
change from yellow or yellow-orange to a deep orange during the
swaying (Figures 15, 16).

The Stretch Display of the male differs from that of the Common
Heron in several respects : ( 1 ) the display of cinerea is performed in
two separate motions, an initial one when the bird stretches its head
and neck upward, then a second when the bird lowers its bill, head,
and neck down and back, and bends at the heel joints, so that the
whole body is lowered toward the nest; the same display is made by
the Green Heron in one smooth movement without bending the heel
joints; (2) during part one of the display of cinerea , the bird emits a
soft “hu” or “huh” call, then utters a soft, gurgling “oooo” call during
part two, whereas the single “aaroo” song of the Green Heron is given
during the swaying movements; (3) the scapular plumes of cinerea
remain depressed (see plate in Holstein, 1927, reproduced in Verwey)
while they are erected to a full fan shape in the Green Heron (see
Figure 16). The modified jugular plumes of cinerea, judging from the
plates, are partly erected during the display, whereas these feathers
are not modified and are not erected during the Green Heron’s display;
(4) eye-bulging and iris color changes, rare in the Green Heron, are
not recorded for cinerea .

Both sexes of the Green and Common Herons show the Stretch, but
the form, time of appearance, function, and orientation of the display
differ between the sexes of the Green Heron. The female Green
Heron assumes the final Stretch Display position exactly as does the
male, that is, in one smooth movement, but the form of the female’s
display differs in the following respects: (1) the female, to my
knowledge, never erects her scapular plumes; (2) the female never
emits any sound at all; (3) the sways of the female are reduced in
number and amplitude, or are completely absent; and (4) I have

46 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 16. The Stretch Display of the male Green Heron as seen from the front.

BREEDING BEHAVIOR OF THE GREEN HERON

47

Figure 17. The Stretch Display of the female Green Heron.

48 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

never observed eye-bulging or iris color changes in her display (see
Figure 17).

The female shows the Stretch Display only after the male has per¬
mitted her to enter the nest, while the male gives his Stretch Displays,
with rare exceptions, before allowing the female to enter the nest
for the first time. The rare exceptions occur when the Stretch Displays
appear during nest relief and just before copulation. The Common
Heron differs strikingly, showing Stretch Displays frequently during
nest relief.

The Stretch Display has three functions in both sexes, two of which
are shared equally by the male and female, that is, nest relief and
synchronizing nest construction. In the female, the display is also a
pre-copulatory invitation or solicitation posture, typically but not in¬
variably shown prior to copulation. I have seen a male show the
Stretch Display immediately before copulation on only a few occasions
(Table 13). The male's Stretch Display also invites the female to
enter the nest; that is, the male signals to the female that hostilities
are subdued or over.

The orientation of the display differs in the two sexes, the female
showing her back to the male, while he characteristically turns his
swaying breast and head toward the female. The scapular plumes of
the male form a fan behind his head and neck (Figure 16) and
emphasize his display; that is, they assist in exaggerating the display
and drawing attention to the swaying, cooing male. The fan does not
function at all in the invitation display of the female. Her scapular
plumes are not erected during the performance, and her body is turned
away from the male (Figure 17).

I agree with Moynihan (1955b) that the causation of appeasement
displays can be quite complex. In the typical Stretch Display, I feel
that the sexual tendency predominates, and a low intensity form of
the Stretch Display supports my belief. During the early stages of
pair formation when males are “skowing” and females are “skeowing,”
males will occasionally perform a low intensity Extended Stretch Dis¬
play which has the following characteristics : ( 1 ) the displaying bird
raises his head and neck as in a typical Stretch Display, but the neck
is not fully retracted; the bill, head, and neck are not vertical, and a
peculiar kink is evident in the partly retracted neck; the alignment of
bill, head, and neck is forward of the vertical by about 3CM:0 degrees;

(2) the male raises his scapular plumes only slightly or not at all;

(3) he sways from side to side only a few times; and (4) he does not
give the “aaroo” call at all.

This Extended Stretch Display (Figure 18) expresses predominantly
hostile tendencies because: (1) males frequently terminate an Ex¬
tended Stretch Display with one or more rapidly repeated “skows”;
(2) appearance of a female on the territory releases threat displays

BREEDING BEHAVIOR OF THE GREEN HERON

49

Figure 18. The Extended Stretch Display of the male Green Heron.

50 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

and/or pursuing or supplanting; and ( 3 ) the Extended Stretch Display
is shown on or off the territory. In the typical Stretch Display the
male never “skows” at the end of the display, the male shows Crooked
Neck Flight, Flap Flight, or Snap Displays when the female approaches
the territory, and the male confines the majority of his Stretch per-
pormances to the area around the nest site. Briefly, typical Stretch
Displays alternate with displays having fewer hostile components,
while Extended Stretch Displays alternate with much more hostile
behavior.

In the Stretch Display the chief heron weapon, the bill, is turned
upward and away from the female, and the neck of the displaying bird
is exposed. There is no crest-erection; in fact, the only feathers erected
are the scapular plumes. Threat display in the Green Heron exhibits
extensive plumage-erection, wing-waving, low, horizontal body posi¬
tions, and is accompanied by harsh, rasping calls. In the Stretch Dis¬
play, on the other hand, the feathers (with the exception of the
scapulars) are depressed, the bill pointed up and away from the
female, and the call is typically low and pleasant-sounding (“aaroo”).
This call is found in no other Green Heron display, and it is confined
to the male.

A male in the Stretch Display, with his lustrous plumage, dark
lores, brilliant coral-orange legs, gentle sways, and gurgling calls,
offers a signal quite the opposite to that of the bristling, rasping fighter
of the Full Forward Display. The female must have a clear signal that
she will be permitted to enter the nest after she has been repeatedly
driven from it.

Another striking peculiarity of the Stretch Display is the individual
variability shown by birds on Rulers Bar Hassock. This variability
involves the number of sways, the number of “aaroo” songs, and the
quality of the song. In 1955, I recorded ten or more Stretch Displays
by each of thirty-four individual males. Of the thirty-four, thirty-one
had soft songs, while three had harsh songs. No males shifted from
one quality to the other during the courtship period; that is, a male
remained soft or harsh. Table 4 lists the records of Stretch Displays
for seven of the thirty-four males observed in 1955. Note that the
number of sways and the number of “aaroo” songs varied from one
to six. No individual, however, showed the entire range of variation
(male 1-31 showed the extreme but concentrated on two sways with
two songs per display). A complete movement from left to right was
counted one sway, and the return movement from right to left was
counted another sway. The “aaroo” song in the vast majority of dis¬
plays is uttered during one movement, but male 1-12 sang once for
every two sways. One male (1-15) in forty-one Stretch Displays
invariably showed only two sways and two songs per display. Male
1-4 was exceptional in that he showed six sways and six songs per

BREEDING BEHAVIOR OF THE GREEN HERON

51

Table 4

Individual variation in components of the

of male Green Herons.

Stretch Display

Male

Song

Quality

No. Stretch
Displays

No. of Sways

No. of Songs

l

2

3

4 5

6

1

2

3

4

5

6

1-1

soft

66

1

45

20

— —

—

1

45

20

—

—

—

1-10

harsh

44

—

40

4

— — -

—

—

40

4

—

—

—

1-15

soft

41

—

41

—

— —

—

—

41

—

—

—

—

1-31

soft

38

—

34

2

1 1

—

—

34

2

1

1

—

1-13

harsh

21

—

12

9

— —

—

—

12

9

—

—

—

1-12

soft

17

—

—

—

17 -

—

—

17

—

—

—

—

1-4

soft

10

—

—

—

— —

10

—

—

—

—

—

10

display, but I have records on this bird for only one day. Most of the
counts point to two, and the vast majority of my single observations
and records of series of less than ten displays are also of two sways
and two songs per display. In the section on Late Pairing Behavior,
activity records are reproduced to show that as the intensity of Stretch
Displays rises, the majority of the males rarely add more than one
sway and one song per display.

I have no data on geographical variation in the Stretch Display, and
the only records I was able to find published (Wheelock, 1906; Sim¬
mons, 1925) are not detailed enough for comparison.

Daanje (op. cit.) discussed the origin of the Stretch Display and
suggested that the second phase of this display in A. cinerea is to be
considered the deep phase of the jump, an intention movement of
flight. Ritualization in both cinerea and virescens has exaggerated the
movements in different ways. In cinerea the ritualization has empha¬
sized the second part of the display, lowering of the body. In virescens ,
exaggeration has affected the side to side swaying movements. Both
species give a pleasant, gurgling call in marked contrast to their threat
calls, an example of the evolution of an appeasing sound.

Billing and other pre-copulatory behavior. Prior to the first copu¬
lation, both of the pair are tense, and a sudden movement releases low
intensity hostile behavior, for example, crest-raising and Forward Dis¬
plays. The tenseness is obvious: the female has just entered the nest,
the male has just permitted another Green Heron to enter the sanctum
sanctorum , and both birds are very wary, leading to rapid shifts in
behavior.

Billing, where both birds extend their head and neck and gently
grasp the mate’s bill momentarily, together with feather-nibbling and
bill-snapping, are preludes to the first bodily contact. Such brief
contacts appear to reduce the attack tendency, inhibiting withdrawal
responses and increasing the sexual tendency. Mutual billing, bill¬
snapping, and feather-nibbling have been recorded for A. cinerea by

52 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Verwey, Witherby et al., (1949), and Lowe (1954), but I have been
unable to find any references for the Green Heron. Similar behavior
has been reported for the Roseate Spoonbill, Ajaia ajaja (Linnaeus)
by Beetham ( 1910) and Allen ( 1942), and for the White Stork, Ciconia
ciconia (Linnaeus) by Schiiz (1942) and Haverschmidt (1949).

Copulation. Mutual feather-nibbling and related behavior are pro¬
longed prior to the first copulation, but after the first copulation pre-
copulatory behavior is reduced to a minimum. During the nest-building
or nest-repairing phase, when copulations are most frequent, the pair
pass twigs and perform occasional mutual Stretch Displays for an
extended period. The female suddenly gives a single Stretch Display
(Figure 17) with her back turned toward the male. The male becomes
greatly excited and moves toward the female, which is usually standing
on the floor of the nest. At the male’s approach, the female assumes
a squatting position low in the nest, with her wings drooped out and
down to the sides. The male enters the nest, nibbles the flank and/or
neck feathers of the female for a few moments, then makes a slight
jump-flight onto the female’s back. Both sexes, especially the male,
flap their wings to maintain balance.

Occasionally, the pair stands quietly side by side in the nest, then
the female suddenly shows a rapid Stretch Display, squats low, awaiting
the male, which then nibbles her head, neck, and flank feathers for
a few moments prior to mounting. The vast majority of copulations
take place in the nest, but occasionally a pair will copulate while stand¬
ing on a branch near the nest. I have never seen Green Herons copulate
away from the nest tree.

The copulation lasts from five to twelve seconds, ten seconds being
most common. Typically, the male leaves the nest after copulating,
perches nearby and preens for a minute or two, then returns to the
nest to engage again in mutual feather-nibbling and billing. While
the male is preening immediately after copulation, the female typically
works on the nest, adding a twig, adjusting others, and so on. Several
hours or longer normally intervene between copulations. I have not
seen one copulation follow another immediately.

The male appears to be much more tense during pre- and post-
copulatory behavior than is the female. In fact, the male at all times
shows hostile behavior more easily immediately preceding and follow¬
ing close contacts between the sexes. The sight of the female engaged
in nest-building or other non-hostile activities, however, apparently
reduces his hostile tendencies and allows him to return to the nest
and engage in mutual nibbling.

After the female has laid one or more eggs of her clutch, the pair
typically copulates when the female has relieved the male on the nest.
I saw no copulations after the last egg in the clutch had been laid.

I saw pseudo-male behavior by a female in copulation only once.

BREEDING BEHAVIOR OF THE GREEN HERON

53

On this occasion the male had been adding twigs to the new nest for
several minutes, and he had retired to a nearby branch to preen and
shake out his feathers. The female flew into the tree suddenly, and
the male adopted at once the Withdrawn Crouch. The female alighted
on his back, wing-waving to maintain balance. She immediately flew
to the nest, gave a single Stretch Display, squatted low in the nest
with wings drooped, and waited. The male entered the nest and
copulated with the female for eleven seconds.

Incomplete copulatory attempts have been described under the
Stiff-necked Upright Display. I found no evidence for promiscuity,
such as Meanley (1955) found in the Little Blue Heron, Florida
caerulea (Linnaeus).

Twig-passing. Before the male allows the female to enter the nest,
he does all the gathering, carrying, and weaving of twigs into the nest.
The nest site is always selected by the male. Once the male has
allowed the female to enter the nest, however, he gathers and carries
most of the twigs and the female does most of the building. Low
intensity Stretch Displays are common early in nest-building before,
during, or after the presentation of a twig. Either sex may display
first, but it is usually the female. These Stretch Displays usually
include two sways, but neither sex utters any sound at all. Here is a
typical sequence: the female inserts a twig in the floor of the nest
as the male looks on from a nearby perch; the female performs a
single, silent Stretch Display; a few moments later, the male walks
and jumps to a neighboring tree, breaks off one twig, brings it back
and passes it to the female standing in the nest. While the female is
“tremble-shoving” (Lorenz, 1955) the twig into place, the male per¬
forms a single, silent Stretch Display on the rim of the nest. Later,
both may show single, silent Stretch Displays side by side on the floor
of the nest. This display and twig-carrying are repeated throughout
the day for several more days, but after an egg or two has been laid,
the bird incubating the egg(s) performs most of the nest-building
activities. Stretch Displays become infrequent and are confined typi¬
cally to nest relief. Early nest-building activities in B. virescens are
strikingly similar to those of A. cinerea, as described by Lowe ( op. cit. ) .

When Green Herons take a twig, they do so by extending their
head and neck fully out and down (in most cases), seizing the twig
crosswise in the bill, then breaking it off by tugging and shaking
motions. Twigs are carried back to the nest in any position. These
twig-grasping movements are the basis for the ritualized Snap Display.

EARLY PAIRING BEHAVIOR

In this section I shall describe the behavior and appearance after
arrival on the breeding areas until the first sexual displays are shown.
There is no clear cut division of pairing behavior into hostile and

54 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

sexual phases. In general, however, there is a decided decrease in
hostility as pairing progresses. Hence, I use the term “pair formation”
broadly to embrace all of the behavior of the two sexes after the
female has ceased to roam and confines her attention to a single
male, until the first copulation ( cf Baerends and Baerends, 1950).

As far as I was able to determine on Rulers Bar Hassock, Green
Herons are unmated when they arrive on their breeding grounds.
However, the time between the first arrival and the laying of the
first egg in the three years 1954, 1955, and 1957 was so short that I
suspect more association of the two sexes than “migratory sociality”
during the northward flight.

Year

Arrival Date of First Green Heron

Date of First Egg

1954

April 20

May 2

1955

April 19*

April 30

1957

April 20

April 29

°Two Green Herons were noted on April 16, 1955, but they flew north out of sight. These
records refer to birds that remained on Rulers Bar Hassock.

Flying Around and Related Activities. Flying Around behavior has
been described as the first reproductive activity after arrival (p. 38).
I am convinced that the females seek out the males, even at this
early stage. They fly past and land near a perched male repeatedly,
even though repelled again and again by threat displays and/or
Pursuit Flights and Supplanting Attacks. The females are attracted
to the males, not to the territories.

Figure 19 records the movements ( diagrammatically ) of one bird
that I took to be a male. The observations were made on April 21,
1957, the day following the first arrival of Green Herons on Rulers
Bar Hassock. I saw the lone male first leaving 2 BA ( that is, breeding
area No. 2; see map, Figure 1) at 1342 hours. It flew unhurriedly to
a perch in the northern part of 1 BA, where it preened briefly. Flying
Around, perching, looking around, and so forth, were repeated until
1435, when the bird flew from the central part of 1 BA to the neighbor¬
ing feeding area. It walked slowly north for a few minutes, then
returned to the northern part of 1 BA. The bird did not make a strike
while it was on the feeding area, and it is my impression that the
bird was uninterested in feeding. I made many similar observations
of this early Flying Around behavior in 1954, 1955, and 1957.

Figure 20 shows the first observed contact between a male and a
female earlier in the day on April 21, 1957. I have used a recording
technique similar to those of Hinde ( 1953 ) and Bastock and Manning
(1955). The notes read from left to right and from the top to the
bottom, as one would read a type-written page. Table 5 is a key to
all of the symbols used in Figure 20 and similar figures. Note that
the female “skeowed” once in flight (1051) just before landing near

BREEDING BEHAVIOR OF THE GREEN HERON

55

\

THE RAUNT '

I

I

I

Figure 19. Flying Around behavior of a lone male Green Heron prior to
settling on a territory. All times are EDST.

56 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

1049

d

9

1054

d

9

1059

d

1104

C?

1109

d

Y

p

T

T

1114

C?

", - r

P

1124 „ i - 1 - 1 - 1 obs. ends

I

Figure 20. An “ethogram” showing the behavior of the male and female
Green Heron on the first contact at the beginning of the breeding season. A key
to the symbols used in this and all following “ethograms” is to be found in Table 5.

BREEDING BEHAVIOR OF THE GREEN HERON

57

Table 5

A KEY TO SYMBOLS USED IN THE “ETHOGRAMs” OR BEHAVIOR RECORDS.

All times are EDST.

1012

1

i i

1012

-

time of observation

c f

-

male

9

-

female

c?i

-

one skow

91

-

one skeow

R

-

stretch display

S

-

snap display

AL

-

alert posture

F

-

forward display

FF

-

full forward display

FL

-

flap flight display

P

-

preening

BS

-

bittern stance

FA

-

flying around

o

nest tree

3 4 5 minutes

—i - 1 - 1

© - nest

□ - nearby tree
Icf - male no. I, etc.

I $ - female no. I, etc.

PF — >■ - pursuit flight

- 1 - ►

skow or skeow in flight

- ►-

bird flying by

- MH - >-

Marsh hawk flies by

I I

i - i - 1

infrequent "skowing"

, or skeowing
i H ii i i

i - 1 - 1

increased frequency
Mill Mill

i - 1 - 1

a rapid burst

lands jtakes off

• •

I lands out A takes off,

1 of sight |obs. terminated

the perched male. After “skeowing” a third time, the female flushed,
landed closer to the male, and “skeowed” a fourth time, at which the
male threatened her with a Full Forward Display. The female re¬
sponded with the lower intensity Forward Display, flushed, and was
pursued by the male, which supplanted her when she attempted to
land (1102). After the female flew out of sight, the male flew around
the area, perched, “skowed” four times, then flew away.

58 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

As more migrants arrive on the breeding areas, Flying Around,
Pursuit Flights, threat displays, and advertising calls increase in fre¬
quency and intensity as a result of more inter-individual contacts. The
initial behavior is purely hostile, even before the males have settled
on territories and the females have started roaming.

Roosting. Both males and females roost on or close to the ground.
Upon their arrival, they roost anywhere in the breeding area. Later,
when the males have settled on territories, the birds roost only on
their territory. I noted roosting sites every night, and during the early
part of the breeding cycle no site was occupied on successive nights.
Green Herons land high and fly or jump directly down to the roosting
spot. They rarely walk down branches to the selected site. When
leaving the roosting spot, they normally fly up, but they walk up
branches to the take-off spot more frequently than they walk down
branches to the roost. Seibert (1951) also states that Green Herons
fly directly down to their roosting spots.

Hostile behavior during early roosting involves an occasional threat
display, apparently to maintain “individual distance” (Hediger, 1942;
Conder, 1949). Fights are rare and usually result from an attempt by
a late arrival to roost in an occupied site. Such fights are brief, and it
is usually the intruder that withdraws. Settling on roosts at this early
stage is typically rapid and uneventful because there are few herons,
and males have not yet established territories.

Once the males have established territories, hostile behavior becomes
more pronounced and more prolonged; and threat displays, fights, and
Supplanting Attacks become common. This activity produces much
milling around, changing of roosts, and a prolongation of settling time.

On the day that I heard the first “skowing” in 1955 I also observed
that many of the males which had advertised during the day also
“skowed” from their roosting perches. I heard such calls even long
after dark when most of the birds were settled for the night.

Territory. I shall follow Noble (1939) in defining territory as “any
defended area,” with the exception that I shall restrict my discussion
to breeding territories. Feeding territory has been mentioned briefly
in the section on hostile behavior.

After the brief Flying Around and/or Pursuit Flight stage, a marked
change appears. Now the males, following these aerial maneuverings,
orient their return flights more and more to a particular portion of
the breeding area. The females now shift to roaming. That is, they
now fly from one ill-defined male territory to another, not at all
restricting their attentions to one particular male. The male’s response
is purely hostile.

At this stage advertising calls begin in earnest: “skow” (male) and
“skeow” (female). At first, the boundaries of the male’s territory are
ill-defined, but the initial area is always larger than the subsequent

BREEDING BEHAVIOR OF THE GREEN HERON

59

territory defended by the pair after pair formation. The initial large
size of the territory depends upon a number of factors: (1) not all
males enter the advertising stage at the same time because migration
is protracted and few males occupy the large breeding areas; these
early arrivals can establish large territories because border clashes are
relatively uncommon at this time; (2) males settling on a territory
for the first time patrol this area, advertising from a number of song
posts, and this additional elbow room is apparently advantageous in
attracting a female. In other words, a male that makes himself con¬
spicuous by moving about and advertising from many song posts stands
a better chance of attracting a female than a male in more crowded
quarters. It may be that the conspicuous calling and moving around
attracts the roaming female; ( 3 ) a larger area will have more perches,
thus allowing a female to stay on a male’s territory longer than if she
was forced to land near the nest site, where she would be vigorously
attacked. Once the pair is formed the large territory has no further
advantage. These points will be treated in greater detail later.

As the season progresses, the number of song posts used decreases
sharply, partly due to the arrival of additional males, and partly as
a result of the female’s behavior (Figure 21). Several things should
be noted in this figure: (1) the male accepted my observation blind
readily, “skowing” directly over my head; (2) at least one old nest
was located on the initial territory; (3) the male used the nest tree
as a song post, in addition to other trees; and (4) as the territory was
reduced in size, the male “skowed” from fewer and fewer song posts,
but the nest tree was always one of these. Eventually, the male only
“skowed” from the nest tree or the nest platform itself.

A variety of evidence indicates that the nest, if an old one is available,
or the nest site, is the primary object around which territorial behavior
revolves: (1) regardless of how much the territory is eventually re¬
stricted in size, the nest or nest site is never abandoned; (2) when¬
ever a male patrols or leaves the territory, he almost invariably alights
in the nest tree first upon returning; ( 3 ) the most intense of the male’s
threat displays are almost always performed from the nest platform;
(4) the song post most frequently used, even when many posts are
used, is the nest or nest tree; ( 5 ) almost without exception, nests that
survive from the previous breeding season are used by the earliest
males; and (6) a female, when visiting a male’s territory during his
absence, will show the greatest number of escape components when
closest to the nest. When peering in toward the nest, or when perched
in the crown of the nest tree, the females appear to be very tense and
nervous.

Tables 6, 7, and 8 substantiate these points with quantitative data.
In all three seasons ( 1954, 1955, 1957 ) , as soon as a male was observed
“skowing” from an old nest platform, this fact was recorded and the

60 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

<Sl

©,

Birch

Bayberry

Birch

O

Large poplar

(D<

-Nest
Bayberry

(D.

.©

©

V — 'm d:

Birch

Bayberry

N

Bayberry

# Birch

o

• Bayberry

Platform

blind

Nest

Platform

blind

Figure 21. “Skowing” perch reduction in the male Green Heron. The upper
third of the figure represents the initial “advertising” area where the male “skowed”
from eleven song posts, one of which was a nest later used by the pair. The male’s
behavior at this stage was almost purely hostile. The middle third of the figure
represents a later period; the male is now beginning sexual displays. Note the
sharp reduction in number of sites used for “skowing.” The lower third of the
figure represents the final “skowing” phase where the male advertises from one
site — the nest.

BREEDING BEHAVIOR OF THE GREEN HERON

61

Significance of the
Green Heron

Table 6

NEST TREE IN THE TERRITORIAL

, Rulers Bar Hassock, breeding

BEHAVIOR OF THE MALE

SEASON OF 1955.

Male

No. Observed Return Flights
to Territory

No. Return Flights
to Nest Tree

1-1

201

159

1-2

147

129

1-10

56

48

1-17

83

63

Table 7

Significance of the nest tree as a song post in the territorial behavior
of the male Green Heron, Rulers Bar Hassock.

Date

Male No.

No.

“skows”

from Song Post

Nest Tree

1

2

3

4

5

6

April 23, 1955

1-1

1

2

1

3

—

—

47

" 23, 1955

1-2

3

—

—

—

—

—

41

" 24, 1957

1-17

3

3

4

5

8

17

196

" 27, 1957

1-23

2

1

—

—

—

—

95

May 14, 1957

3-3

10

13

7

—

—

—

286

Table 8

Re-use of

NESTS AT THE BEGINNING

OF EACH BREEDING

season. Green Heron.*

Year

No. Nests

No. Surviving

No. Nests Tagged at

Surviving

Nests Re-used

End of Breeding Season

1954

unknown

unknown

63

1955

59

53

102

1956

unknown

unknown

none

1957

68

67

none

®A11 data from Rulers Bar Hassock. No observations were made in 1956; the nests reused at
the start of the 1957 season were tagged survivors from the end of the 1955 season.

nest was tagged. In every case, eggs were laid in the surviving tagged
nests. Because no birds were color-banded, I cannot rule out nest-
abandoning or territory-shifting, but males closely observed every day
of the breeding season gave no impression of shifting and abandoning
territories.

Although behavior revolves around the nest, it is not clear why an
old nest or a possible nest site should be so critical. There seems to
be an abundance of trees and building materials for nests. The impor¬
tance of the nest may be tied up with the fact that twig-passing plays
a limited role in courtship. In contrast, twig-passing is much more
important in the mutual displays of Black-crowned Night Herons dur¬
ing courtship. Black-crowns have a much more mobile courtship, and

62 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

old nests are infrequently used for first nestings. Also, the male
Green Heron does a considerable amount of repairing or building
prior to pair formation, while both sexes build the bulk of the nest
together in the case of the Black-crowned. Nest removal and transfer
experiments are necessary to determine the significance of the nest
in the Green Heron’s territorial behavior.

As more males arrive and start to settle on territories, border clashes
increase sharply, and the boundaries of the territories become defined
temporarily. But the boundary of a Green Herons territory shrinks
steadily in size. Several factors are concerned with this shrinkage:
arrival of new males, behavior of the females, the rough synchronization
of neighboring males’ behavior, and the shift from hostile to sexual
displays. This shift in motivation is, I believe, the key factor.

Figure 22, though diagrammatic, represents the actual territorial
behavior of seven male Green Herons during the breeding season on
Rulers Bar Hassock in 1955. At stage 1, males A, B, C, and D have
started territorial behavior at about the same time. Note that A’s
territory is large compared to his subsequent area. The behavior of
the males at this stage is purely hostile: “skowing,” threatening,
patrolling, and fighting at the borders. Each male had at least one
old nest on his territory; these nests are represented by a large black
dot. At stage 2, males A, B, C, and D are now “skowing” much less
frequently and from fewer song posts, and, more important, each of
these males is engaged in activities, both hostile and semisexual, with
one particular female. Note that males E and F have arrived, and
settled on large initial territories, and are at the purely hostile stage.
Contacts between E and F and the four earlier males, ABCD, are
infrequent, but ABCD do threaten and attack all male intruders coming
into their reduced territories. The factor which reduces E-ABCD or
F-ABCD conflicts at this stage is that when males A, B, C, and D return
to their nest trees after foraging or other flights, they fly directly to
their small territories. This and their reduced “patrolling” minimizes
the number of contacts between them and E and F. Too much fighting
on the part of ABCD at this stage would seriously hamper their relation¬
ships with the females; hence, the advantage of the direct flights.
Allen ( 1951 ) suggests that such direct return flights by nesting Snowy
Egrets reduce clashes between neighbors, and his photograph shows
adults with eggs and young. This type of direct return flight occurs
even earlier in the breeding cycle of the Green Heron.

At stage 3 in Figure 22, males ABCD are at the nest-building or
nest-repairing stage, while males E and F are now negotiating with
individual females. Male G has arrived on the scene, and he starts
to patrol a large initial territory. It is of interest and importance to note
that even though G engaged in fewer actual fights than did the other
males which arrived earlier, G’s territory also shrank steadily in size.

BREEDING BEHAVIOR OF THE GREEN HERON

63

Stage I

Stage 4

Stage 2

Stage 5

• Nest

Stage 3

® Nest tree
X Observed contact

**• Original boundary of
As territory

0 75 150

i - 1 - 1

Scale in feet

Figure 22. Territorial behavior in the Green Heron. Full discussion in text.

64 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Stage 5 completes the picture. On the area initially defended by A
(Stage 1), four pairs of herons are now breeding, and the respective
pairs now defend only the nest, its contents, and the few feet imme¬
diately surrounding this one indispensable object.

I have seen incubating birds completely ignore a male “skowing” in
the crown of the tree five feet above the incubating bird’s head,
behavior which would have resulted in immediate attack just a week
previous. Again, these incubating birds seem to sneak back to their
nests, running a gauntlet of threatening males. During flight, and
shortly after alighting in their nest tree, these birds show many escape
or general alarm responses, that is, rapid crest-erection and depression,
sleeked plumage, and tense appearance, with no calls or threats. The
males at the hostile stages seem to ignore these trespassers after the
latter land in their own tree. It may be that failure of the incubating
birds to respond to threat displays while in flight, and their direct,
tense return flights are signals which inhibit attacks, i.e., the trespassers
exhibit a type of “submissive behavior.”

How the intrustion of late arrivals on well established territories
takes place has presented an intriguing problem. I feel that the shift
in behavior from hostile to sexual displays has been little explored,
and that this shift is the key to the problem in the Green Heron. The
rigid territorial boundaries which are maintained long after pair for¬
mation, reported in so many species, especially Passerines, are simply
not found in the Green Heron.

Functions of territory. The recent Ibis symposium on territory in
birds (Hinde et al., 1956b) clearly emphasized the diverse functions
of territory. With the Green Heron, territory has several functions, and
these vary from stage to stage in the breeding cycle. These are intra¬
colony functions, not long-range or ultimate, as suggested by Lack
for A. cinerea (1954a, b). First, the aggressiveness and advertising
of the male, together with patrolling, that is, his mobility in the fairly
large initial territory, make him more conspicuous to the roaming
females. Second, once a female confines her attentions to a particular
male, the pair then has an area which facilitates pair formation. The
early mutual activities of the pair include a large number of aerial
displays (for example, Circle Flight, Crooked Neck Flight, and Flap
Flight), and too sharp a reduction in territory size at this stage would
make contacts with neighbors too frequent, thus seriously interfering
with the behavior of the pair. Later, the two sexes confine their displays
to a small area around the nest or nest site, and the displays of the male,
especially, are stationary ones, such as the Stretch and Snap. Third,
most copulations take place on the nest or only a few yards from it,
and apparently the vigorous defense of the nest site prevents inter¬
ference with copulation. Fourth, the territorial behavior of the male
secures for the pair a nest site or a previous year’s nest.

BREEDING BEHAVIOR OF THE GREEN HERON

65

0645

2 c?

III

Id1

I

1

- 1 -

1

imiii

I

0650

2d*

Id*

1

1

i

•

0655

2d1

III

III

in

i

in

Id1

immi

Hill

in

ii

iiiiii

0700

2 c?

1° 1

. \

Id*

n

r

0718

d*

i i i

i i 1 1

%

i i

i

i i

i i

ii i i

0723

d*

Mil

Mil

Mil

Mil

m

0728

9

!□ i

i

d*

II

INI

1 1 1 1

1

i

mu

i

0733

9

<?

I l

INI

1

1

i

0738

9

tni

II

c?

i l l

in i

1

Hill

i

0743

9

ii

1

1

i

d*

T77

iiiiii

Hill

Hill

i

llll 1

1

Q

i

0748

▼

C?

1

i

i

nr

0753

i

1 1

c?

to 1

'll

1

1 '

Figure 23. The early territorial advertising of the Green Heron. Note the
complete absence of sexual displays. See Table 5 for a Key to the symbols.

66

COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

1750

1755

9

d*

9

d*

1800

d*

1805

2d*

Id1

in mi ill 1 1 1 '1 i iiiimiiiiii i i II 1 1 1 1 1 1 1

ii

III R R R R I I I R m

1810

2d1

Id*

1815

2d*

Id1

2d1

1820

Id*

2d*

1825

Id*

I_ III I

P llll P I R FL - K) III R MIR I R

II I III II III _ Ml _ III

* ii I iiili nm iiiii m iiii

ini iiiiii nm

r mTr iiimii r s iiiiis r

I

i - 1 - 1 - 1 - 1 - '

R

1830

d*

r

T

R

1835 • - 1 - 1 - 1 - 1 -

<f llll lllllll R II

1840

1845

1850

R Mini FL -

-^0

ill

R ' III

- 1

R

Jo

i 1 1 1 1 1 1 ilium

FL — -of

r — - 1 -

1

R

i

imi

- p-

s ii r siip

- 1 -

llll

<f II ^FFX R R

Figure 24. An “ethogram” showing the effect of “skowing” on neighboring
territorial male Green Herons. Note particularly the “skow-trading” between 1809
and 1824 hours. The diagonal arrows at 1750, 1756, and 1850 indicate that the
male walked up to the crown of the nest tree, threatened the female with a Full
Forward Display, then walked back down to the nest after the female flew away,
as shown at 1750, or remained in the crown of the nest tree (1756), or flew past
the nest tree in a Flap Flight Display.

BREEDING BEHAVIOR OF THE GREEN HERON

67

Mutual hostile behavior. Once the males have settled on territories,
both males and females become highly vocal. Periods of “skowing”
are interrupted by short preening bouts, twig-grasping, and an oc¬
casional Pursuit Flight when a roaming female approaches too closely.
This early advertising of the males is shown in Figure 23. This male
was “skowing” from the floor of an old nest, and he remained there
until about 0800, when a female landed in the crown of the nest tree.
The female left immediately after a Full Forward Display by the
male, which then pursued the female. Note that the male remained in
the nest even when the female landed at the borders of his territory
at 0739; also that “skeowing” by the female stimulated the male to
“skow” (e.g., at 0739, 0746, 0747, and so on).

Shortly after a Pursuit Flight the male typically returns to his
territory and resumes “skowing,” as shown in Figure 23. The fre¬
quency of “skowing” varies considerably, from as low as one
“skow” per hour to sixteen “skows” per minute. Towsend (1928)
records a frequency as high as ten “skows” in a quarter of a min¬
ute, but thirteen “skows” in half a minute was the only record of
such a burst I was able to make. Bursts of 10-14 “skows” per minute
were quite common, however. “Skowing” frequencies vary with the
time of the day, “skowing” of neighboring males, and presence or
absence of a female. “Skowing” males definitely stimulate each other,
and one such bout of “skow trading” is shown in Figure 24. Such “skow
trading” territorial males soon learn the exact location of their neighbors
and their song posts. A “long distance threat” function, however, is
another possibility, because the males on distant territories also engage
in such “duels.” The threat function of the “skow” is indicated by the
reaction of a male that landed in the territory of a rival when the
intruder was unaware of the owner’s presence because he was con¬
cealed by reeds. A single “skow” from the owner released immediate
withdrawal on the part of the intruder, even though the owner was
still out of sight (cf, Lack, 1953, on the threat function of song in
the English Robin, Erithacus rubecula ( Linnaeus ) ) .

The advertising call of the male clearly attracts females, and I believe
that female roaming can be explained partly on the basis of this attrac¬
tion. From a high wooden platform I could watch a female fly from
one “skowing” male to another, and it was quite evident that the
female settled, at least temporarily, on the fringes of the territory of
that male which engaged in a “skow-skeow” bout with her.

The female’s advertising call seems to be a stimulus that releases
the male’s “skow,” thereby informing the female of the exact number,
location, and “behavioral disposition” of all males. Figure 25 shows
that the female’s “skeow” is stimulating to the males. Note the bursts
of “skowing” after each “skeow” from the female. Also note that the
frequency of the male “skowing” increased sharply after the arrival

68 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

0645

0650

0655

0700

0718

0723

0728

0733

0738

0743

0748

0753

2 c T
Id*

2c?

Id*

2c?

I<?

2c?

I<?

c?

<?

9

<?

9

c?

9

<?

9

c?

9

<?

c?

III

1

1

- 1 -

1

iiniii

— i —

i

I

1

l

i

i

III

III

m

i

in

iiiimi

iiiii

in

lot

i

ii

. 4

iiim

i ■ ■ ■

ii

•

/F\

*

r

i i i

....

1

1 1

■

i i

i i

INI

Mil

Mil

INI

1 1 1

i

1 1 i

!□ i

i

1

II

II I I

Mil

i

i

Mill

i

1 1

1 1 1 1

1

1

1

1

tni

n

1 1 1

in i

1

iiiii

i

1

1

ii

1

1

1

III

mm

1

iiiii

mil

i

till 1

i

i - I - 1 - 1 - r

I I - PF -

• - T - 1 - 1 - ' - r

1 0 I II I

Figure 25. An “ethogram” showing the effect of female “skeowing” on the
“skowing” frequency of a territorial male Green Heron. Note particularly the
period between 0718 and 0748 hours.

BREEDING BEHAVIOR OF THE GREEN HERON

69

of the “skeowing” female. When a female is threatened or attacked
and then leaves the territory of the hostile male, she “skeows” from
the fringes of his territory or from a neighboring, momentarily un¬
occupied territory, as if to test his readiness for her return. If the
male is ready to respond he “skows,” thus releasing a return flight by
the female. If the male does not “skow,” however, the female remains
quietly perched for a few moments, then repeats her “skeow.” Finally,
if the male still fails to “skow,” the female may return to the close
vicinity of the male anyway, or she may fly away. Apparently the
females approach the males right from the start, and their persistence
in returning again and again, even though repeatedly attacked, is a
striking feature of their behavior. Experiments might establish what
I suspect, that the “skeow” call modifies the reaction of the male even
though the change is not observed at once. Briefly, the conspicuous
differences in the calls are hardly accidental.

At the same time that the male shows the first sexual displays,
typically low intensity Stretch and Snap Displays, the females stop
roaming and confine their attentions to these particular males. Figure
26 shows the behavior of a “pair” at this stage. Note that the female,
perched on the territory of the male, but not close to the nest tree
where the male was displaying, attempted to land in the nest tree
at 1630, but that she was repulsed by the threatening male, which
then pursued her. The male returned to the nest tree at 1632 and
resumed “skowing.” The female again attempted to land in the tree
occupied by the male, was forced to withdraw, and returned again at
1742, shortly after a Stretch Display by the male. The female’s
“skeowing” released two short bursts of “skows,” then a series of
“skows” and Stretch Displays by the male. At 1811, the female at¬
tempted to land once more in the nest tree and was threatened by
the male; but note that the male showed a Flap Flight Display this
time rather than a Pursuit Flight (1812). Figure 27 shows the later
behavior of the same pair. Note the sharp increase in frequency of
Stretch and Flap Flight Displays; also that the male’s behavior was still
hostile, especially when the female tried to land in the nest tree ( 1150),
or when she flew too close to the nest tree (1148, 1204, and 1229).

It is evident from my discussion that the “skow” is not inhibited by
the arrival of a female. Verwey states that the Liebeslockruf or “mating
call” of the Common Heron is instantly inhibited by the arrival of a
female. By contrast, the arrival of the female Green Heron definitely
stimulates the male to “skow” at a much greater rate than previously.
Only after the male begins sexual displays does “skowing”’ decrease.
Another striking difference between cinerea and virescens is that the
mating call of cinerea is restricted to males, while both sexes of virescens
have advertising calls. It is possible that the advertising call of the
female Common Heron, if such a call exists, has been overlooked by

70 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

1612

1617

1622

1627

1632

1637

1642

1741

1746

1756

1811

9

d

9

d

9

d

9

d

d

9

d

9

d

9

d

9

d

9

d

9

d

□

0 I

II

/

FF - PF-

fOil

it

I I 1 1

I I

□

III I
it

I I I I I III II II

\o

A

i i I

FF - PF-

/

\n i i

r

foil R
I

lllllll

lllllllllllll

71

\r

1 1

in

M R I R II

I

A

r

1 1

\o

R III I
I

/

FF - FL — *-0 llllllll III

Figure 26. An “ethogram” showing the first appearance of male sexual displays
and cessation of roaming by the female. Note the persistent return behavior of the
female, even though she is repeatedly repulsed by the male. The diagonal arrows
at 1630, 1643, and 1812 indicate that the male walked up to the crown of the nest-
tree and displayed a Full Forward Display.

BREEDING BEHAVIOR OF THE GREEN HERON

71

1140

1145

1150

1155

1200

1205

1210

1215

1220

1225

1230

1235

1240

1°

R P-

I R

97-

< S S I

9 jo |

& R yFFv R R R

X \

R R R R

- — - □ FL —0~ 111 FL - «-□ I

<f R FL-

R FL-

9

O FL - *-0 III R F

la' t

d* R

9 \n I

I

FL-

f 1°

□ FL-

0 II

9

d*

mill Mill R R R

fl - i i i

III III II

I

II

9 II

c? I I I I

9

lllll

I

II

I I FL-
II

-I

□

I I I I I I lllll
I

lllll R F
II

c? i i min

9 I

III I I III

I

c? Ill

9

I I I

INI

MH-| —

<S III

I I

I BS I

Figure 27. An “ethogram” showing the sharp increase in sexual displays of
the male Green Heron after the female has ceased to roam. The diagonal arrows
at 1150 indicate that the male walked to the crown of the nest tree, showed a
Full Forward Display, then walked back down to the nest when the female
flew away.

72 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

European observers because many of these displays are fleeting in
nature.

The previous emphasis on male hostility during the early pairing
behavior does not mean that females are showing primarily sexual
behavior at this time. Frequently, when the females attempt to land
in the nest tree occupied by the male, the females show just as intense
threat displays as the males. Occasionally, the sexes engage in vicious
fights in the crown of the nest tree before the female leaves, but the
female always retreats. In general, the behavior of the female is less
aggressive than that of the male because an important function of her
behavior is to stimulate the male to react in a sexual rather than in a
hostile manner.

Nest-building before pair formation. As mentioned earlier, the female
takes no part in nest-building or nest-repair until after pair formation.
The male makes extensive repairs to an old nest or constructs the
foundation of a new one. At first, the male may merely adjust the
twigs in the old nest. Later, the male occasionally grasps, then quickly
releases, twigs near the nest tree or in the nest tree itself. Then, a
twig may be broken off, only to be dropped. These dropped twigs may
be retrieved repeatedly, only to be dropped after “mouthing.” Finally,
a twig is secured, brought to the old nest, and inserted with the

Table 9

Morphological and behavioral characters used in sex discrimination by

MALE AND FEMALE GREEN HERONS.

Character Male Female

Advertising call
Site tenacity
Snap Display

Stretch Display — time of
occurrence

Stretch Display — vocal
quality

Twig-grasping

Nest-building

Stiff-necked Upright
Display

Soft-part coloration
Plumage “luster”

Response to threats of
territory-holding male

ic I »

skow

tenacious

present

common before and dur¬
ing pair-formation

with “aaroo” song

common prior to pair-
formation

starts repair on old nest
or lays foundation for
new nest prior to pair-
formation

present

brilliant

lustrous

threatens, fights, or
withdraws

skeow

roams

absent

only shown after pair-
formation

always silent

absent prior to pair-
formation

no nest-building activities
until after pair-formation

absent

duller than male
duller than male

if interested in male,
withdraws and returns
repeatedly

BREEDING BEHAVIOR OF THE GREEN HERON

73

Effect

Table 10

OF SOME ENVIRONMENTAL FACTORS ON THE BEHAVIOR OF NEWLY ARRIVED

Green Herons. Rulers Bar Hassock, early spring, 1955.

Date

Temperature

°F.

Wind Velocity (mph)

Behavior

1955

Max.

Min.

Av.

DiffA

Av.

Max.

DiffA

Apr.

20

64

45

55

—

7.6

22

—

First Flying Around, First
Pursuit Flights

ft

21

61

43

52

-3

6.8

16

-0.8

Increased Flying Around,
Pursuit Flights

//

22

69

48

59

+7

7.5

27

+0.7

First “skowing,” First
fighting

//

23

69

49

59

0

4.3

15

-3.2

Great activity, including
first Stretch Display, SA’s,
Flap Flight Displays

//

24

57

47

52

-7

7.5

17

+3.2

Occasional Flying Around

ft

25

48

44

46

-6

12.5

29

+5.0

An occasional “skow”

n

26

45

42

44

-2

15.4

22

+2.9

No activity

H

27

54

44

49

+5

11.8

23

-3.6

An occasional fight

//

28

56

46

51

+2

15.1

22

+3.3

Activity increases, nest-
repair noted

//

29

65

48

57

+6

16.6

33

+1.5

Greatest activity to date

H

30

73

51

62

+5

9.2

20

-7.4

Increased activity, first egg
laid

May

1

65

49

57

-5

6.7

17

-2.5

Very active, additional
nests and eggs

° Average difference from the previous day.

characteristic heron “tremble-shoving” movements. Frequently the
male will sit in the nest up to one hour, even during the early phases
of courtship. While sitting in the nest, the male adjusts the twigs
of the floor and rim. He continues until the nest is almost completely
repaired.

Sex discrimination. The first response of a territory-holding male
Green Heron to intrusion by either sex is always hostile, ranging from
stationary threat display to pursuit and fighting. Pair formation is
eventually effected, however, so the two sexes must some how tell
each other apart. I believe that Green Herons discriminate sex by
the same characters that I used, that is, a combination of morpho¬
logical and behavioral characters (Table 9). The most reliable are
sexual displays, position in copulation, and role in nest-building.

Influence of weather. Tables 10 and 11 show that in both 1955 and
1957 there was an initial burst of activity, then a lull, and then a
renewal. In both years the period of lessened activity occurred shortly
after a sharp drop in the average daily temperature, or during a brief
period of low temperatures. The decrease in activity was most notice¬
able in 1955 when the average daily temperature dropped steadily

74 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Table 11

Effect of some environmental factors on the behavior of newly arrived
Green Herons. Rulers Bar Hassock, early spring, 1957.

Date

Temperature

°F.

Wind Velocity (mph)

Behavior

1957

Max.

Min.

Av.

Diff.*

Av.

Max.

Diff.°

Apr.

20

65

49

57

10.3

26

First arrival on area

ft

21

82

61

72

+15

19.2

34

+8.9

First Flying Around, first
Pursuit Flights, first “skow-
skeowing”

tt

22

65

49

57

-15

13.7

26

-5.5

Occasional Flying Around,
Pursuit Flights, first low in¬
tensity Flap Flight Display

tt

23

68

49

59

+2

7.1

17

-6.6

Increased Flying Around,
Pursuit Flights

tt

24

80

58

69

+10

9.0

16

+1.9

Greatly increased “skow-
skeowing,” threat displays,
fighting, occasional Flap
Flight

tt

25

59

50

55

-14

10.1

20

+1.1

Greatly decreased activity

ft

26

78

50

64

+9

9.1

18

-1.0

Slight increase in activity

rr

27

72

54

63

-1

7.6

17

-1.5

First Stretch Display, in¬
creased activity, prolonged
nest-repair

tt

28

77

53

65

+2

6.6

16

-1.0

General increase in activity

tr

29

75

61

68

+3

13.0

26

+6.4

First egg laid

tt

30

71

57

64

-4

20.1

36

+7.1

Much activity, nest repair,
more eggs laid

May

1

80

58

69

+5

20.6

43

+0.5

Much activity, additional
nests with eggs

* Average difference from the previous day.

for three days. Although the average wind velocity increased during
this period, it is apparent that the drop in temperature was the crucial
factor because, following the rise in temperature, activity was renewed
even though the wind continued to rise. The same increase in activity
with rise in temperature was noted in 1957, and then, too, the average
wind velocity increased during the active period. When temperature
is rather constant, however, the wind is an important factor. During
periods of blustery weather, the earliest arrivals remain inactive and
quiet, perched low in the protected parts of the breeding areas. In
the later stages in the breeding cycle the wind reduces mutual adver¬
tising displays.

During rain storms most of the birds stop flight displays and go
to protected perches on the breeding areas. On several occasions,
however, I saw threat displays during relatively heavy downpours.
In general, low temperatures, high winds, and moderate to heavy rains
reduce activity.

Coloration of soft-parts. “Soft-parts” refers to the unfeathered por-

BREEDING BEHAVIOR OF THE GREEN HERON

75

tions of the body, i.e., the bill, lores, iris, legs, feet, and skin surrounding
the eye. An early reference to the "red” legs of virescens is in Wilson
and Bonaparte (1878). Christy (1929; cited in Roberts, 1936), after
flushing a Green Heron in the month of May, 1928 ( ?), states, “He was
extraordinarily handsome, or so he seemed to me, and particularly I
remarked his coral-orange legs.” Allen (1934) and Pough (1951) both
note the orange-red color of the legs and feet of the male during the
breeding season, and Allen’s book contains a color plate by George M.
Sutton ( opp. p. 141 ) , which shows well the brilliant coloration of the
legs and feet. However, Allen’s statement ( 1934:144) that ‘hers remain
yellow or greenish-yellow” does not hold for the females I observed in
Rulers Bar Hassock, eastern Massachusetts, Florida Bay, and Puerto
Arista, Chiapas, Mexico. Roberts (1936) and Pough (op. cit.) similarly
restrict the orange-red coloration to the males. Leg-color changes
during the breeding season have been noted in the closely related
Butorides striatus in Surinam by Haverschmidt ( 1953 ) , and by Stan¬
ford (1936) and Smythies (1953) in Burma.

Table 12 lists the breeding and non-breeding soft-part coloration
of both sexes, with brief remarks on plumage. The only data I have

Table 12

Soft-part coloration of male and female Green Herons.

Area

Male

Female

Non-breeding

bm

brownish-black, basal part of lower man¬
dible yellowish

same

lores and orbital
skin

greenish-yellow

same

legs and feet

bright yellow to yellowish-green

same

iris

yellow

same

general plumage

greenish (may appear bluish) wings and

same

characteristics

back, maroon neck, greenish-black crest

Breeding

biU

lustrous blackish-brown to shiny jet-black

same, but less
intense

lores and orbital

intense blue-black but variable

dull blue-black,

skin

highly variable

legs and feet

brilliant coral-orange

same, not as
intense

iris

yellow to orange, deep orange on occasion

yellow

general plumage

rich, lustrous green wings and back.

similar coloration,

characteristics

rich maroon neck, glossy greenish-black
crown; modified scapular plumes have
silvery tone

but much duller

76 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

relative to geographical variation in soft-part coloration is the observa¬
tion that the male Green Herons of the Puerto Arista area of Chiapas,
Mexico, are the most brilliantly colored I have ever seen. Individual
variation was marked, especially on Rulers Bar Hassock.

I have found no references to color changes in the bill, lores, and iris
for virescens or striatus. In virescens these parts change in color only
during a brief period during courtship. Iris color changes noted in
some of the displays are especially transitory. The coloration of the
lores, too, changes rapidly, especially in the female, where it also shows
much individual variability.

These soft-part color changes enhance and emphasize the various
hostile and sexual displays. The colors of both sexes fade rapidly to
the winter color after egg-laying. The female’s color has faded com¬
pletely by the time the last egg has been laid. At this time the male’s
coloration is dull, but not as “washed-out” as that of the female. By
the time the young hatched I could not recognize the sex by any differ¬
ences in coloration.

LATE PAIRING BEHAVIOR

Once the female begins to confine her attentions to one male, her
roaming ceases, and the male’s behavior rapidly becomes more and
more sexual. Instead of only making “skows” along with threats and
Pursuit Flights, the male begins to perform more and more Stretch,
Snap and aerial displays, such as the Crooked Neck and Flap Flight
Displays. Figure 28 shows the behavior of the “pair” at this stage.
Note the increase of sexual displays and the sharp reduction in
“skowing,” except when a neighboring male “skows.” Note also the
sharp reduction in female “skeowing” which is no longer functional.
Most of the female’s behavior consists of attempts to land in the nest
tree, and short flights from tree to tree around the displaying male.
The female now lands in the nest tree occupied by the male, and she
remains perched there even though threatened with high intensity
hostile displays (1743, 1756, and 1917). The female is not always
tolerated in the nest tree, however, as can be seen at 1750 and 1850
in the same figure.

Part of the function of the female’s short flights, both normal flights
and display flights (e.g., Circle Flights), seems to be to get the
male to follow her in such a way that the female can land first in
the nest tree upon returning. The female is frequently successful in
this ruse, but her stay in the nest tree is usually brief, especially when
she tries to perch too close to the nest. As the mutual flights increase
in frequency, however, the female is permitted to stay in the nest tree
for longer and longer periods.

A common sight at this time is that of a male performing one Stretch
or Snap Display after the other, from the floor of the nest, while the

BREEDING BEHAVIOR OF THE GREEN HERON

77

1634

1639

1651

1700

1705

1710

1715

1720

1725

1730

1735

1740

1745

9

d

9

d

d

d

d

d

d

d

d

d

d

9

d

9

d

□ 1

i

1

6 i

i

R

i

R

h

R

- D AIM

I III

/ < RAIN

-« RAIN

R

l

R

1

* 1 1

FI

i

1

l 1

P

• 4

D

r l_

rx

i i i

1

R

i

1

1

R

1 1

1 1

i

i

l

1

i i i

ii i i i i

IRS

R S RS

s

R

R

ii

i

s

s s

S S

R

R

in

S R

PSSS

9

Cl

1 1 1 1 1 1 1 iiii

1 1

till

r L

r V

\o

iiiiii

III

R

R

- i— i _

• l

_ n

l_J ~ ■

• LJ

i

R R

i

</>

>

□

- 1 - 1

FL -

Figure 28. An “ethogram” showing the late stages of pair formation behavior
in the Green Heron. Note: Figure 28 consists of three continuing “ethograms.”
The double-headed arrow at 1743 indicates that the male started to walk up to
the crown of the nest-tree (where the female was perched), hesitated, showed a
Forward Display briefly, then walked back down to the nest.

78

COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

1750

1755

1800

1805

1810

1815

1820

1825

1830

1835

1840

1845

1850

9

d

9

d

d

2d
I d

2d

\d

2d
I d

2d
I d

2d

Id

d

d

d

9

d

9

d

Jof

R ^FF

o

/

— i -

- i —

R

— i -

t

lllllllllllll

/N FL

i —

i

►0

i

lllllll

MINI

i

i i

lllllllllllll

1 1 1 1 1 1 J 1 III

1 1

1 1

1 R R

R R

1"

1

1 1

1

R

in

1

III

1

i

P

Mil P 1

R FL-

III R

III R 1

R

II

1 III

II

III

1 II

III

I

II

I ' mu

Mill

mil

i

IIIIII

mi

mi iiiiii

Hill

f

lii r iiiiiiii

"i

R

S

inns

R

i

R

i

i

•

■

< - 1 - 1 - - 1 - r

R

i • 1

- 1 -

1 1 II

lllllll

R II

R IIIIII FL -

-^0

III ' R

,)\\ R

1°

i 1 1 1 1 1 1 ilium

•

R

inn s ii r

SUP INI

FL — -of

1 - - 1 -

— i -

r

II R R

Figure 28. (Continued).

BREEDING BEHAVIOR OF THE GREEN HERON

79

1855

1900

1905

1910

1915

1920

c?

9

c?

2d1

9

9

cf

9

c?

ii n i

R II R

I

II I II I II III I FL - *-0 III

III II I INI

llll II

R I III mill

| n - *-□

1 I 1

FL-

□ - -0

•0 III R R

R R ^FFv S S

— t

FL-

Figure 28. (Continued).

female, perched in the crown of the nest tree, peers down intently at
the swaying, softly “aarooing” male. Sometimes the female tries to
approach the male during such a display, only to be repulsed, but
she is rarely forced to leave the tree. The male simply stops the sexual
display, threatens her briefly, and when the female has retired to the
crown of the tree, he resumes his sexual displays. When the male is
away, the female shows an active interest in the unoccupied nest,
peering at it intently, walking slowly down a branch toward the nest,
then retreating. Her anxiety is clearly indicated by rapid tail-flicking
and crest-raising, and by her reluctance to enter the nest.

Finally the crucial moment comes when the female is permitted to
enter the nest for the first time. If any act can be considered as the
turning point in the pair formation of the Green Heron, it is this.
I have witnessed this performance on many occasions, and the behavior
of the two sexes is almost invariable. Both are obviously tense, but
the male seems to be more highly agitated because his tail-flicking and
crest-raising are more pronounced. If the male is standing in the nest
before the female enters it, he shows a low intensity Forward Display
as she approaches. The female, which is moving cautiously toward the
nest, shows a fleeting Forward in return, but her feather-erections are
extremely slight. As the female enters the nest, any sudden movement
by either bird instantly triggers rapid feather-bristling by both sexes.
Typically, the male leaves the nest as the female steps in, and he perches
on a nearby branch, peering intently at the female in the nest. The
male characteristically moves about the surrounding branches in a low,
crouched position, tail-flicking and crest-raising steadily. As soon as

80 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Table 13

Frequency of various pre-copulatory activities by one pair of Green Herons

(Pair 1-13), Rulers Bar Hassock, spring of

1955.

Period

Bill-snapping

Billing

Date

of Obs.

Mutual

Male

Female

Mutual

Male

Female

May

9

8 hrs.

—

—

—

2

—

1

//

10

3

n

1

—

—

1

—

—

n

11

3

n

2

—

—

1

—

—

ft

12

11

n

6

—

—

6

—

—

bn

13

12

n

2

—

—

5

—

—

n

14

4

n

—

—

—

—

—

—

bn

15

10

n

—

—

—

—

—

—

n

16

2

n

—

—

—

—

—

—

bn

17

8

n

—

—

—

—

—

—

bn

18

1

n

—

—

—

—

—

—

n

19

3

n

—

—

—

—

—

—

Total 11 days

65 hrs.

11

—

—

15

—

1

Date

Period

F eather-nibbling

Stretch Displays

Copulations

of Obs.

Mutual

Male

Female

Mutual

Male

Female

May

9

8 hrs.

1

—

—

1

—

1

1

n

10

3

n

1

1

—

1

—

1

1

n

11

3

n

2

2

—

—

—

1

1

n

12

11

n

7

—

1

1

1

1

—

bn

13

12

n

2

20

3

—

—

3

1

n

14

4

n

—

—

—

—

—

—

—

bn

15

10

n

—

6

1

—

—

—

1

n

16

2

n

—

—

—

—

—

—

—

«//

17

8

n

2**

—

—

—

—

—

—

bn

18

1

n

—

—

—

—

—

—

n

19

3

n

—

—

—

—

—

—

—

Total 11 days

65 hrs.

16

29

5

3

1

7

5

^Deposition of one egg.
°°Nest relief ceremony.

she has gained admittance to the nest, the female starts to fidget with
the nest twigs, but she does so nervously, picking at a twig here, then
looking at the male, turning around in the nest, and then touching
another twig.

If the male is perched in the crown of the nest tree at the female’s
first entrance into the nest, he adopts the Forward Display and stalks
toward the female. Occasionally, the male gives a few very soft “raaah-
raaah” calls as he nervously approaches the female, and as above, any
sudden movement releases brief hostile displays. This is a trying
period for both sexes, and the nervous moving about the nest and
nearby branches, fidgeting with nest twigs, bristling, and watching
each other, continue for the rest of the day. If one or both of the pair
leaves the territory (that is, to feed or if flushed by a sudden noise),

BREEDING BEHAVIOR OF THE GREEN HERON

81

the sequence is repeated upon their return. Regardless of the intensity
of the male’s threat displays, the female now remains standing in the
nest, and for a brief period, the observer gets the distinct impression
that now it is the male that is the intruder!

After the first entrance of the female into the nest all male “skowing”
and Snap Displays, all female “skeowing,” and all aerial displays cease
completely. After a few encounters as described, the first pre-copulatory
activities appear, such as bill-snapping, billing and feather-nibbling.
Pre-copulatory behavior is prolonged before the first copulation, and
then these mutual activities decrease in frequency and intensity with
each copulation, apparently because of "familiarization.”

Table 13 shows the frequency of pre-copulatory activities for one
pair from the first entrance of the female into the nest until the com¬
pletion of the clutch ( four eggs ) . In general, bill-snapping and billing,
which have hostile components, drop off sharply at the time the first
egg is laid. These are mutual displays, while feather-nibbling becomes
more of a male display and Stretch Displays more of a female display.
Both of these latter displays, however, are shown mutually. Most Green
Herons carry over feather-nibbling into nest relief ceremonies. Also,
the pair occasionally show brief, always silent Stretch Displays at nest
reliefs, which can hardly be called “ceremonies” because they are so
brief, and because so little ritualization is involved. This contrasts
sharply with the case of species such as A. cinerea and A. herodias,
where elaborate nest relief ceremonies occur after egg-laying. As
incubation progresses, Green Herons eventually relieve each other
without any display whatever. The relieving bird enters the nest
quickly and settles on the eggs as the relieved bird rises and leaves.

Following nest-building and egg-laying sexual behavior disappears
suddenly and completely, and the pair enters the parental phase of
the reproductive cycle. Even during the period of egg-laying, the pair
spend little time in purely sexual activities, as shown in Table 14.
Several activities shown are of interest. Note that both members of
this pair ( 1-13 ) spent a great deal of time away from the nest before
egg-laying, which is rather atypical for the Green Heron. Other mated
pairs on Rulers Bar Hassock spent many hours in the nest together,
standing quietly side by side. This companionship lasts until the
deposition of the second egg, when the incubation routine of “one on,
one off” starts. Note in Table 14 that the attentive periods on the first
and second eggs are intermittent, that is, there are fairly long periods
during which the eggs are not covered by either bird. With the laying
of the third egg, however, a regular incubation rhythm is established
in which the male incubates mostly in the midday and midnight hours,
while the female sits dining the early morning and late evening hours.
This schedule, however, was highly variable from pair to pair on Rulers
Bar Hassock.

82 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Behavior

ENTRANCE

Table 14

OF A NEWLY-FORMED PAIR OF GREEN HERONS (PAIR 1-13)
OF FEMALE INTO NEST UNTIL COMPLETION OF CLUTCH,

Hassock, Spring of 1955. All times are in minutes.

, FROM FIRST

Rulers Bar

Date

Pre-

Period copulatory
of Obs. and Related-
Behavior

Preening

Nest-building

Incubation

Time Away
From Nest

n

o

?

cf?

d

?

d

?

d"

?

May 9

480

30

20

5

17

5

6

— —

—

390

420

" 10

180

12

5

7

5

30

—

—

—

120

150

" 11

180

26

4

—

—

5

—

—

—

120

150

" 12

660

41

34

60

20

19

3

—

—

540

540

*" 13

720

63

3

20

14

20

28

150

90

450

510

" 14

240

—

10

—

—

12

—

180

—

30

240

*" 15

600

29

9

60

2

34

2

360

60

165

450

" 16

120

—

5

—

—

3

—

105

—

7

120

*" 17

480

17

4

3

7

4

420

53

30

420

*" 18

60

—

2

—

2

1

45

9

13

48

" 19

180

—

6

—

—

—

—

165

—

5

180

^Deposition of one egg.
°°Nest relief ceremonies.

Egg-laying dates and time of deposition are shown for four pairs
in Table 15. To the best of my knowledge, no eggs were laid by any
Green Heron on any of the New York City study areas outside the
period 0500 to 1100. Note that the first egg is laid later in the morning
than the remainder of the clutch; this is very characteristic. It was also
characteristic for the female to be relieved by the male shortly after
deposition of each egg. In every nest at which I observed the deposition
of the first egg, the male settled on the egg without hesitation as soon
as the female left.

The egg-laying intervals shown for pair 1-13 in Table 15 are char¬
acteristic for the birds breeding on Rulers Bar Hassock in all three
years of this study. The first and second eggs were always laid two
days apart in those nests checked in detail, but the other intervals
varied with different pairs. In the majority of nests, the third egg was

Table 15

Egg-laying intervals and time of deposition for four nests. Rulers Bar
Hassock, spring of 1955. All times are EDST.

Nest

Date of Laying Egg Number

Time

of Laying Egg Number

Number

1 2 3 4 5

1

2

3

4 5

1-12

May 11 May 13 May 14 May 15 May 16

1014

*

0621

* 0610

1-13

May 13 May 15 May 17 May 18 -

0938 0700

0630

0650

1-40

May 29 May 31 June 1 June 2 -

0716

0652

0605

0629

1-41

May 27 May 29 May 31 June 2 -

0830

0805

0746

0751

°Exact time of laying not observed, but both eggs thus marked were laid prior to 1200 hrs.

BREEDING BEHAVIOR OF THE GREEN HERON

83

laid two days after the second, and all other eggs followed at one-day
intervals. Pair 1-41 in Table 15 were most unusual in that all eggs
were laid at two-day intervals.

Before pair formation Green Herons pay practically no attention to
other species in and around the territory, but a marked change occurs
shortly after the pair has been formed and before any eggs are laid.
Then and for the remainder of the nesting phase both show intense
hostile behavior toward all Passerines, but especially toward the two
most important local egg predators, the Grackle and the Fish Crow.
The threats and attacks also alert all incubating and brooding birds
within earshot.

Part II

COMPARATIVE BEHAVIOR
OF OTHER NORTH AMERICAN HERONS

Introduction

An enormous number of publications on animal behavior, especially
bird behavior, have appeared in the past two decades. This writing
stems primarily from the stimulation provided by the numerous theories
and hypotheses of Konrad Lorenz and Niko Tinbergen. One of the
many results of this “school of comparative ethology” is the renewed
interest in, and the application of ethological findings to, systematics.

One of the major findings of the comparative ethologists is that
evolutionary relationships are reflected in behavior patterns as clearly
as in morphology. Furthermore, in many groups where morphological
criteria alone have given two or three alternate classifications, be¬
havioral characters have led to unequivocal decisions. As a result the
taxonomist who formerly relied solely on structural characters now
has at his command in many groups a body of behavioral information
to assist him in solving taxonomic problems. The radical revision of
the family Anatidae by Lorenz ( 1941 ) and Delacour and Mayr ( 1945,
1946) used behavioral information extensively.

The first task of the ethologist is to make an inventory of all the
behavior patterns of each species being studied. This descriptive
phase is essential before any thought can be given to experimental
analyses of isolated problems. Much effort has already been wasted
by experimentalists who considered this purely descriptive phase
trivial. Aside from the compilation of behavior catalogues, compara¬
tive ethological studies provide information on the causation, function,
and evolution of behavior patterns. Answers to these three problems
for any given display may be completely obscure when such a display
is observed in but one species.

Recent work in “systematic ethology” has centered on the lower
taxa, that is, species and genera. The behavioral characters so far
described, analyzed, and used in assessing relationships of species and
genera are the so-called “Fixed Action Patterns” shown during threat
and courtship activities. Great emphasis is placed on hostile and
sexual displays. These displays are excellent subjects for study because
they are frequently very striking and highly characteristic of a given
species. Furthermore, when groups of species are closely related, the
ethologist usually has little difficulty in determining those displays
which are homologous.

84

COMPARATIVE BEHAVIOR OF NORTH AMERICAN HERONS

85

Ethological studies require a great deal of time to be thorough and
comparative, as do most evolutionary studies. Most ethologists, more¬
over, find that the intensive study of one species is necessary before
an extensive study of a group can be planned. I shall use the Green
Heron as a basis for comparison throughout the remainder of this
paper. In Part II of this study the species are treated following the
plan adopted for the Green Heron, but none of them have been
studied in as great detail as the Green Heron.

The behavioral data gathered to date only permit provisional sug¬
gestions as to systematic relationships. Morphological studies, such
as Bock’s (1956), in which the student has museum skins readily
available for most of the species, have been much more broadly com¬
parative than my own research. Final decisions must await more
detailed studies of the behavior of the majority of non-North American
species of herons.

GREAT BLUE AND GREAT WHITE HERONS

Recent contributions to the systematic status of the Great White
Heron ( Ardea occidentalis Audubon of the A.O.U. Check-list of North
American Birds, 1957) confirm Mayr’s view that it is conspecific with
the Great Blue Heron, Ardea herodias Linnaeus (Mayr, 1956; Meyer-
riecks, 1957b; Meyerriecks and Meyerriecks, 1958). No evidence in
the literature, or field observations, suggest significant behavioral dif¬
ferences; hence, I treat them as color phases. Where appropriate, I
refer to the Great White Heron as “Florida Bay herodias ” and to the
Great Blue Heron as “mainland herodias”

METHODS

All observations of Florida Bay herodias were made in the Florida
Keys, especially Cotton Key (see map in Meyerriecks, 1957b). Most
of the observations of mainland herodias were also made in the Florida
Bay area, but important data were also gathered on Rulers Bar Hassock.
I include important observations of both color phases made in the
Florida Bay area by Robert P. Allen, and where appropriate I refer
to the important study of Michigan herodias by Cottrille and Cottrille
(1958).

The same techniques used for the Green Heron studies were em¬
ployed, but many important observations were also made from an
open boat and from an exposed spot on U.S. Route 1, the main road
through the Florida Keys.

STUDY AREAS

Observations of mainland herodias on Rulers Bar Hassock were con¬
fined to migration and feeding behavior because breeding of the Great
Blue Heron on Long Island, New York, is unknown ( Cruickshank,
1942).

Florida Bay is roughly triangular in shape, and it is bounded on
the north by the Florida mainland, on the south and east by those
keys traversed by U.S. Route 1, and on the west by the Gulf of Mexico.
I did most of my field work in the shallow eastern portions of the bay
(rarely more than ten feet deep). This area is dotted with numerous
keys (see map in Meyerriecks, 1957b). Broad reefs, fully exposed or
only partially covered with water, run from key to key, and hundreds
of individuals of seven species of herons can easily be compared on
these feeding areas.

DISTRIBUTION AND DESCRIPTION

The white birds have a very restricted range, being known only
from Florida Bay, Cuba, Isle of Pines, Jamaica, and the coast of
Yucatan. The blue birds, however, range as breeding birds throughout

86

GREAT BLUE AND GREAT WHITE HERONS

87

North America, from southern Alaska, Alberta, and New Brunswick in
the north, to southern Mexico, the West Indies, and the Galapagos
Islands in the south. The closely related Cocoi Heron, Ardea cocoi
Linnaeus, replaces herodias throughout South America, while the
Common Heron, Ardea cinerea, replaces it throughout Europe, Asia,
parts of Africa, and Madagascar. On the smaller keys both phases
nest solitarily or in small groups (2-3 pairs) in the red mangroves,
the only suitable vegetation. The larger keys support many more
breeding pairs, which characteristically nest in the black mangroves
that make up the interior of the island, rarely in the red mangroves
on the shore. The mainland birds may nest singly or in very large
colonies.

Ardea herodias is the largest North American heron. It is approxi¬
mately fifty inches (127 cm.) long with a wing spread of about six
feet (183 cm.).

The white birds are immaculate and have short occipital plumes
(which may be absent); and short scapular and jugular plumes. The
blue birds are bluish-gray on the back and wings; the primaries are
darker than the rest of the wings; forehead and crown are white,
with the feathers on the side of the head black; the blue birds have long,
black occipital plumes; scapular and jugular plumes are longer than in
the white phase; the brownish-white throat is streaked with black; thighs
are chestnut. The sexes of both phases are indistinguishable in the
field except during the courtship period. Mayr (1956) has discussed
the significance of bill and plume lengths in the two phases.

MAINTENANCE ACTIVITIES

Locomotion. No detailed studies of take-off, flight, or landing were
made, but these differ only in minor details from those of the Green
Heron. A. herodias does not normally erect its crest when taking off
or landing as does the Green Heron. The large size of herodias pre¬
sumably limits its agility.

The large body and long neck make the bird appear to fly more
slowly than the smaller Green Heron, but published flight speeds are
roughly the same (Wetmore, 1916, and Wood, 1933 for A. herodias ;
and Lowe, 1954, for A. cinerea ) . The flight of herodias is steady, strong,
and direct, with deep wing beats, in contrast to the apparent undulating
flight of virescens. I have seen the spectacular spiral descent flight of
herodias in many places. These flights were similar to those of the
Green Heron.

On short flights, or when alarmed near the nest, herodias character¬
istically flies with head and neck almost fully extended and the legs
dangling below. Lowe describes similar flight behavior in cinerea.
On long flights the neck is withdrawn in the characteristic heron
S-shape, and the legs are held in line with the general body axis,

88 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 29. Great White Heron “peering over” while feeding.

trailing behind. On migration herodias flies higher than does virescens ;
the same was true of herodias when flying to and from distant islands
in Florida. Unlike the Green Heron, herodias frequently glides during
a sustained flight. Long glides before landing are common in both
species, but those of herodias are quite spectacular at times.

Unlike the Green Heron, which sneaks along in a crouched position,
herodias strides forward with body erect, head and neck extended.
Each step is seemingly measured, the advancing leg being brought
forward smoothly and unhurriedly. Toes are relaxed and then spread
wide as the foot is carefully placed down. The bearing is majestic.
Of the North American herons which I know, only herodias and the
Common Egret, Casmerodius albus (Linnaeus), have these attributes,
although they are approached by the Little Blue Heron.

Feeding behavior. The basic type of heron feeding behavior Stand
and Wait, together with a slight modification Wade or Walk Slowly,
is common to all herons. A feeding Great Blue Heron shows the basic
method to perfection. Whether standing on some exposed shoal or
among the arched prop roots of the Red Mangrove, these herons are
the essence of immobility. They may hold the body upright, with head
and neck fully extended at about a 45 degree angle, or they may
crouch low, body held horizontally, the bill almost touching the sur¬
face of the water. The only movement is that of the large, yellow
eyes as they follow every motion of the intended prey. The statuesque
immobility is maintained for long periods, but eventually the bird
either strikes or moves slowly away. From either position, crouched
or erect, the strike is swift and direct.

Both phases of herodias show, as do all herons, a slight modification
when they wade or walk slowly. Wading is slow and each step is

GREAT BLUE AND GREAT WHITE HERONS

89

measured; the body may be erect or low and horizontal. The latter
position is typical just before a strike when wading forward. Peering
over (Figure 29) is very common, as if the bird were trying to locate
prey which has moved behind something. This attitude may be held
for several minutes at a time.

In one modification of Wade or Walk the bird wades forward very
slowly in an erect position and then suddenly extends and withdraws
its wings about a foot in a short, rapid flick. Such wing-flicks may be
repeated as many as five or more times, but usually two or three
flicks in rapid succession are made, then the bird resumes wading.
I have seen wing-flicking only on bright days, in open, shallow water,
when each wing-flick created an obvious sudden shadow on the surface
of the water. The function of wing-flicking is to startle prey. It is
true that the shadow of the bird’s body is always present on the surface,
but this moves slowly, while the wing-flicks suddenly appear and dis¬
appear as two extensions of this shadow. I have seen wing-flicking in
exactly the same fashion in the Common Egret and the Little Blue
Heron. Wing-flicking is, I believe, a low intensity form of Open
Wing Feeding, beautifully shown by the Snowy and Reddish Egrets
and the Louisiana Heron, Hydranassa tricolor (Muller).

I have not seen hovering over the water, skipping over the surface,
or swimming and driving for prey, all recorded for mainland herodias
(Bent, 1926) and the Common Heron of the Old World (Lowe, 1954).
This presumably reflects only lack of observations of unusual feeding
methods.

Audubon (quoted in Bent, 1926) records that herodias pierces its
prey with its bill, but I have never seen this or any species of heron
secure food by stabbing. The prey is usually taken in scissor fashion;
that is, it is grasped between the mandibles. Lowe makes similar
statements for the Common Heron. Richard Borden ( pers . comm.)
informs me that he has a motion picture record of a Florida Bay herodias
stabbing its prey.

If the prey proves to be too large for immediate consumption, herodias
and other species will hold it firmly between the mandibles, then
batter it against the ground, a rock, or a root, or they will repeatedly
drop, strike, pick up, and drop it, until the prey is reduced to insensi¬
bility. Small prey is tossed directly into the gullet with one smooth
movement of the head. Bill-dipping and head-shaking after swallowing
prey are just as characteristic of herodias as of virescens, and no sig¬
nificant differences in these movements could be found.

Care of the Body Surface

Preening. I made no detailed observations of preening, but my
notes suggest no striking differences between herodias and virescens.
Only two points need to be mentioned: ( 1 ) herodias appeared to droop

90 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

the wings more frequently and further when preening the upper neck
(compare Plates 1 and 8); and (2) herodias only occasionally fanned
the wing to any appreciable extent when underwing preening; normally
birds merely extended the wing slightly and then put the head between
the body and the partly open wing ( compare Plate 2 ) . Plate 9 shows
a nestling white phase herodias preening one of its primaries.

Scratching. A. herodias scratches by bringing its leg up directly,
the wing being held closed. The crest is always erected fully, and the
head is rotated as the bird works the side of the head and neck with
the middle toe.

Shaking. This series of movements is much like those of virescens ,
although it is my impression that tail-wagging is more often a terminal
component of shaking in herodias than in virescens.

Bill-cleaning and other maintenance activities. Cleaning and wiping
the bill, pecking the feet, and stretching the wings and legs showed
no significant differences from the Green Heron. Plate 10 shows a
nestling white heron in the sun-bathing position. Adult herodias show
the same movements while sun-bathing, but I have never seen a
sun-bathing herodias tilt the head upward as virescens often does.

The other maintenance activities ( defectation, sleeping, yawning,
and relaxation ) differed in no significant aspects from virescens.

Alarm, “Freezing,” and Escape Responses

Both phases of herodias show a variety of responses to danger. These
may be grouped into two main categories: (1) flight; and (2) alert
postures. I have never seen the Bittern Stance in herodias.

Flight. When fully aware of the approach of humans, herodias
usually responds with an unhurried flight; such leisurely flights are
usually silent. On occasion, however, a low, rather harsh “frahnk” or
“frawnk” call may be uttered as the bird takes wing. When suddenly
startled by a human, herodias explodes into flight, usually giving one
or more “frawnk” calls.

These large herons vary a great deal in their responses to the close
approach of a human to the nest. The incubating or brooding bird,
if well aware of the intruders presence, rises from the nest and stands
in one of the alert postures described later. If suddenly surprised on
the nest, however, these birds flush directly, and their subsequent
behavior is highly variable. Some individuals land near the nest and
keep the intruder in sight at all times, while others leave the area and
circle about the nest site, sometimes landing hundreds of yards away.
Others may fly back and forth over the nest, and each time they spot
the intruder they fly rapidly away again. It is my distinct impression
that those birds which fly around the nest call “frawnk” or “frahnk”
more frequently and more intensely than those which flush and land
nearby. As with the Green Heron I have never seen herodias hover

GREAT BLUE AND GREAT WHITE HERONS

91

over the nest or the intruder, as is so characteristic of the Black-crowned
Night Heron.

Alert postures. The measure of the escape tendency in any alert
posture is the degree to which the neck is extended and the feathers
are sleeked. In general, the greater the escape tendency, the greater
the extension of the neck and the greater the sleeking of the plumage.
In addition, the bird depresses the rear portion of its body more and
more as it readies itself for flight. A bird in the normal perch posture
is relaxed, with the head and neck withdrawn or only partly extended,
and its feathers are neither erected nor sleeked.

My observations of herodias were not as complete as those of
virescens , but crest-raising and tail-flicking are clearly not components
of these alert postures in herodias.

An extreme escape response was released among white and blue
phase birds which frequently fed in close proximity to a nest occupied
by a pair of Ospreys, Pandion haliaetus (Linnaeus). Both adult
Ospreys normally remained close to the nest, one incubating or brood¬
ing, the other guarding. Whenever a feeding heron approached too
closely, the Osprey on guard would dive at the heron. As the Osprey
swooped, the heron suddenly ducked down to an extremely low, flat
position, head and neck withdrawn, feathers completely sleeked, the
ventral parts of the body actually plunging into the water. As the
Osprey veered away, the heron adopted an extreme alert posture, head
and neck fully extended; in addition, the heron turned its body so as
to keep the Osprey continually in full view. Occasionally, as the heron
rose from the flattened position, it would utter one or more harsh
“frawnk” calls. Only once did I observe a white phase herodias stab
at one of the Ospreys as the latter swooped at the heron's head. The
stab was made from the extreme crouched position.

Feeding herodias adopted moderately intense alert postures as they
approached the vicinity of the Osprey’s nest. Many times they by¬
passed the Osprey nest by feeding in roughly a circular path around
the nest. The dives of the Ospreys were directed at the large herons
(for example, A. herodias , C. albus , D. rufescens ); the small species
were ignored.

“Freezing” responses. Occasionally both phases of herodias will
adopt an extreme alert posture, with head and neck elongated to the
utmost, and bill horizontal. The birds freeze in this position for long
periods. Sometimes the source of danger seems to be hidden, but the
blue phase especially adopts this extreme alert posture when standing
in reed-choked areas; these birds are fully aware of the nature of
the disturbance. They seem to be trying to conceal themselves as much
as possible. Stone (1937) gives a good description of these freezing
postures in the blue phase of herodias in the marshes near Cape May,
New Jersey. This posture is not to be confused with a Bittern Stance.

92 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Other responses. I have never observed predation on A. herodias.
The large size, powerful striking apparatus, and formidable threat
displays of this species appear to be sufficient to intimidate most
potential predators, but there are occasional references to adults being
taken by predators (for example, Monson, 1951; Cottrille and Cot-
trille, 1958).

The Florida Bay populations of herodias were habituated to the roar
of the numerous outboard motors, and other boat noises so common
in this region, especially along the well-traveled intracoastal water¬
way. Away from the main boat lanes, however, these birds were very
wary, and they usually responded to the sudden noise of a boat motor
starting by showing an extreme alert posture or by flushing wildly.

Habituation to the click of a camera was quite rapid with one pair
of herodias nesting on Cotton Key in Florida Bay (Meyerriecks and
Meyerriecks, 1958). When the first migratory herodias arrived on
Rulers Bar Hassock in 1957, the birds which fed near the tracks flushed
wildly at the approach of the trains, but habituation was rapid. Later
it was a common sight to see these and numerous smaller herons feed¬
ing within a few feet of the speeding trains.

My notes from the Florida Bay and Rulers Bar Hassock regions
include many observations of the responses of A. herodias to the alarm
notes of other birds. Table 16 lists the species and their calls which
released alert postures or flight (unusual). Most passerine alarm
notes were ignored, but occasionally a particularly loud note, given
close to the heron, would elicit a very low intensity alert posture.

Table 16

Birds and their calls which release alert postures and/or

flight in Ardea herodias.

Species

Call

Redwinged Blackbird, Agelaius phoeniceus

Green Heron, Butorides virescens

Little Blue Heron, Florida caerulea

Reddish Egret, Dichromanassa rufescens

Common Egret, Casmerodius albus

Snowy Egret, Leucophoyx thula

Louisiana Heron, Hydranassa tricolor

Black-crowned Night Heron, Nycticorax nycticorax
Yellow-crowned Night Heron, Nyctanassa violacea
American Bittern, Botaurus lentiginosus

“check”

« i »

skeow

aaah

“crog”

“frawnk”

t< yy

aarg

cc i yy

raah

tc 1 yy

squok

squak

“wok-wok”

MIGRATION AND BEHAVIOR ON ARRIVAL

Florida Bay herodias are non-migratory, but little is known about
movements around their restricted range. At no time during my stay
in Florida Bay (December 10, 1955-May 20, 1956) did I notice any

GREAT BLUE AND GREAT WHITE HERONS

93

changes which might indicate movements into or away from this
area. Occupation of the breeding sites is protracted, and high winds
and low temperatures inhibit courtship activities. The same applies
to the blue birds in the Florida Bay area. Breeding may take place
at any season of the year, but most eggs seem to be laid during the
period September-March. On Cotton Key all eleven nests discovered
in February, 1956, contained young; no nests had eggs. The young
ranged from newly hatched birds to those capable of sustained flights
(probably 50-60 days old). Courtship activities were noted around
Cotton Key in March, however, so it is possible to observe at one time,
even on one tiny key, birds in various stages of the reproductive cycle.

Many populations of mainland herodias are highly migratory. Stone
(1937:113) suggests that male herodias breeding in southern New
Jersey arrive before the females, but his description does not seem to
support his conclusion. Detailed studies of the closely related A. cinerea
in Europe reveal that both sexes arrive on the breeding grounds at the
same time (Verwey, 1930; Lowe, 1954). The occurrence of dancing
ground behavior ( see p. 100 and Lowe op. cit. ) suggests that there is a
delay in the occupation of the breeding areas, but Roberts (1936:175)
states that Great Blue Herons in Minnesota move directly into nest
sites on arrival, and Cottrille and Cottrille ( 1958 ) show that courtship
activities do not begin at once upon the return of the herons in spring
to the Michigan colonies, even though they occupy the breeding sites
on arrival.

This suggests behavioral differences between sedentary and migra¬
tory populations. Dancing ground behavior is indulged in by herodias
in Florida Bay, but I have only been able to find two brief notes out¬
side of the Florida Bay region (Mitchell, in Bent, 1926:102; Smith,
1894 ) . Smith’s observation was of sedentary populations in California.
Both descriptions are too limited to be of use.

In the British Isles A. cinerea has been observed dancing prior to
colony occupation (Lowe), but this behavior is unrecorded on the
continent. The British populations are relatively sedentary, while the
continental ones are highly migratory. There remains the distinct
possibility that the migratory populations of both herodias and cinerea
dance during migratory stops.

HOSTILE BEHAVIOR
Non-aerial, Non- vocal Displays

Crest-raising. The crest of the Green Heron is its most mobile feather
unit, but the back, neck, and pectoral plumes of both mainland and
Florida Bay herodias are much more mobile than the crest feathers.
Crest-raising appears to be a component more of hostile than sexual
displays in herodias.

The crest is fully erected in the Aggressive Upright, Forward, and

94 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Full Forward Displays. The crest is not raised in any alert posture
or in the Upright Display, both of which express high escape tendencies,
or at least greater escape than attack tendencies. During the Snap
Display the performing male erects its crest fully; however, I believe
that this crest-raising is not a hostile component but functions to
emphasize a sexual display.

A bristling blue phase bird, especially one in an intense hostile
display (for example, the Forward Display), has the bill and the
bulging yellow eyes accentuated by two triangular patches of black
feathers on the sides of the head. These patches appear to point
toward the eyes. This pattern is nicely shown for the similarly marked
A. cinerea in Lowe (1954: Plate 4). The crest of herodias probably
functions to increase the apparent size of the head of the attacking
bird, thus accentuating the intimidatory function.

Tail-flipping. As with crest-raising, tail-flipping in this species is
a component of those hostile displays which express greater attack
than escape tendencies. I have seen tail-flipping only in the Aggressive
Upright, Forward, and Full Forward Displays.

On occasion, but less frequently than with the Green Heron, a feeding
herodias will flip its tail just before a strike. Perhaps tail-flipping is
indicative of preparation for sudden movement at the sight of the prey.

Upright Display. A bird in this display (Figure 30) shows the
following components:

1. the head and neck are extended upward to their fullest extent, and are
held at about a 45 degree angle

2. the plumage is markedly sleeked, the neck especially appearing extremely
thin

3. the bill remains closed, and the eyes seem to bulge

4. the wings are held tightly closed

5. no calls are given at anytime, and tail-flipping is absent.

On numerous occasions I saw both white and blue phase herodias
perform mutual Upright Displays in the Florida Bay region. Such
mutual threat displays afforded direct comparisons of the two phases,
and in all cases the two forms were practically images of each other.

I believe that the escape tendency is relatively high. When con¬
trasted with more hostile displays (Aggressive Upright or Forward,
for example), the sleeked plumage, tightly closed wings, upright pos¬
ture, and complete lack of calls indicate escape components. An
increase in the attack tendency is expressed in the assumption of the
Aggressive Upright Display, together with walking or wading toward
the opponent. An increase in the escape tendency is shown by a
turning to one side and slight extension of the wings, an intention
movement of flight.

The Upright Display is most commonly seen where large numbers
of herodias are feeding in close proximity, and assumption of this
display usually indicates some sort of territorial trespass, or at least

GREAT BLUE AND GREAT WHITE HERONS

95

Figure 30. Adult Great White Heron in the Upright Display.

96 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 31. Adult Great Blue Heron in the Aggressive Upright Display.

GREAT BLUE AND GREAT WHITE HERONS

97

too close approach. Upright threats are also common during “dancing
ground” activities (p. 100).

Aggressive Upright Display. This display differs from the preceding
in the following respects: (1) the head and bill are arched downward
by a shallow arching of the upper neck; (2) the crest, pectoral, back,
and especially the neck feathers are all erected; this gives the neck
a thick appearance; (3) very frequently the bird opens its bill and
utters one or more harsh “rok-rok” calls; and (4) the bird moves
toward the opponent in a stiff, tense fashion. Figure 31 shows a blue
phase herodias in a typical Aggressive Upright Display, while Plate 4
in Lowe shows a striking similar, if not identical, display in cinerea.

Aggressive Uprights are much more frequently seen during feeding
territory disputes, presumably because of the concentration of birds,
but they are commonly seen, too, both in the breeding and non¬
breeding seasons, away from feeding areas.

The Aggressive Upright is an expression of a higher attack tendency
than the Upright, and I believe that the attack tendency is relatively
and actually much stronger than the escape tendency. The arched
neck (preparation for a strike), feather erections, and harsh calls are
all attack components, but the upright neck and tense attitude of the
displaying bird strongly suggest conflict.

The Aggressive Upright Display of herodias resembles the Stiff¬
necked Upright Display of the Green Heron in the thick, stiff, upright
attitude of the neck, and the downward inclination of the bill ( compare
Figures 10 and 31). The display of the Green Heron, however, occurs
only under special circumstances during the breeding season, and
only males perform this display. The Aggressive Upright of herodias
may occur in any hostile clash, breeding season dispute or otherwise,
and both sexes show this display.

Forward Display. From the Aggressive Upright, a bird assuming
the Forward Display: (1) inclines the body forward to a horizontal
position; (2) retracts its head and neck in preparation for a strike;
( 3 ) erects all of its feathers, especially the back plumes, to an extreme
degree; (4) walks rapidly toward the opponent with its wings partly
spread; ( 5 ) opens its bill wide and utters a series of rasping “rok-rok”
calls; and (6) the bird’s eyes appear to bulge from its head (Plate 11).

During the breeding season, when the soft-part colors change
radically, an attacking herodias is a formidable creature, taking full
advantage of its brilliant red-orange bill, bulging yellow eyes sur¬
rounded by lime-green lores and orbital skin, bristling plumage and
bright red legs. However, the primary function of such soft-part color
changes is to enhance sexual displays; their role in hostile displays
seems secondary (see Part III).

The Forward is an expression of the strongest attack tendency shown

98 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 32. Great Blue Heron performing a Snap Display.

by herodias, short of the Full Forward which immediately precedes
attack.

Full Forward Display. I doubt that this display is distinct because
it is a forward strike with the head from the Forward posture. However,
if the opponent retreats somewhat, the attacking bird will hold its
head and neck in the extended position, which, as a definite modifica¬
tion of the Forward Display, is assigned a descriptive name.

The hostile series, Upright — > Aggressive Upright — > Forward — »
Full Forward, forms a sequence showing an increasing attack tendency
and relative decrease in the escape tendency. In general, the displaying
bird shows two major movements with a rising attack tendency: (1)
increasing forward inclination of the body, and ( 2 ) increasing erection
of plumage. The reverse movements are shown with an increasing
tendency to escape; that is, the plumage is sleeked and the bird adopts
an upright posture with head and neck extended fully.

Fighting. When fighting, herodias attempt to buffet their opponents
with their wings and to strike them with their large bills. Although
the initial thrust with the bill is typically made from the horizontal
Full Forward position, subsequent strikes are delivered downward
as each bird tries to get above the other. I have never seen a fight
which resulted in damage to either of the combatants. Indeed, actual
physical contacts are quite rare, most of the encounters being rather
short-lived sparring matches.

Snap Display. Verwey’s (1930:21) description and illustration of
the Schnapphewegung of A. cinerea fit the same display in A. herodias
exactly. With the exception of bobbing, which is a movement added

GREAT BLUE AND GREAT WHITE HERONS

99

to the Snap Display, I was unable to find any difference between
herodias and cinerea.

The displaying bird, always a male as far as I was able to determine
(see p. 33; the account of Cottrille and Cottrille is not at all clear on
the Snap Display), typically performs while perched on the platform
of an old nest. The male extends its head and neck fully out and
downward until the bill is level with, or slightly below, the level of
the feet. Then the male bends slightly at the ‘11661” joints, lowering
the body toward the floor of the nest. Just as the head and neck
reach their fullest extent the male snaps his mandibles together, pro¬
ducing a loud “bok” sound. The crest feathers, together with the
occipital plumes, are fully erected during this display, but erection of
the neck feathers is highly variable. After making the “bok” sound
the male holds the extended pose for a few moments, then rises to
a normal perched position (Figure 32).

One variant of the Snap Display, the Low Bow Display, is frequently
shown by males which have been performing repeated Snaps. I believe
that the Low Bow variant is a high intensity form of the typical Snap
Display, but I lack quantitative data. The displaying bird assumes
the extended posture of the Snap, except that all feathers remain de¬
pressed, no “bok” sound is made, and the neck is held in a shallow,
inverted U-shape. In this position the bird bobs rapidly up and down,
bending at the “heel” joints with each bob.

The Snap Displays, together with the bobbing, of herodias and
virescens are strikingly similar. The similarity is heightened by the
fact than an occasional male herodias will grasp a twig momentarily
at the end of a Snap Display, just as male virescens grasp, then release,
twigs after a Snap. My observations of the Snap in herodias were less
detailed than those of virescens , but I believe that the causation of
this display is similar in both species ( see discussion in Part I ) .

Lowe describes a variant of the Snap Display in cinerea which ap¬
pears to be entirely hostile in its causation. This hostile Snap apparently
occurs only during territorial disputes, but the form of the display is
identical with the Snap shown by the male toward a sexually active
female. I have not seen this hostile form of the Snap in herodias , and
no mention of such a display by herodias is made by Cottrille and
Cottrille.

Bill-snapping. Verwey has described this behavior for cinerea , but
I have not seen it in herodias. However, bill-snapping and several other
important pre-copulatory displays are described by Cottrille and Cot¬
trille. These displays in herodias resemble closely similar displays
in cinerea (Lowe).

Withdrawn Crouch. The only behavior of herodias that resembles
the Withdrawn Crouch of virescens is the extreme crouch described on
page 91. It is possible, however, that something similar to the With-

100 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

drawn Crouch of the Green Heron might be observed in a large colony
of herodias where birds are continually landing in the crowded nesting
areas.

Dancing ground displays. This behavior has been referred to briefly,
and Lowe gives a good account of similar activities in English popu¬
lations of cinerea.

I have seen this behavior only in the Florida Bay region, but casual
notes in the literature appear to deal with dancing ground behavior
in mainland herodias (Smith, 1894; and Mitchell, in Bent, 1926).
Burleigh (1929) describes an observation made near Renton, Wash¬
ington, on March 14, of a circling, tumbling flight in herodias which
might be an aerial prelude to dancing ground activities. The flight
resembled those described by Lowe for English cinerea.

An observation I made near Bottlepoint Key, eastern Florida Bay,
on December 26, 1955, is typical of dancing ground behavior. Four
white and one blue herodias were standing quietly in the shallow water
immediately south of Bottlepoint Key. One of the whites and the
blue had brilliant soft-part coloration, and these two appeared to start
the dancing activities. The brilliantly colored white bird raised its
head and neck to about a 45 degree angle, erected its back plumes, and
strutted toward one of the other white birds. Immediately, the blue
bird performed in the same fashion, but the display of the blue bird
was abruptly stopped when the bird (dull white) toward which the
display was directed threatened the displaying blue. Both the latter
and the dull white bird assumed the Upright Display posture, which
they held for almost one minute; then both threatening birds relaxed
to an extended alert posture. At no time did the two birds that
danced display toward each other, and, because of their dancing, soft-
part coloration, and subsequent behavior, I took them to be males.

A short time later, the blue bird extended its wings very slightly,
erected its head and neck as before, and danced toward a dull-colored
white bird. The dance was a combination of a skip and a slight jump-
flight, and the blue bird seemed to float momentarily. After it had
gone about ten yards, the blue bird took off and circled around the
white bird toward which its dance had been directed, and its circular
flight reminded me of the Circle Flight Display of the Green Heron.

On two other occasions I saw this dancing ground behavior in the
Florida Bay region, and Robert P. Allen, of Tavernier, Florida, informs
me ( in lift. ) that he has seen this performance many times in the same
region. Both Allen and I have seen strutting and dancing in birds
that ranged in soft-part coloration from brilliant to very dull (that is,
non-breeding coloration). Two facts, the dullness of the soft parts
of some “dancers” and the frequency of Upright Displays during the
performance, lead me to conclude that dancing ground activities are
the first indications of breeding season activities in A. herodias , at

GREAT BLUE AND GREAT WHITE HERONS

101

least in the Florida Bay area. Although more observations are necessary,
I believe that the upright posture and strutting walk of the dancers
has evolved from the Upright Display; this is suggested by the extended
head and neck, sleeked plumage (except for the back plumes), and
orientation of the dancing display. Hostile displays are common pre¬
cursors of many sexual displays in birds.

Non- aerial. Vocal Displays

Advertising calls. Verwey (1930) and Witherby et al., (1949) record
a pairing or advertising call for A. cinerea. I have no information on
such calls for A. herodias, although Cottrille and Cottrille state, "this
eerie howl of the heron is perhaps its nearest counterpart to the territory-
advertisement and mating songs characteristic of many birds.” The
eerie howl is the call that accompanies the Stretch Display. They list
the calls of cinerea and state, "those of the Great Blue Heron [are]
very similar,” but no details are given.

On several occasions in Florida Bay, I observed a male, standing on
an old nest, open and close his bill repeatedly, but I was unable to
approach close enough to establish the nature of the call. Judging
from the behavior of the Green Heron and other heron species, an
advertising call was appropriate for the stage of the breeding cycle
and the location of this male.

Attack call. Other than some harsh croaks, difficult to describe, heard
during a fight, the only attack call I have ever heard in herodias is the
harsh "rok-rok” (p. 97).

Verwey describes several hostile calls for cinerea , among them a
loud "gooo” uttered by a bird as it stabs at an opponent, and a harsh
"schaah” uttered by the birds in a Pursuit Flight. Cottrille and Cottrille
state that similar calls are made by herodias.

Escape and alarm calls. The only notes associated with escape and
alarm that I heard were the “frawnk” and "frahnk” calls (p. 90).
When merely aware of the presence of a source of threat, the alarmed
bird utters one or more barely audible, low "frawnks.” As the escape
tendency increases, however, the "frawnks” are louder, harsher, and
more frequent.

Cottrille and Cottrille describe a cry of “pain or terror” ( herodias )
heard on one occasion, and Verwey describes an "anxiety” note for
cinerea (a nasal “go-go-go”). This is uttered especially when a dis¬
turbance, such as a man near a nest, is located below the calling bird
( Witherby et al.).

Aerial, Non- vocal Displays

I observed no Flying Around, Pursuit Flights, Supplanting Attacks,
or ritualized display flights by herodias. The evidence in the literature
suggests that there is little or no aerial behavior preceding territorial

102 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

activities. Cottrille and Cottrille mention a pursuit call ‘made by
both pursued and pursuer.” One observation by the Cottrilles ( 1958:5)
suggests a mutual aerial display, but no details are given. The circular
flight which I observed on December 26, 1955 (p. 100) indicates that
a more detailed study might reveal aerial displays in herodias. It is
my impression that few aerial displays are shown by the larger species
(e.g., A. herodias, A. cinerea, C. albus), while aerial displays are very
common with the smaller species (e.g., B. virescens, L. thula). I ob¬
served no aerial, vocal displays by herodias.

SEXUAL BEHAVIOR

I observed only one sexual display, the Stretch Display, in the
Florida Bay region, but Cottrille and Cottrille describe a number
from Michigan.

The Stretch Display is the same as the Reckbewegung of Ardea
cinerea, so well described and figured by Verwey, Holstein (1927),
Witherby et al., and Lowe. Cottrille and Cottrille, who call what is
obviously a Stretch Display “howling,” make no comparison with
the same display in cinerea, although their figures beautifully show
that the display they observed was the Stretch.

I had Verwey ’s description and illustration before me in the field,
and I was unable to detect the slightest difference between the displays
of the two species. I was, however, unable to hear clearly the call
which accompanied the Stretch Display of herodias, but the description
of this call given by Cottrille and Cottrille shows that the calls of
herodias and cinerea are strikingly similar. Verwey and Lowe state
that males of cinerea utter a two-parted “hu-oooo” or “hoo-oooo,” and
Cottrille and Cottrille state that herodias emits a ‘low o-o-o-o”; at
the height of the Stretch the sound becomes higher and louder —

€C1 77

h-o-o-o-o-o.

The males I saw performing the Stretch Display did so from the
floor or rim of an old nest, less frequently from a nearby branch;
similar observations were made by the Cottrilles in Michigan. When
displaying the Stretch, the males I saw made the following movements:

1. the bird stretched its head and neck to their fullest, bill pointing
straight up

2. it depressed all of its feathers except the pectoral plumes

3. it held this position momentarily, then lowered its head and neck down
and backward so that the occipital plumes almost touched the middle
of the back

4. as the bird lowered its head and neck, it bent its legs at the “heel”
joints and lowered its body toward the nest

5. it held this lowered position for a few moments, then rose to a normal
standing position (Figure 33).

All the males of herodias I saw perform the Stretch Display in the
Florida Bay area had brilliant soft-part coloration. The description

GREAT BLUE AND GREAT WHITE HERONS

103

Figure 33. Great White Heron performing the Stretch Display. The figure to the left represents
stage 1 of the Stretch, while stage 2 is shown in the figure on the right.

104 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

given by the Cottrilles for Michigan herodias is similar, except that
the birds they saw appeared to erect their neck feathers to a great
extent, and when the displaying bird arose from the crouch, it held
its bill pointing upward; furthermore, no remarks are made by the
Cottrilles on the coloration of the birds they observed.

I believe that the Stretch of herodias is simpler and more highly
ritualized than the same display of virescens. Nothing that I noted
resembled the Extended Stretch Display of virescens. I get the im¬
pression from the descriptions given by Cottrille and Cottrille that
the Stretch Display (“howling”) of Michigan herodias is primarily
sexual in nature.

I conclude that the origin and function of the Stretch Displays of
virescens and herodias are similar.

EARLY PAIRING BEHAVIOR

I have no information on pairing behavior in herodias after the
initial occupation of the breeding areas. Details on many aspects of
pairing biology of this species are to be found in Cottrille and Cottrille.

No detailed studies of roosting behavior were made, but herodias
usually alights in the crowns of the trees, then walks or jumps slowly
down to the final roosting spot, while virescens characteristically flies
directly down to the final spot, or jumps down to it.

A vast literature, mainly anecdotal, on coloration of the soft-parts
of herons has accumulated, but little pertains to North American
species. Most of the data on soft-part coloration I gathered in the
Florida Bay region, but I also made some important observations of

Table 17

Soft-part coloration of Ardea herodias and A. cinerea.

Area

A. herodias
(white phase)

A. herodias
(blue phase)

A. cinerea

bill

dull yellow

Non-hreeding
dull yellow

yellow

lores and
orbital skin

pale grayish-blue

pale grayish-blue

yellowish- green

legs and feet

yellowish-green

brownish or greenish black

brownish

iris

yellow

yellow

yellow

bill

Breeding

bright orange to red bright orange to red

orange to red

lores and
orbital skin

bright lime-green

bright lime-green

red ? ?

legs and feet

bright red

bright red

pinkish to red

iris

yellow

yellow

yellow

GREAT BLUE AND GREAT WHITE HERONS

105

migratory herodias on Rulers Bar Hassock in the spring of 1957. No
color changes were noted by Cottrille and Cottrille in Michigan.

Table 17 lists the breeding and non-breeding coloration of the soft-
parts of both color phases of herodias and, for comparative purposes,
A. cinerea. I have not listed sexual differences because of insufficient
observations. However, A. herodias males are more brilliantly colored
than the females and acquire their brilliant breeding coloration earlier
than the females.

REDDISH EGRET

Bock ( 1956 ) , on the basis of a few plume characters, places Dichro-
manassa rufescens , Hydranassa tricolor , and Florida caerulea in one
genus ( Hydranassa ), and he puts Leucophoyx thula in another
( Egretta ). My observations of the behavior of rufescens and thula
indicate that they are congeneric. Bock admits, “The lack of characters
distinguishing Hydranassa from Egretta presents a major problem.
Other than the difference in plumes, I have been unable to find any
good character that separates the two genera.”

METHODS

With the exception of a few minor observations near Puerto Arista,
Chiapas, Mexico, I made all of my observations of the Reddish Egret,
Dichromanassa rufescens , around the Bottlepoint, Stake, Low, and Cow-
pens Keys area of Florida Bay ( see map in Meyerriecks, 1957b ) . I used
the same techniques as with the previous species. However, rufescens
is an extremely tame species, and I was able to make numerous obser¬
vations from an open boat anchored in the shallows. The areas have
already been described.

DISTRIBUTION AND DESCRIPTION

The Reddish Egret ranges as a breeding bird from southern Florida,
Louisiana, the coast of Texas, and Baja California, south to Sinaloa,
Yucatan, Cuba, Isle of Pines, and the Bahamas. The species winters
south to Guatemala, El Salvador, and Venezuela. A more detailed
treatment of the breeding range and frequency of the white and
dark phases is to be found in Allen (1954^55). The Reddish Egret
nests singly or in small to large colonies. The largest breeding colony
known (several thousand pairs) is on Green Island, in the Laguna
Madre, off the coast of Texas. At all seasons of the year this bird is
almost completely confined to salt or brackish water.

D. rufescens is medium-sized, length about thirty inches, wing spread
about four feet. Most field guides state that it occurs in two color
phases, light and dark, but this is misleading because the Reddish
Egret is extremely variable in color and pattern, at least in the dark
phases. My field notes clearly indicate that there are at least three
color phases (white, intermediate, dark), a statement which Bock
also makes, although he gives no details. Allen (1954-55) also recog¬
nizes three phases. I have numerous notes on highly aberrant forms
from the Florida Bay region, including observations on two melanistic
birds. These notes suggest even greater complexity than three phases,
but this information will be published elsewhere.

Adults have numerous lanceolate plumes covering the entire head
and neck, these plumes being especially long in the occipital and

106

REDDISH EGRET

107

pectoral regions. Long scapular plumes extend beyond the tip of
the tail in all phases. The dark birds ( except the two melanics noted )
have the head and neck colored a vinaceous-brown, with the body and
wings neutral gray. The intermediate birds are neutral gray in color
all over, and the white phase birds are typically immaculate; some
white phase individuals, however, may have a few to many of the
flight feathers tipped with slate gray. The basal half of the bill in
adults is a pale flesh color (non-breeding season), while the tip is
black. All phases have blue legs and feet.

MAINTENANCE ACTIVITIES

Locomotion. No detailed studies of take-off and landing were made,
but rufescens does not appear to differ significantly from the previous
species in these activities. Except in hostile situations the Reddish
Egret does not erect its crest or flip its tail conspicuously while landing
or taking off.

The flight is strong and rapid. Except for some unusual aerial
displays to be described later, the flight of rufescens is direct. On long
flights the head and neck are retracted, but on short flights, and in
certain specialized displays, the head and neck are extended.

Unlike herodias and albus, rufescens is extremely active. In the
grace and agility of its movements the Reddish Egret is matched only
by the Snowy Egret and the Louisiana Heron, among North American
herons.

This species has a very striking spiral descent flight which most
closely resembles that of the Snowy Egret and the Louisiana Heron.
Spiral descent flights are extremely common during the courtship
period.

Green Herons walk by sneaking forward from a crouched position;
herodias and albus wade forward smoothly and unhurriedly from an
upright position; rufescens strides along with rather short, jerky motions
from a peculiarly upright position. No other heron with which I am
familiar walks in the same manner.

Feeding behavior. The Reddish Egret is unique among North Amer¬
ican herons in its varied feeding behavior. Not only does it show the
previously described Stand and Wait, Wade or Walk Slowly, Peering
Over, and Wing-flicking types, but it possesses an almost incredible
number of other types. These other types all involve a great deal of
activity, and all (with the exception of Canopy Feeding) are variations
of the general category called Disturb and Chase by Meinertzhagen
( 1949 ) , who used this term with reference to the feeding behavior of
the Reef Heron, Demigretta gularis ( Bose ) .

The basic types, Stand and Wait and Wade or Walk Slowly, are
shown rather infrequently. Wing-flicking, in which the feeding bird
moves forward slowly while partly extending and retracting its wings

108 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

rapidly, is quite common, but the most characteristic feeding technique
used by rufescens is a lurching, weaving type of half -run, half -jump
progression. As the bird reels forward, it stabs rapidly to the right
and left, attempting to seize any prey disturbed by its activities. This
method is used primarily in very shallow water. One characteristic
variant of Wade or Walk Slowly might be termed “head-tilting.” The
bird moves rather slowly through the water with its head and neck
strongly inclined to one side as it peers intently at the surface (see
Plate 12). If a school of tiny fish is disturbed, the egret chases the
prey over a wide area, and it holds its head and neck tilted to one
side or the other throughout the chase.

Open Wing Feeding is another characteristic feeding technique of
rufescens. The feeding bird begins by running slowly, wings partly
extended. Then at the sight of the prey it runs rapidly and extends
the wings fully. As the prey dash about, the egret turns and twists,
wings still extended fully, and the bizarre performance may continue
for several minutes. On occasion the bird may halt suddenly in the
middle of a run, retract one wing, rapidly extend and retract the other
wing, and then resume its forward run. Open Wing Feeding frequently
ends in Canopy Feeding or Underwing Feeding.

Canopy Feeding has been described for the Black Heron, Mel-
anophoyx ardesiaca (Wagler), of tropical Africa and Madagascar, by
numerous authors (for example, Ayres, 1878; Loveridge, 1922; Rand,
1936; Jackson, 1938; Delacour, 1946; Winterbottom, 1957). Superb
photographs of this unusual feeding technique are to be found in Dela¬
cour (1946) and Winterbottom (1957). To my knowledge this type
of feeding behavior has not heretofore been described for the Reddish
Egret. In fact, Delacour, speaking of ardesiaca , states, “it has a very
peculiar way of catching fish, which I think is unique in the family.”
Although ardesiaca shows Canopy Feeding to an extreme degree, it still
closely resembles the feeding of rufescens.

When showing the Canopy Feeding method, rufescens runs forward
with wings extended, then halts and peers into the water, and then
brings both wings forward over its head, forming a canopy over the
head and neck. This pose is held for a few moments to several minutes.
I could clearly see the rapid fish-catching movements the bird made
under the canopy of the wings. This species differs from ardesiaca
in that the latter typically places the tips of the wings in the water,
while all the Reddish Egrets I saw Canopy Feeding held the tips of
the wings clear of the surface. Plates 13 and 14 show two expressions
of Canopy Feeding by the Reddish Egret.

Rand (1936), describing the canopy of ardesiaca , concluded that
the function was to bewilder the prey by the darkness of the canopy.
My observations of rufescens at extremely close range (three feet) indi¬
cate that the shadow provides a false refuge for the fish startled into

REDDISH EGRET

109

motion by the previous dashing activities. Reddish Egrets typically
hold their wings in the canopy attitude for a minute or two before
they make a strike. I could clearly see many fish enter the shade of
the canopy in the shallow water, and when a number of prey had thus
“fallen for the ruse,” the egret would stab rapidly under its extended
wings.

Very often Reddish Egrets engage in feeding which I believe is low
intensity Canopy Feeding. I call it Underwing Feeding. The egret will
run forward, wings fully extended, then stop suddenly, extend one or
both wings fully and horizontally, then stab rapidly under one or both
wings. This type differs from Canopy Feeding in that the wings are
not brought forward and the bird does not place its head under the
raised wings except when it strikes.

I conclude that the following series of feeding types represents an
evolutionary sequence: Wing-flicking — » Open Wing — » Underwing
— > Canopy Feeding. The Great Blue Heron, Common Egret, Reddish
Egret, Snowy Egret, Louisiana Heron, and the Little Blue Heron all
show the initial stage, Wing-flicking. Sutton ( 1936 ) gives an excellent
description of what is obviously Wing-flicking feeding behavior in
the Least Bittern, Ixobrychus exilis (Gmelin). The Reddish Egret,
Snowy Egret, and the Louisiana Heron show further developments of
the series, but the Reddish Egret shows all the steps. From the de¬
scriptions of Canopy Feeding by the Black Heron, it appears that
this species has carried the evolutionary series to its extreme develop¬
ment. It would be of the greatest interest and importance to discover
what other heron species show any or all of these stages.

Although there are minor variations in all of these feeding types,
only two other methods warrant description. These are foot-stirring
and hovering-stirring. In a recent paper by Rand ( 1956 ) the Reddish
Egret was not listed as one of the North American herons that show
foot-stirring. However, N. B. Moore (in Baird et al., 1884) gives an
excellent description of such behavior in rufesceens. I saw foot-stirring
by the Reddish Egret on numerous occasions in the Florida Bay region.

As the egret waded forward rather slowly, it would vibrate its feet
over the surface of the mud, imparting a scraping motion to the feet;
then it would stop and peer at the surface of the water and either strike
at prey or move on, usually resuming the scraping motions. Hovering-
stirring is an aerial variant of typical stirring or scraping. The feeding
bird moves forward slowly, then suddenly launches into flight, hovers
over the surface of the water, and very gracefully scrapes the mud or
aquatic vegetation with one foot. This behavior is repeated while the
bird continues in flight. The strike is made from the hovering position.
A more detailed treatment of foot-stirring and hovering-stirring by
rufescens and other heron species is to be found in Meyerriecks ( 1959 ) .

This brief treatment of the amazing array of feeding activities of

110 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

rufescens emphasizes my statement that the Reddish Egret is the most
active of the North American herons.

Care of the body surface. I made detailed observations on most of
the other major maintenance activities of rufescens (for example, preen¬
ing, head-scratching, and sunning), but this species does not differ
significantly from the species already described. The Reddish Egret
does, however, engage in underwing preening more frequently and
more extensively than any other heron species with which I am familiar;
in addition, such preening is most typical after very active feeding
behavior. I believe that the greater frequency of underwing preening
and the active feeding methods are correlated.

Alarm , “freezing,” and escape responses. In its alarm and escape
responses the Reddish Egret does not differ significantly from other
heron species. Crest-raising in this species, as with herodias and albus ,
is not a typical component.

When caused to fly by the sudden appearance of a human, the
Reddish Egret may utter a not unpleasant “crog” call note; on other
occasions the startled bird may utter a rather harsh “raaah” note.
Griscom (1926), after flushing Reddish Egrets from their nests on
Culebra Key, Ascension Bay, Quintana Roo, recorded a nasal, croaking
“ wonk” call note.

The extension of the head and neck, together with thinness of the
neck, are keys to the strength of the escape tendency in any alert
posture. I saw nothing resembling a Bittern Stance in the Reddish
Egret.

MIGRATION AND BEHAVIOR ON ARRIVAL

In Florida this species is migratory, but in many parts of its range
it appears to be sedentary. I arrived in the Florida Bay area on Decem¬
ber 10, 1955, and a few Reddish Egrets were already present. A definite
movement into the Keys was noted on December 24, 1955, and the first
signs of breeding activity were also noted the same day. Although I
made no daily counts, I feel that the movement into the area was
protracted. Allen (1954-55) states, for the Green Island population,
“Usually the first birds arrive at Green Island from somewhere in
Mexico in March, but most of them show up in April. There is con¬
siderable spread to these arrivals.”

High winds and low temperatures definitely inhibited breeding ac¬
tivities in the Florida Bay region, and Allen ( 1942, 1954-55 ) noted the
marked effect of temperature changes on the breeding behavior of
rufescens off the coast of Texas.

HOSTILE BEHAVIOR
Non- aerial. Non- vocal Displays

Crest-raising. Of all the North American herons the Reddish Egret
is easily the “bristliest,” and this “bristling” is particularly striking in

REDDISH EGRET

111

crest-raising. Moderate to extreme crest-erection occurs in a variety
of hostile and sexual displays.

The non-aerial displays in which the crest is erected are the Upright
(moderate erection), Aggressive Upright (full erection), and For¬
ward (extreme erection) Displays. The degree of crest-raising is an
excellent indicator of the strength of the attack tendency.

The Reddish Egret resembles the Green Heron in that the crest is
its most mobile feather unit. When rufescens erects its crest, it appears
strikingly "big-headed.” Although crest-raising certainly functions to
increase the apparent size of the weapon-bearing part of an attacking
bird, I believe that hostility may be a secondary function. The extreme
development of plumes and their erection have evolved chiefly to
emphasize certain sexual displays in which head-tossing is a com¬
ponent (p. 118).

Tail-flipping. The only hostile displays in which the tail is flipped
are the Upright and Snap Displays. On several occasions I saw a
feeding Reddish Egret flip its tail just before a strike, but this movement
was so rare that I suspect it is not a typical component of the strike
as it is with the Green Heron.

Upright Display. A Reddish Egret in this display makes the following
movements:

1. extends the head and neck upward to their fullest, inclines the head and
upper part of the neck to the rear so that the occipital region is directly
over the middle of its back — the bird is leaning backwards

2. holds the bill closed and horizontal

3. sleeks the body plumage to an extreme, but erects the crest and neck
feathers moderately

4. holds the wings tightly closed, and may or may not flip the tail in¬
frequently

5. gives no calls at any time

6. bulges its eyes from the head (Figure 34).

It should be evident that the Upright of rufescens differs significantly
from the similar display of herodias , which shows no crest and neck
feather erections, and inclines the body and extended head and neck
forward to about a 45 degree angle.

I conclude that the escape tendency of a Reddish Egret showing this
display is strong because a slight increase in the escape tendency almost
invariably leads to immediate flight. Orientation away from the op¬
ponent, slight extension of the wings, and tail-flipping all indicate a
rise in the escape tendency.

Aggressive Upright Display. This display, which shows a relatively
greater attack than escape tendency, has the following components:

1. the bird holds its head and neck as in the Upright, but inclines them
forward so that the occipital region is above the forward part of the back

2. holds its bill horizontal but opened, and the bird may utter one or
more rather harsh “raaah” calls

112 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 34. The Upright Display of the Reddish Egret.

REDDISH EGRET

113

Figure 35. The Aggressive Upright Display of the Reddish Egret.

114 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

3. erects its body plumage, especially scapular plumes, moderately; erects
crest, neck, and pectoral plumes strongly

4. holds wings partly extended

5. faces opponent frontally and walks slowly toward him

6. bulges its eyes from the head (Figure 35).

Orientation toward the opponent, wing extension, harsh calls, full
feather erections, and forward inclination of the head and neck indicate
a rise in the attack tendency. If the escape tendency of a bird in the
Aggressive Upright increases, neck, pectoral and scapular plumes are
depressed, and the head and neck are inclined rearward.

Forward Display. This display, which is the most intense hostile
behavior shown by rufescens short of actual fighting, differs from the
previous display in that the feathers of the head, neck, pectoral, and
back regions are all erected to an extreme. Furthermore, the displaying
bird extends its wings fully and runs directly toward the opponent,
uttering a series of very harsh “raaah” calls as it rushes forward. A bird
in the Forward Display fairly bristles all over, and the headlong rush
toward the opponent is quite spectacular. The only conceivable tend¬
ency being expressed in this display is intense attack.

Fighting. I saw fighting on many occasions in the Florida Bay region.
All of the fights were feeding territory disputes or clashes at the start
of pair formation. Reddish Egret fights are truly spectacular, frequently
ranging over a hundred yards or more. Mutual Forward Displays lead
to headlong crashes. Both participants attempt to strike at the opponent
from above; at the same time they jab viciously with the bill and
flail with buffeting blows from the wildly flapping wings. Almost all
of the blows made with the bill are aimed at the opponent’s head. The
Reddish Egret differs from fighting Green Herons in that the latter
species strikes and recoils almost always from a horizontal body posi¬
tion, while rufescens fights from an upright position. The only damage
I ever saw was the loss of a few feathers, but the force and tenacity of
their fighting indicates real damage may be expected.

The hostile series, Upright — » Aggressive Upright — > Forward Dis¬
plays, forms a sequence showing increasing attack tendencies. In
general, as the attack tendency rises, the bird increasingly inclines its
head and neck forward and increasingly erects its crest and other
plumage.

Snap Display. This display in rufescens differs from the same display
in virescens and herodkis in the following components:

1. instead of a simple forward and downward extension of the head and
neck, rufescens bows and extends at the same time, first to the right,
then to the midline, then to the left, and then repeats

2. the bird erects the crest and neck feathers fully as it bows forward

3. rufescens shows tail-flipping as a typical component.

The Snap Display of the Reddish Egret appears to be more
specialized in that only the bowing type is shown, while virescens and

REDDISH EGRET

115

herodias show both the bowing type and the simple forward Snap with¬
out any bowing components. Furthermore, virescens and herodias
typically erect only the crown feathers, while rufescens erects the
neck feathers in addition.

The causation of the Snap Display of the Reddish Egret appears to
differ significantly from that of the other two species. The Snap of
rufescens is much more of a hostile display because: ( 1 ) the bird shows
the Snap during the very early stages of pair formation; (2) the Snap
is preceded and/or followed by other hostile displays; and (3) any
individual approaching a bird giving the Snap is threatened and/or
attacked. In contrast, virescens and herodias frequently alternate
Stretch and other sexual displays with their Snaps, and the approach
of another individual frequently triggers a bout of intense sexual dis¬
plays. Study of a large population of rufescens (for example, Green
Island, Texas ), however, might reveal the occurrence of a Snap Display
with greater sexual tendencies. It would be of great interest to deter¬
mine the degree of tail-flipping, if any, in such sexual Snaps.

Dancing ground displays. On two occasions in the Florida Bay
region I saw what appeared to be a dancing ground display by the
Reddish Egret. Audubon (in Bent, 1926) describes what may be
dancing ground behavior. In my first observation of this behavior
six birds were feeding in one group on a large shoal. Two of the birds
had brilliant soft-part coloration. A seventh Reddish Egret flew in
from a nearby shoal and landed about fifty yards from the group. This
new arrival also had brilliant soft-parts. Just before landing, “seven”
raised his head and neck to about a 45 degree angle and began low
intensity head-tossing. When head-tossing, the displaying bird rapidly
raises and lowers its head in a vertical plane and fully erects the crown
plumes.

“Seven” continued head-tossing after it had landed, and then it
slowly moved toward the group. As it approached one of the dull-
colored birds (a female?), “seven” opened its wings slightly, raised
its head and neck upward and forward so that the head and bill were
held at about a 45 degree angle, and then it began very intense head¬
tossing. At the same time “seven” began to utter a series of low, not
unpleasant “crog-crog” calls. However, the displaying of “seven” was
abruptly ended when one of the other brilliantly colored birds (a
male?) threatened “seven” with the Upright Display. “Seven” im¬
mediately responded by adopting an Aggressive Upright; the two birds
flew at each other and engaged in a brief fight. “Seven” succeeded in
driving the other ( male? ) from the shoal, but I saw no further dancing.

The only other observation of dancing ground displays I made in¬
volved five birds feeding together on a small shoal. One of the birds
(a male?), with very brilliant soft-part colors, started the dancing
activities. This bird paused in its feeding, gazed briefly at one of the

1 16 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

other Reddish Egrets, then raised its head and neck, extended its wings,
and began to head-toss rapidly. The “actor,” still head-tossing, moved
slowly toward the egret it had been looking at, and then suddenly
flew and began a most bizarre aerial performance. It landed, still
head-tossing, on one side of the standing egret, then flew up and over
the standing bird, landed on the other side, and then repeated the whole
performance. The standing bird assumed a low intensity Upright
posture each time the displaying male (?) flew over its head. This
jumping over display was continued for a few minutes, then the
“actor” landed and began to walk toward one of the other birds. Its
walk was more of a ritualized strut as it moved very slowly toward the
other bird. The dancing ground display was ended when one of the
other egrets threatened the “actor.” Mutual threat displays followed,
then all five birds resumed their feeding.

Such dancing ground displays were the first indications of breeding
behavior. My preliminary belief is that they are essentially hostile in
nature.

Non- aerial. Vocal Displays

On several occasions I heard a bird, presumably a male, utter a
series of “arg-arg” calls from the top of a large mangrove tree. These
birds were always brilliantly colored, and all of them had previously
shown rather intense Snap Displays. Other individuals, presumably
females, were attracted to the vicinity of the calling males. Further¬
more, the “arg” call of rufescens very closely resembles the “aarh” call
of the Snowy Egret, which is the advertising call of that species. This
“arg” call of rufescens may be the advertising call of this species, but
I do not have enough data for proof.

Aerial, Non- vocal Displays

Flying Around. This behavior, one of the first indications of terri¬
toriality, is very similar to that of the Green Heron. I saw individual
Reddish Egrets Flying Around repeatedly in the Cowpens Keys area
of Florida Bay. They flew from one key to another, around the periphery
of one key, or typically, back and forth from one end of a key to the
other. All of the birds engaged in Flying Around were brilliantly
colored, and all were especially responsive to the presence of other
Reddish Egrets. I conclude that the function of Flying Around in
rufescens is the same as in the Green Heron — to familiarize the bird
with potential nesting sites and with those birds which are ready to
engage in breeding activities.

Pursuit Flight. Once individual males had established themselves
on one part of one key, they engaged in Pursuit Flights with any other
Reddish Egrets which approached their territory. Most of the Pursuit
Flights at the start of the breeding season were short-lived, mainly

REDDISH EGRET

117

because the pursued did not appear to be ready to engage in any further
activities. These birds were typically dull-colored. When the pursuer
engaged a brilliantly colored bird in a Pursuit, however, the flight was
usually quite prolonged and ranged over the entire length of one key,
or from one key to another. Crest-raising by both pursuer and pursued
was typical, but the crest-raising of the pursuer was more intense. An¬
other characteristic component of lengthy Pursuit Flights was bill¬
snapping.

Supplanting Attack. As with the Green Heron the Supplanting At¬
tacks of the Reddish Egret were most noticeable during the early phases
of the reproductive cycle. The frequency and intensity fell off sharply
once the pair began to engage in mutual sexual displays. The only
difference between the Supplanting Attacks of rufescens and those of
virescens is that rufescens , when attacking, erects all of its plumage as
it supplants.

One variant of the Supplanting Attack of the Reddish Egret involves
head-tossing by the supplanting bird during the short flight toward
the bird to be supplanted. The attacking bird launches from its perch,
then about midway through its flight it elevates its head and neck
slightly and shows very low intensity head-tossing. I noted that in
almost every case in which a Supplanting Attack was preceded by
head-tossing, the attack was mild, and I suspect that the attacking bird
was expressing a very low sexual tendency in its head-tossing.

Bill-snapping. This behavior is most commonly seen during pro¬
longed Pursuit Flights, and the pursuer is the only individual observed
to bill-snap. As the pursuer approaches the pursued, the former opens
and shuts its bill rapidly, producing an audible snapping noise; this
snapping may be repeated over and over during one Pursuit Flight,
but it is typically shown on close approach. The usual response of
the pursued is to elevate the crest strongly, and to turn and peer intently
at the pursuer.

I believe that bill-snapping in Pursuit Flights is strictly a hostile
component, because many of the birds which showed this aerial per¬
formance attacked the pursued when the birds landed, and they fre¬
quently attempted to nip the rear portions of the body of the pursued
during the flight.

Aerial, Vocal Displays

On many occasions I heard individual Reddish Egrets utter the
“raaah,” “crog,” and “arg” calls during flight in what appeared to be
hostile activities, but I did not observe any aerial, vocal displays which
appeared to be ritualized to any extent.

SEXUAL BEHAVIOR

The sexual displays of the Reddish Egret are given with striking
frequency in both aerial and non-aerial situations. This is especially

118 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

true of the Stretch Display. Another striking characteristic is the
frequency of mutual sexual displays.

Stretch Displays. There are three distinct Stretch Displays: (1) a
Stationary Stretch Display; (2) an Aerial Stretch Display; and (3)
a Circle Stretch Display. Many low intensity forms of all three occur.

Stationary Stretch Display. The bird performs this display while
perched in an exposed situation, and it makes the following movements:

1. it extends the head and neck fully upward, then brings the head slightly
back and down so that the occipital region is over the upper part of the
back — this is done in one smooth motion

2. it points the bill upwards to the zenith

3. it erects the feathers of the crown, neck, pectoral, and scapular regions
to their fullest

4. it bulges its eyes from the head

5. just as the bill points to the zenith, the bird performs an intense bout
of head-tossing, the head being moved rapidly up and down in a vertical
plane (Figure 36).

I was unable to determine whether or not any call accompanies the
Stationary Stretch Display, but the displaying birds kept their bills
closed during the performance. A soft “crog” call, however, is a com¬
ponent of the other Stretch Displays.

In low intensity displays the bird holds the head slightly from the
vertical and reduces the number and amplitude of the head-tosses. In
some low intensity forms the head-tossing is eliminated. The erection
of plumes varies greatly, but even in low intensity forms the crown
plumes are always erected. Both sexes show the Stationary Stretch
Display, and it figures prominently in nest relief. Figure 6 in Pemberton
( 1922 ) shows a high-intensity form of the Stationary Stretch Display.
All birds observed performing Stretch Displays had brilliantly colored
soft-parts.

Unlike the Extended Stretch Display of the Green Heron, the Stretch
Displays of the Reddish Egret show sexual tendencies. The extreme
erection of plumage might be considered a hostile element in this
display, but I believe that such movements, particularly the vigorous
head-tossing, function to emphasize the movements of the head and
neck.

Aerial Stretch Display. This display is performed in full flight.
Shortly after launching from its perch, the displaying bird extends its
head and neck forward and upward, bill at about a 45 degree angle,
and then it head-tosses vigorously. As the displaying bird approaches
the object of the display, it raises its head to the vertical and increases
the frequency and amplitude of head-tossing. Each toss is accompanied
by a soft, not unpleasant “crog” call. Both sexes show the Aerial Stretch
Display, and mutual bouts are very common, especially when the pair
have been associating for several days.

A possible hostile component, if indeed it is a hostile element, occurs

REDDISH EGRET

Figure 36. The Stationary Stretch Display of the Reddish Egret.

120 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

in the aerial form of the Stretch. Occasionally a bird will bill-snap
during flight, and it alternates such snapping with head-tossing and
“crog” calling. This might be a low intensity hostile element, but the
display is primarily sexual. Alternatively, bill-snapping in the Aerial
Stretch Display may have been borrowed from hostile snapping (for
example, in Pursuit Flights), and now serves to emphasize a sexual
display.

Circle Stretch Display. This is the most bizarre of the sexual displays.
I am convinced that this form is performed solely by the males, although
no birds were marked. If so, this is the only sexual display of rufescens
which is performed by one sex only.

The usual sequence of events is as follows: a male and a female
are standing quietly some distance apart on an open shoal. Suddenly
the male launches into an Aerial Stretch Display, flying directly toward
the female. As the male approaches, the female responds with a low
intensity Stationary Stretch Display. Immediately upon landing, the
male circles the female at a distance of from five to ten yards, but as
the intensity of his display rises, he draws closer and closer until he
almost touches the female with his extended wings. During his circling
the male alternately raises his head to about a 45 degree angle, head¬
tossing vigorously, then depresses his head until his bill touches the
surface of the water. He holds his wings fully extended, alternately
extending and closing one wing, usually the one closest to the female.
As the male shortens the diameter of his circle, the intensity of his
movements increases. Now his head-up motions carry the head and
bill over backward, then the downward movements cause his head and
bill practically to touch the water. All this time the female has been
motionless except for rather low intensity head-tosses. The Circle
Stretch Display is usually ended when the male rises to a standing
position, very close to the female, and the pair then engage in mutual
Stationary Stretch Displays.

I believe that a male performing a Circle Stretch Display is expressing
an extremely strong sexual tendency, exceeded only by copulation. The
male appears to be ecstatic during such a performance, and the female
never lets her attention leave the circling, bowing, head-tossing male.
During such a spectacular performance the two birds are easily
approached.

Peering Down Display. The causation of this unusual display is
obscure. A pair which have been displaying mutual Circle Flights,
Stationary and Aerial Stretch Displays, and other sexual activities,
will be seen frequently to stand side by side in very shallow water.
The soft-parts of both sexes were always brilliantly colored. The
motionless pair will then bend over, extend their heads down, bills
almost touching the water, and then they will peer into the water,
sometimes for a few moments but typically for a minute or two.

REDDISH EGRET

121

The first time I saw this display, I thought that one of the pair, or
both simultaneously, had seen some prey close to its feet. However,
the highly stereotyped bending, immobility in the bent-over attitude,
and the complete absence of any feeding movements tend to eliminate
feeding motivation.

Occasionally one of the pair will bend and peer first, but the partner
follows suit almost immediately. Because it is associated with sexual
displays before and after, 1 believe that mutual peering down is a
sexual display. Furthermore, I saw no indications of hostility before
or after this display.

Jumping Over Display. Jumping over has already been described
during dancing ground activities, but the present display differs in that
both of the pair show it. It appears to express sexual tendencies when
shown mutually. Suddenly one of a pair which has been standing
quietly in shallow water flies up and over its mate. During the jump-
flight, the displaying bird head-tosses vigorously and erects the feathers
of the head, neck, pectoral, and scapular regions fully. It may repeat
jump-flights as many as ten times in succession. During this display
the partner toward which the performance is directed shows high
intensity head-tossing in an upright position. In contrast, the stationary
bird in dancing ground activities responds by assuming an Upright
Display. I believe that the jump-flight during dancing ground activities
is of low intensity because it does not involve any feather erections or
head-tossing.

After one of the partners has displayed for a few minutes, the other
usually jumps over in return. Mutual jumping over may last for ten
or fifteen minutes, and it may be followed by a mutual Circle Flight
and several mutual Aerial Stretch Displays. I conclude that a strong
sexual tendency is being expressed in this display; it is usually preceded
and/or followed by other sexual displays. In this mutual display I
observed that both birds had brilliantly colored soft-parts.

Circle Flight Display. This display closely resembles the Circle
Flight of the Green Heron. The pair is perched quietly in an exposed
situation; suddenly one of the partners launches into flight, closely
followed by the mate. They fly almost invariably in a circle. One or
both birds may show low intensity head-tossing in flight, and several
times I heard both birds utter very soft “crog-crog” calls. On their
return, the pair may engage in Stationary Stretch Displays and other
mutual sexual activities, or they may launch into another Circle Flight
Display. Circle Flights are most common during the early stages of
pair formation, and I believe that they are expressions of rather low
sexual tendencies. No nipping, bill-snapping, or other hostile behavior
was noted during this display.

Twig-passing Display. Unlike that of the Green Heron, the Twig¬
passing Display of rufescens is highly ritualized. Either member of

122 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

the pair may act as “gatherer” or “receiver.” Allen ( 1954-55 ) describes
this display as follows:

“The reddish egrets, already paired, were getting their nests in order
quite rapidly. One member of the pair (usually the female, as determined
by subsequent observations) occupied a perch convenient to the nest site
while the other (usually the male) wandered about gathering nesting
material. When the male returned, with, or quite frequently without, a
stick or small branch, he began his recognition and greeting ceremony while
still on the wing. This consisted of a series of “back bends,” as I called
them in my notes. Without missing a wingbeat he raised his head so that
the bill pointed upward, neck arched and head bent back, the stiff feathers
of the mane only slightly extended. He did this several times so that the
effect was one of a series of nods or bows. The female, for her part, raised
the mane from crown to breast and pointed her bill straight towards the
zenith. As they joined each other on the top of a cactus plant, both birds
fluffed out their manes, rubbed their bills together excitedly and uttered
low “chicken-like” notes and a flat snapping sound apparently produced by
clacking the upper and lower mandibles together. At the same time they
kept up a mutual display of short, grave, head nods, one to the other.
Generally this was followed by a period of working on the completion
of the nest.”

Where Allen says “recognition and greeting ceremony,” I would
substitute “Aerial Stretch Display,” and where he uses “a series of nods
or bows,” I use “head-tossing.” In Allen’s description the display of
the female on the nest would be a Stationary Stretch Display, using
my terminology.

In general my observations of the Twig-passing Display of the
Reddish Egret agree closely with Allen’s account. Observations of
the nest-building behavior of three pairs of rufescens on Cowpens Key,
however, showed that the sexes played equal roles in gathering, re¬
ceiving, and weaving twigs into the nest. Also, the gatherer began his
or her head-tossing while wading in water near the nest. A return
to the nest in flight was not invariable. At one nest on Cowpens Key,
located only a few feet from the edge of the key and close to the surface
of the water, the returning bird almost always returned to the nest
walking directly, then awkwardly scrambled up the branches of the
nest tree. In every such case the gatherer began its head-tossing
immediately after it had seized the twig, and this display continued
until the twig had been passed to the mate. Even as the returning
bird scrambled and fell among the lower branches of the nest tree,
it continued to display. As Allen suspected, the flat snapping sound
is made by the rapid movement of the mandibles against each other.
Bill-snapping in Pursuit Flights is purely a hostile element, but I
believe that bill-snapping in the Twig-passing Display, as in the Aerial
Stretch Display, is not a hostile component.

I conclude that the Twig-passing Display of rufescens is a variant
of the Stretch Display which has been carried over into the nest-building

REDDISH EGRET

123

stage of the breeding cycle. Virescens and herodias also show Stretch
Displays during the nest-building stage.

Movements of the head and neck play a prominent role in the vast
majority of sexual displays of the Reddish Egret. It is no accident, in
my opinion, that the feathers of the head, neck, and pectoral regions
of the dark birds are sharply set off from the rest of the body plumage
in over-all coloration and plume type. I agree with Lorenz that such
feather coloration and structure have evolved to emphasize the exag¬
gerated movements of these regions. Even though the white birds lack
this color separation, the bristly plumes and their erection during sexual
displays emphasize the upper parts of the body.

EARLY PAIRING BEHAVIOR

Territory. After the dancing ground activities, the males begin to
perch for long periods in particular parts of the potential nesting areas.
At first these males, always brilliantly colored, threaten and/or attack
and pursue other Reddish Egrets which approach their perch. The
first response is hostile. The initial area defended is large, but in a
matter of days the male’s territory has been reduced considerably.
Unlike the Green Heron, the Reddish Egret males show Stationary
Stretch Displays almost from the beginning of territoriality.

Displaying, threatening males are approached by the females, and
the response of the male is at first hostile. Soon, however, the male
begins to perform low intensity sexual displays, and then the first
mutual sexual displays begin to appear.

The striking feature of the courtship at this early stage is its mobility.
Although the male typically returns to his territory after aerial displays,
mutual displays may be performed as much as one-quarter of a mile
from the male’s territory. This mobility may be “permitted” by the
open nature of the habitat and by the small breeding population in the
Keys, both of which reduce contacts between courting pairs. It would
be of great interest to determine the mobility in a crowded population,
such as the one on Green Island, Texas.

The first entrance of the female into the nest was the moment of
pair formation in the Green Heron. I was unable to determine the
exact time of pair formation in the Reddish Egret, but it is my impres¬
sion that when the pair begin mutual Jumping Over and Peering Down
Displays, they are paired. I could not detect hostility between the
sexes after this stage. Twig-passing and mutual Stationary Stretch
Displays at the nest complete the pairing stages.

Coloration of soft-parts. Table 18 lists the breeding and non-breeding
soft-part coloration of rufescens. It is my impression that the coloration
of the males is more intense than that of the females. The males ap¬
peared to acquire their coloration earlier than the females. Allen
( 1954-55 ) has published a detailed description of the soft-part colora-

124 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Table 18

Soft-part coloration of the Reddish Egret.

Area

Coloration

Non-breeding

bill

distal third black; proximal two-thirds pale flesh-color to
pale violet

lores and orbital skin

pale flesh-color to pale violet

legs

blue

feet

blackish

iris

white to pale straw

Breeding

bill

distal third lustrous jet-black; proximal two-thirds brilliant
pink

lores and orbital skin

brilliant violet-blue

legs

brilliant blue (somewhat more greenish-blue in some
white phase birds)

feet

blackish

iris

white to pale straw

tion of the Reddish Egret, and I agree with his statement, “the soft
parts are striking in color and apparently uniform in all plumage
phases.” In the Florida Bay region, however, several white phase birds
in breeding coloration had legs somewhat more greenish-blue than
bright blue.

SNOWY EGRET

Study areas, methods of observation, and other details were the
same as with the other species. Occupation of the breeding areas
was not studied in detail, but in my experience thula resembles albus
in that the birds moved into the nesting areas over a long period
extending weeks after arrival on the breeding grounds.

DISTRIBUTION AND DESCRIPTION

As a breeding bird the Snowy Egret, Leucophoyx thula , ranges from
California, Idaho, and Colorado in western United States east to Long
Island, New York, and Massachusetts, south to Mexico, the Gulf
Coast of the United States, and through Central America to Chile
and Argentina in South America. In recent years this egret has been
steadily expanding its breeding range northeastward in the United
States. At the periphery of its breeding range the Snowy Egret may
nest singly, but large colonies are typical.

The Snowy Egret may be recognized by behavior, size, and pure
white plumage. It is one of the smaller North American herons, about
twenty-five inches in length, with a wingspread of about three feet.
The plumage is immaculately white, the bill is entirely black, as are
the legs, while the feet are a bright yellow, except during the breeding
season. In the nuptial plumage the Snowy Egret has a crest of short,
aigrette-like plumes; long aigrette scapular plumes recurved at the
tips; and it has similar plumes, but not recurved, in the pectoral region
of the ventral tract. These unusual plumes figure prominently in
many hostile and sexual displays.

MAINTENANCE ACTIVITIES

Locomotion. In flight thula closely resembles virescens in the typical
upward and forward extension of the head and neck just before
landing, and in the almost invariable crest-raising just before alighting.
Snowy Egrets frequently erect their crests when taking off, as do Green
Herons.

The flight is rapid and direct, and thula appears to beat its wings
more rapidly than do the other North American herons. The Snowy
Egret resembles rufescens, herodias, and albus in that it walks rather
slowly, except when feeding, in an upright position, unlike virescens
which walks from a low crouch. In its other locomotory activities thula
does not differ from the larger herons previously described.

Feeding behavior. The strenuous activity of the Snowy Egret when
feeding has been observed by a host of authors ( for example, Audubon,
in Bent, 1926; N. B. Moore, in Baird et al., op. cit .; Forbush, 1929).
Even though it is typically very active, I have seen thula use the basic
heron feeding types Stand and Wait and Wade or Walk Slowly on

125

126 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

many occasions. Sprunt and Chamberlain (1949), speaking of the
Snowy Egret, state, “It is no still-hunting bird.” I have observed indi¬
vidual Snowy Egrets perched for long periods on logs and mangrove
roots, and I have watched them feed frequently by wading very
slowly. Their more active hunting techniques, however, are usually
seen.

Wing-flicking is a very common feeding technique, and a Snowy
Egret when feeding in this manner closely resembles rufescens. That is,
the bird moves forward slowly or at a rapid pace, then periodically
extends and retracts its wings in a rapid flicking motion. The wing-
flicking frequently shifts into Open Wing feeding, where the bird rims
forward, or dashes wildly about, both wings widespread, and then stabs
to the right and left. When feeding in this manner, thula closely re¬
sembles rufescens and tricolor.

I have never seen a Snowy Egret use Canopy Feeding, but it will
use a stationary open wing stance where the bird comes to a halt
from a rapid run, fans one wing, and then stabs at any prey disturbed.

One of the most characteristic feeding methods used by thula is
foot-stirring. In Florida, “This heron (that is, the Snowy Egret), more
than any other kind, is what Mr. N. B. Moore designates as a scraper,
or raker, because it uses its legs and claws to start from their hiding-
places such animals as it wishes to seize for food — namely, crawfish,
tadpoles, suckers, aquatic insects, etc. In this movement it far sur¬
passes all other species, and manages its legs with greater adroitness
and rapidity” (Baird et al.). My observations of the foot-stirring of
thula , rufescens , and tricolor support Moore’s statement ( Meyerriecks,
1959).

Numerous observations of the foot-stirring of thula have been re¬
corded (for example, Walsh, 1929; Jones, 1936; Mcllhenny, 1936;
Hickey, 1937; Haverschmidt, 1948a; Rand, 1956), and most of the
descriptions agree in general. The feeding bird extends one leg, then
rapidly vibrates it so as to impart a raking, scraping, shuffling, or
stirring motion to the foot, and then snaps at any prey disturbed.

The bright yellow toes of thula are sharply set off from the black legs,
and this distinctive pattern may have evolved in conjunction with foot¬
stirring. It is of interest to note that the Little Egret, Egretta garzetta
(Linnaeus), of the Old World, also has dark legs and bright yellow
feet, and that this species also engages in foot-stirring (White, 1947;
Hobbs, 1957 ) . However, the feet of thula turn a brilliant coral-orange
and those of garzetta turn a brilliant crimson-pink during the early
part of the breeding season, and the feet of thula are prominently dis¬
played during courtship, so the primary function of the distinctively
colored feet is not entirely clear. Other details of foot-stirring, hovering-
stirring, and foot-paddling in the Snowy Egret are to be found in
Meyerriecks (1959).

Rice (1954, 1956) has described the close association of Snowy

SNOWY EGRET

127

Egrets with cattle as they fed on grasshoppers and other insects dis¬
turbed by the grazing animals. This behavior is similar to the associa¬
tion of Cattle Egrets, Bubulcus ibis Linnaeus, and Little Egrets with
cattle and other grazers. Stoddard (in Howell, 1932) observed a flock
of about forty Snowy Egrets following a drove of pigs on a Florida
prairie and picking up grasshoppers disturbed by the pigs.

Care of the body surface. The Snowy Egret agrees closely with the
Reddish Egret in the frequency of underwing preening. This behavior
may well be correlated with the use of the wings in a variety of activ¬
ities. Through the courtesy of Samuel A. Grimes I am able to reproduce
his beautiful photograph of underwing preening by the Snowy Egret
( Plate 15 ) . The Snowy Egret does not differ from the species already
described in any significant details in other maintenance activities.

Alarm , “freezing” and escape responses. In its alarm and escape
responses the Snowy Egret resembles the Green Heron more than
any other North American heron. Like the Green Heron, thula shows
moderate to extreme crest-raising in a variety of alarm situations. The
Snowy Egret also agrees with virescens in that crest-raising is most
frequently seen from one of the extended alert-postures, that is, with
head extended fully upwards. Where virescens shows rapid crest-
erection and depression from this attitude, however, thula typically
maintains the crest in an erect position, only slowly depressing the
crest as the source of the alarm (a human, for example) departs.

When flushed suddenly, thula will erect its crest to an extreme degree
and utter several loud, rather high-pitched “aarg-aarg” call notes. Such
calls typically alert all other Snowy Egrets within hearing range (a
social warning signal).

Alert postures in thula agree closely with those of virescens and
other herons in that the thinness and extension of the neck are the
keys to the strength of the escape tendency.

With respect to one other escape response, the Snowy Egret tends
to behave like the Green Heron. Whenever a Snowy Egret was sud¬
denly surprised by the appearance of a Marsh Hawk, the egret adopted
a low intensity Bittern Stance. The bird momentarily froze with its
neck extended fully, but unlike virescens , it held the bill up at about
a 45 degree angle. Green Herons hold the Bittern Stance for a long
period, but the egrets adopted this pose only for a few moments and
usually flushed when the hawk approached closely. To be anthropo¬
morphic for a moment, it appeared as though the egrets had momen¬
tarily “forgotten” that they were white and most conspicuous.

HOSTILE BEHAVIOR
Non- aerial. Non- vocal Displays

Crest-raising. The crest is erected in many displays, both hostile
and sexual. The non-aerial displays in which the crest is erected are

128 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

the Upright (erection moderate to full), Aggressive Upright (always
full erection), Forward (extreme erection), and the Snap Display (full
erection). During fights, which are common, the crest is typically
erected to an extreme. The degree to which the crest is erected in any
hostile display is a good indication of the strength of the attack
tendency, as is the case with rufescens. I believe that the crest-raising
shown in the various alert postures of the Snowy Egret is an expression
of conflict ( see discussion of the causation of crest-raising in the Green
Heron, p. 26).

The Green Heron and the Reddish Egret resemble each other in
that the crest is the most mobile feather unit. However, the Snowy
Egret differs in that the crest, pectoral plumes, and the scapular plumes
all appear to be linked in their mobility. That is, whenever one of
these feather units is erected, then the other two are erected also,
particularly during hostile displays, and in sexual displays also. These
facts suggest that erection of these modified plumes in hostile displays
increases the apparent size of the displaying bird, while in sexual
displays the feather erections emphasize certain movements of the
body.

Tail-flipping. From my limited observations, tail-flipping does not
appear to be a characteristic component of hostile displays. I have
seen Snowy Egrets flip their tails in the Upright, Aggressive Upright,
Forward, and Snap Displays, and I saw tail-flipping during some
extreme alert postures, but it did not appear to be a characteristic
element.

Upright Display. A Snowy Egret in this display shows the following
movements :

1. extends the head and neck upward to their fullest extent, with the neck
held vertical

2. holds the bill typically horizontal, but may incline it slightly downward

3. erects the crest partly or fully, and erects the pectoral and scapular
plumes moderately

4. holds its wings tightly closed, and may flip the tail infrequently

5. gives no sound, or may utter several rather harsh, low-pitched “aarg”
or “aarh” calls.

The Upright Display of thula resembles the Upright of rufescens
except for the vertical position of the head and neck and the erection
of the pectoral and scapular plumes. I conclude that the Uprights of
these two species agree closely in their causation; that is, the escape
tendency, although actually low, is relatively stronger than the attack
tendency. A slight increase in the escape tendency leads to orientation
away from the opponent, or unmistakable intention movements of
flight (sleeking of plumage, retraction of head and neck, and leg¬
bending). In contrast, a slight increase in the attack tendency leads
to the next higher expression form, the Aggressive Upright Display.

The Upright is a common sight, especially during the very early

SNOWY EGRET

129

stages of pair formation and territoriality. Occasionally the threatening
birds will move very slowly toward each other, both birds in the
Upright, but typically one or both birds will assume the Aggressive
Upright as they move closer to one another.

Aggressive Upright Display. As a bird assumes the Aggressive, it
shows the following components:

1. extends head and neck upward to their fullest, and inclines them
slightly forward from the vertical

2. inclines the bill downward at about a 45 degree angle

3. erects the crest fully, and erects the pectoral and scapular plumes either
moderately or fully

4. bulges its eyes from the head

5. holds its wings either tightly closed or partly extended

6. typically utters several harsh, rasping “aarg” or “aarh” calls

7. remains stationary or moves very stiffly toward the opponent.

The Aggressive Uprights of thula and rufescens are similar except
that forward inclination of the neck is more extreme in thula.

Forward Display. This is the most intense of the hostile displays,
and when assuming it the bird shows the following components:

1. retracts its head and neck slightly, so that the occipital region is directly
over the upper part of the back, neck in an “S”-shape

2. inclines its body forward as it bends slightly at the “heel” joints

3. erects its crest, pectoral and scapular plumes to an extreme, the tips of
the scapulars almost touching the fanned plumes on the back of the head

4. inclines its bill downward at about 45 degrees

5. bulges its eyes from the head

6. partly extends its wings and frequently “fans” them slightly

7. gives harsh calls, typically rasping “aargs,” or an even harsher “raah” call

8. faces the opponent, either stationary or while moving stiffly toward
the opponent

9. may show color changes in the lores and orbital skin in a matter of
seconds (Figure 37).

A Snowy Egret in the Forward Display is “all feathers.” The apparent
body size is very much increased by the extreme erection of all the
plumage, especially the highly modified plumes. The retraction of
the head may express an increasing escape tendency, but I believe
that this makes the bird ready for a strike at the opponent.

The change in color of the lores and orbital skin is unusual. The
Snowy Egret is the only heron I have seen undergo such rapid changes
in soft-part coloration. The lores and associated areas may change
in a few seconds from yellow to orange to cherry-red. I believe that
these rapid changes are correlated with the increasing attack tendency
because the cherry-red rapidly shifts to orange or yellow once the
bird has ceased the hostile display. In addition, I have seen such rapid
color changes only during hostile clashes, never during sexual displays.
Lowe observed rapid color changes in the bill of A. cinerea on its
dancing grounds, and this same author cites George Edward’s observa-

130 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 37. Snowy Egret performing the Forward Display.

SNOWY EGRET

131

tion of rapid color changes in a Little Bittern, lxobrychus minutus
(Linnaeus), during a nest relief ceremony.

Fighting. I have observed more fights in thula than in any other
North American heron, even though I have spent far less time watching
thula than virescens. Typically, two fighting Snowy Egrets fly head¬
long toward one another from the Forward Display, collide in the
air, and then tumble to the ground. Characteristically, they attempt
to strike their opponents from above, aiming their vicious jabs at the
head. They flap wildly trying to buffet each other. Occasionally one
of the fighters jumps into the air and tries to land on the others back,
and the jabbing, whirling, flapping opponents may range over a hun¬
dred yards or more in their struggles. During a fight, as Storers
(1948:89) photographs show, the birds may actually fly backwards.
Fights rarely lead to physical damage except for the loss of one or
more plumes.

Both the Reddish Egret and the Snowy Egret fight from an upright
position, strike from above, fight furiously and over a wide area, and
enter the engagement with a headlong dash.

Snap Display. The Snowy Egret, like the Green Heron, shows
“cinerea” type Snap Displays (no bobbing components) as well as
variants in which the bird bobs rapidly up and down. The Snap
Display of thula which lacks the bobbing motions has the following
components :

1. the bird extends its head and neck fully forward and downward so that
the head and bill are on a level with or slightly below the feet

2. raises the crest fully, and the feathers of the neck partly or fully

3. makes a loud snapping sound by clicking the mandibles together as it
extends its neck

4. bulges the eyes from its head

5. may flip its tail, but not characteristically

6. holds the Snap Display position for a few moments, then rises to a normal
perched position.

The bobbing variant is as just described except that the displaying
bird, with its neck fully extended, bobs rapidly up and down and gives
the neck a peculiar bowed appearance during the bobbing movements.
The Snap Displays of thula most closely resemble those of virescens ,
but the Snap of thula appears to be much more of a hostile display
because it appears only early in the breeding season, because the dis¬
playing bird alternates Snaps wth much more intense hostile behavior
(Aggressive Uprights and Forwards), and because conspecific indi¬
viduals are attacked if they approach the displaying bird. Furthermore,
Green Herons tend to confine bouts of Snapping to their own territories,
but Snowy Egrets may Snap on or off their territories.

In short, the form of the Snap Displays resembles those of virescens ,
but causation and orientation resemble those of rufescens.

132 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Non-aerial, Vocal Displays

The characteristic advertising call is a nasal, fairly high-pitched
“aarh,” uttered repeatedly from a prominent spot. One variant I heard
was a rather high-pitched “arg,” very similar to the alarm call. Another
advertising call, heard most frequently before the performance of sexual
displays, sounded like “arg-obble.” The “obble” sounded like a low
intensity form of a call that characteristically accompanies the Stretch
Display.

Although I collected and sexed no Snowy Egrets, it is my impression
that both sexes utter the “aarh” and “arg” advertising calls. I believe,
however, that the “arg-obble” variant is restricted to the males.

The Snowy Egret is highly social throughout the year, and extreme
sociality carries over into the early pairing period. Groups of from
five to ten egrets engage in advertising bouts, all of them being perched
in one small area, whereas Green Herons engage in “skowing” bouts
with one (rarely more) neighboring territorial male. Unlike the Green
Heron, the Snowy Egret appears to have an ill-defined territory, at
least during the first few days. Typically, one male begins to advertise
from a conspicuous song post, and then he is joined by several males
and females. A bout of calling back and forth ensues, and the little
party of advertisers moves about, not settling on one particular spot
for very long. Again unlike virescens, advertising in thula is highly
social.

Aerial, Non- vocal Displays

Flying Around. Flying Around is strikingly similar to that of
virescens , except again that thula displays are highly social. Single
birds show this display, but it is much more common to see small
groups of from five to ten egrets Flying Around. Most of the birds
engaging in this display show signs of changes in soft-part color, the
feet especially being a brilliant coral-orange. A bout of Flying Around
may take an individual or the group over hundreds of yards, and un¬
like the Green Heron, which usually returns to the starting point, these
little flocks may settle far from their starting point. Later in the
breeding cycle, Flying Around bouts become restricted to smaller and
smaller areas, and eventually the flights end at the take-off spot.

The function of Flying Around in thula appears to be the same as
in virescens and rufescens.

Pursuit Flight. Social Pursuit Flights are much more common than
flights which involve only two birds. During such a social flight the
pursuer may chase first one bird then another, only to be pursued in
turn by some other individual. Pursuit Flights involving two birds,
however, are not unusual. Most Pursuit Flights of two birds result
from the too close approach by one bird to an advertising individual;
the latter first threatens the intruder and then pursues it. In such cases

SNOWY EGRET

133

the pursuer typically returns to a place close to its former song post.

During the social Pursuit Flights crest-raising, “arg”-calling, and
nipping in flight are very common. The social nature of such flights
makes their causation difficult to analyze, but the occurrence of crest¬
raising, calling, and nipping indicate that the flights are primarily
hostile.

Supplanting Attacks. This, of all the hostile displays, most closely
resembles the same display in virescens. The upward and forward ex¬
tension of the neck of the supplanting bird, and the occurrence of
crest-raising in both individuals agree closely in the two species. In
addition, I saw Snowy Egrets make incipient copulatory attempts
during Supplanting Attacks, just as male Green Herons do on occasion.
In both species Supplanting Attacks are most frequent early in the
breeding season, their intensity and frequency falling off rather sharply
as the breeding season advances.

Aerial, Vocal Displays

Although I heard individual Snowy Egrets call “arg” or “aarh” during
Pursuit Flights or Supplanting Attacks, I saw no hostile, aerial, vocal
displays which appeared to be ritualized.

SEXUAL BEHAVIOR

With respect to three displays, the Stationary Stretch Display, the
Aerial Stretch Display, and the Jumping Over Display, the Snowy
Egret resembles the Reddish Egret more than any other North Amer¬
ican heron with which I am familiar.

Stretch Displays. The Snowy Egret shows two distinct Stretch Dis¬
plays, a stationary one and an aerial version, and there are several low
intensity forms of both types.

Stationary Stretch Display. From a very conspicuous perch the dis¬
playing bird performs the following movements:

1. raises the head in one smooth, rather rapid motion, then brings the
head back and downward so that the occipital region is directly over the
lower part of the back

2. points the bill straight upwards

3. erects the feathers of the crown moderately, erects the pectoral plumes
fully and fans them laterally, and erects the scapular plumes to an
extreme so that their recurved tips touch the erected plumes of the crest

4. bulges its eyes from the head

5. bends the legs at the “heel” joints, then begins to pump its head
vigorously up and down, the occipital region touching the lower part
of the back on the downstroke

6. as the bird pumps its head, it utters a series of gurgling “a-wah-wah-
wah,” “a-wah-wah-wah” calls (Figure 38).

In low intensity forms of the Stationary Stretch Display the bird
may utter only one or two “a-wah-wah-wah” calls or none at all, and

134 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Figure 38. Male Snowy Egret performing a Stationary Stretch Display.

SNOWY EGRET

135

it may reduce the number of pumps of the head to one or two. In full
intensity displays the bird may pump ten or more times and utter a
long series of calls.

I have seen the Stationary Stretch Display performed only by what
I took to be males. The extreme erection of the scapular plumes cer¬
tainly emphasizes the pumping movements of the bird’s head, and
the brilliant cherry-red lores and orbital skin draw the observer’s eyes
toward the rapidly moving head. In addition, the laterally-fanned
pectoral plumes and the brilliant coral-orange feet, sharply marked
off from the lustrous black legs, emphasize the exaggerated movements.

The “a-wah-wah-wah” call which accompanies this display is a rather
low-pitched, not unpleasant gurgling call which may also be uttered
by the bird just before performing the Stationary Stretch Display. In
fact, the bird may give this call, or a less intense variant, rather fre¬
quently days before it first performs the Stationary Stretch Display.

One of the most interesting aspects of this display is its amazing
attraction function. As soon as a male assumes the Stretch Display
position, Snowy Egrets within sight and hearing fly to perches near
the performing bird, and all of these "spectators” watch the performing
bird intently. I have seen one displaying male ringed by as many as
ten other "fascinated” Snowy Egrets. These gatherings also indicate
the degree of sociality found in this species.

Aerial Stretch Display. From a conspicuous perch, the displaying
bird launches into flight as follows:

1. the bird stands erect, neck partly extended forward at about a 45 degree
angle; crest and scapular plumes are slightly erected

2. it bends forward from this position so that the body is almost horizontal,
neck extended forward fully, crest moderately erect, scapular plumes
fully erect, and the wings slightly drooped

3. droops the wings strongly, and then raises its head upward and begins
to utter the “a-wah-wah-wah” call

4. from this position the bird launches into flight, sways the head and neck
from side to side several times, then brings the head and neck down and
to the rear into the Stationary Stretch position except that the bill is
pointing forward at about a 45 degree angle

5. the bird then flies slowly in a large circle, continuously calling the
“a-wah-wah-wah” call with its head and neck back, all of its plumes,
but especially the scapulars, streaming in the wind

6. about three-fourths of the way around the circle, the bird brings its
head and neck forward but still partly extended, lowers its plumes,
and stops calling

7. just before landing, the bird makes a sudden short flight upwards,
dangles its legs, elevates its crest and scapulars, then “pounces” down
to its perch (Figure 39).

Everything about this spectacular performance is highly exaggerated,
and all of the plumes, soft-part coloration, and extremely bizarre move¬
ments, together with the continuous calling, render this a unique
display. I believe that only males show this aerial performance, but

136

COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

unlike the stationary version, this display is performed before a single
female. She follows every movement of the performing male intently
all through his flight until he lands near her (following the short up-
down flight).

I have seen the Aerial Stretch Display only after a male and female
had been engaged in a variety of activities together for several days,
and it is my impression that the aerial form is an expression of much
stronger sexual tendencies than the stationary type. During the aerial
performance the displaying male appears to be ecstatic, and the female
is so intent on watching the male’s activities that she may be approached
rather closely.

Circle Flight Display. The Snowy Egret may engage in individual
or mutual Circle Flight Displays, and the performances are very
similar to those of the Green Heron.

When performing singly, the displaying bird launches into a typical

SNOWY EGRET

137

flight, flies in a circle around the mate, and then lands near the starting
point. Occasionally the bird may utter one or two “a-wah-wah-wah”
calls during flight, but this is not typical. Several times I have seen
a bird performing this display raise its head slightly in what appeared
to be low intensity form of the Aerial Stretch Display, but these move¬
ments are very slight.

In mutual Circle Flights one of the partners launches into flight and
is typically followed at once by its mate. The two birds fly in a circle
around the take-off spot and then land near this point. Sometimes the
last bird to fly overtakes its partner in flight, but I have never seen
any signs of hostility on such occasions. Individual Circle Flights are
much more common than mutual ones early in the breeding cycle,
but once a male and a female have been associating for several days,
mutual Circle Flights are the rule. Such flights are sharply reduced in
frequency following the first performance of an Aerial Stretch Display.

Tumbling Flight Display. One very outstanding display, which
appears to be a variant of the Circle Flight Display, involves some
spectacular aerial maneuvers. The performing bird launches into a
typical Circle Flight, but instead of “once around, then land,” the
bird continues to circle its mate, rising higher with each turn. When
it has reached a point about fifty to a hundred yards ( very variable )
above the ground, the bird suddenly tumbles over and over as it hurtles
downward, all of its plumage streaming in the wind. The displaying
bird may actually turn completely over three or four times during
its fall. After tumbling, the bird rights itself, then lands in a standard
manner near its partner. I have seen such tumbling flights outside the
breeding season when Snowy Egrets were approaching a roosting site.
This species characteristically approaches a roost flying high over it
and then, twisting and whirling, plunges downward to the tops of the
trees, “braking” to a sudden stop.

Jumping Over Display. This display resembles closely the Jumping
Over Display of the Reddish Egret, except that the Snowy Egret
does not show any head-tossing. Typically, a pair of Snowy Egrets
engage in a mutual Circle Flight Display, but instead of returning to
the take-off spot they land in the shallow water ( Florida Bay ) or on
the marshes (Rulers Bar Hassock) near the nesting sites. Both stand
quietly for a few moments, then suddenly one makes a short jump-
flight over the other, lands, then repeats the jump-flight. During the
jump-flight the displaying bird erects its crest, pectoral, and scapular
plumes fully, and these feathers stream and toss about as the bird
moves up and down over its partner. Sometimes a few jumps by one
of the pair ends the display, but I have seen the partner display in
return on several occasions. The behavior of the pair after a bout
of jumping is very variable; sometimes they start to feed, or they may
return to the breeding sites, or they may stand quietly for several

138 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

minutes before flying away. The fact that this display is shown by
a pair which has been engaging in mutual activities for several days,
plus the fact that no hostile behavior was noted, leads me to conclude
that it is a sexual display.

Sherwood ( 1957 ) makes the only previous reference to what appears
to be this display: “One day in January [in southern Florida] three
pairs of Snowy Egrets put on a mating display in the now shallow
water, jumping over one another with plumes flying. . however, no
further details are given. It would be most interesting to discover
whether or not Snowy Egrets engage in social Jumping Over Displays,
as Sherwood’s observation suggests.

EARLY PAIRING BEHAVIOR

Territory. The establishment and maintenance of territory by the
Snowy Egret resembles such behavior in the Reddish Egret, except
that territorial advertising is much more conspicuous. With respect
to the gradual occupation of the breeding areas, thula resembles albus
closely.

The first indications of breeding behavior are the return of certain
individuals from the feeding areas early in the day to the potential
breeding sites. These birds perch quietly in conspicuous spots. Most
have bright orange feet and yellow lores. An occasional bout of Flying
Around alternates with prolonged preening, but these birds are very
alert to the appearance of other Snowy Egrets. Should one land in
the breeding areas, the perched birds typically Supplant the new¬
comers, and Pursuit Flights are common. Rarely, one of these birds
engages in a low-intensity Snap Display. Half-hearted twig-grasping
also occurs, but the twigs are merely grasped and then released at
once.

As soon as several individuals spend more of the day in the breeding
areas, the first advertising calls are heard. Bouts of “aarh” or “arg”-
calling become common, and the first hostile clashes occur. At times
one of the calling birds shows a low intensity Stationary Stretch Display,
thereby attracting a small party to it. The usual response of the dis¬
playing bird is to threaten the “audience,” whereupon the latter
retreats until the attacker again displays a Stationary Stretch. This
gathering and retreating is especially conspicuous during the early
phases of the breeding cycle.

Soon after the appearance of the first Stationary Stretch, individual
Circle Flights become common, and occasionally one of the flying
birds will show a Tumbling Flight Display. As more and more egrets
move into the breeding areas, threats and fights increase in frequency,
and the original territories of the first arrivals become restricted. On
Rulers Bar Hassock an old nest was not necessary for the establishment
of territory, in sharp contrast to the Green Heron.

SNOWY EGRET

139

Once the male has selected a spot, his Stationary Stretch Displays
rise in frequency and intensity. Now, one or two birds of the small
groups that gather retreat very slowly when the male threatens, and
I took these individuals to be females. The hostile displays of the
male decrease to the point where these females are no longer threatened,
and then the male performs his first Aerial Stretch Display. At this
point the females begin to confine their attentions to one displaying
male rather than flying from one gathering to the next.

Soon the pair begin to perform mutual Circle Flights, and the
male shows more and more Aerial Stretch Displays. Advertising calls
fall off abruptly and aerial displays, such as the Tumbling Flight
Display, rise sharply in frequency and in intensity. I believe that pair
formation is effected when the pair show mutual Jumping Over Dis¬
plays. After such performances the pair may return to the nesting site
and engage in nest-building activities. I made no observations of the
pair after this stage.

Coloration of soft-parts. Table 19 lists the breeding and non-breeding
coloration of the soft-parts of the Snowy Egret. No sexual differences
are listed, but I believe that the males show more intense coloration
and acquire their color changes earlier than the females; this is es¬
pecially true for the changes in the color of the feet. Huxley ( 1922 ) ,
as far as I have been able to determine, was the first to note the change
in lore color from yellow to red during the breeding season. Cruick-
shank (1948) has recorded the cherry-red coloration of the lores of
thula early in the breeding season, and several authors have commented
on the orange appearance of the feet of breeding birds, but no detailed
descriptions of such changes have been published previously.

I have already described the rapid change in color of the lores and

Table 19

Soft-part coloration of the Snowy Egret.

Area

Coloration

Non-breeding

bill

black, basal third somewhat yellowish

lores and orbital skin

bright yellow

legs

black

feet

bright yellow

iris

yellow

Breeding

bill

shiny black throughout

lores and orbital skin

cherry-red

legs

shiny black

feet

brilliant coral-orange

iris

yellow

140 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

orbital skin of this species in the Forward Display. Such rapid
changes seem, however, to be restricted to the early phases of the
breeding cycle, because birds in Stationary and Aerial Stretch Displays
normally have brilliant coloration.

When Snowy Egrets first occupy the breeding areas, the lores are
of the same bright yellow as they are outside the breeding season,
but in a matter of days, the lores have become a bright orange.
Later, the lores become a bright cherry-red, and this red color lasts
until the clutch of eggs is completed, when it fades. The fading may
progress through dull red to orange, then to bright yellow, or from
bright red to yellow directly.

The feet also change, from bright yellow to brilliant coral-orange;
then there is a gradual fading back to yellow once the eggs have been
laid. This shift in color of the feet is an excellent indicator of the
onset of breeding behavior. Many of the birds that begin advertising
early in the breeding season have bright orange feet but yellow lores.

Part III

SOME EVOLUTIONARY TRENDS IN HERON BEHAVIOR

INTRODUCTION

It is difficult to discuss with any degree of finality the origin and
evolution of heron behavior patterns because only a few of the more
than sixty species of the family Ardeidae have been studied in detail.
For example, the tiger bitterns of Mexico and Central and South
America are considered to be the most primitive members of the family.
These species would appear to be rather unspeciailzed in their displays
and general behavior. The lack of well developed plumes in tiger
bitterns tends to support the view that the displays associated with
such plumes would be simple in form. In brief, the tiger bitterns
would provide significant information as to the probable origin of
many of the displays of the more advanced forms. However, the
breeding behavior, and indeed the general biology, of tiger bitterns
is poorly known. Hence, the student interested in evolutionary origins
and trends must use behavioral data gathered from studies on the
more advanced species.

I shall discuss in this section some evolutionary modifications based
primarily on the behavior of the North American species treated in
Parts I and II. Wherever appropriate I shall cite examples of key
species whose study would greatly assist in the clarification of many of
the points under discussion.

SOCIALITY

As stated in the Introduction, the family Ardeidae is particularly
suitable for a comparative ethological study, and numerous examples
were given in support of this contention. One was the extreme degree
of diversity shown with respect to social behavior. With A. herodias
and A. occidentalis considered to be conspecific, twelve species of
herons breed in North America north of Mexico. They exhibit in
striking fashion many degrees of sociality. I recognize five social cate¬
gories with respect to the North American herons (Table 20).

I do not intend to imply that this is the only possible classification
of social behavior among herons, or that the species listed are rigidly
delimited by one category. For example, solitary Snowy and Cattle
Egrets, and Little Blue Herons are commonly observed, particularly at
the periphery of their ranges, but these species are much more typically
seen in small to large flocks. Furthermore, nesting pairs of such highly
social species as the Louisiana Heron ( Meyerriecks, 1957a), are corn-

141

142 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Table 20

Degree of sociality of North American herons. Ardea cinerea is included

FOR COMPARATIVE PURPOSES.

Degree

Species

Solitary throughout the year, including solitary
breeding

Solitary outside breeding season, solitary to
semisocial breeding

Solitary or semisocial outside breeding season,
highly social breeding

Mainly social throughout year
Highly social throughout year

Botaurus lentiginosus

Butorides virescens,
Ixobrychus exilis
Ardea herodias,

A. cinerea,

Casmerodius albus,
Dichromanassa rufescens
Florida caerulea,
Hydranassa tricolor
Leucophoyx thula,
Bubulcus ibis,

Nycticorax nycticorax,
Nyctanassa violacea

monly seen singly or in small numbers at the periphery of the breeding
range. The true social status of these pioneers, however, is revealed by
an examination of the behavior of such a species toward the center
of its range.

The probable course of social evolution in herons has been from the
primitive solitary status exemplified by Botaurus , through a transitory
semisocial phase ( Butorides and Ixobrychus ) , to extreme, year around
sociality, represented by Leucophoyx , Bubulcus, and Nycticorax. The
Night Herons ( Nycticorax and Nyctanassa) must have evolved their
present highly social structure independently of such species as thula
and rufescens, because they differ in numerous other respects, both
behavioral and structural.

The change from solitary to semisocial is shown by the American and
Least Bitterns, and the Green Heron. The American Bittern is typically
solitary throughout the year. On migration or in suitable nesting areas,
however, this species may assemble in small groups. Green Herons and
Least Bitterns are typically solitary outside the breeding season, and
in many areas virescens and exilis are known only as solitary nesters.
On migration, however, small to large flocks are not uncommon, and
in suitable areas many pairs (hundreds in virescens) may nest in close
association.

Those species which are typically solitary or semisocial outside the
breeding season provide examples of the next stage in the evolution
of a highly social structure. A. herodias and C. albus are commonly
seen feeding singly or in small groups, but on migration, when flying
to and from roosting areas, and during the breeding season, both
species are highly social. It is but one further step to the full develop¬
ment of a species that is highly social in all of its activities (feeding,

SOME EVOLUTIONARY TRENDS IN HERON BEHAVIOR

143

migration, roosting, nesting, social displays). For the North American
herons this high degree of sociality is best shown by thula, ibis , and
nycticorax.

POLYMORPHISM

Polymorphism, as color phases, is widespread in the family Ardeidae.
Of the twelve breeding herons of North America, four have distinct
color phases ( herodias , rufescens, caerulea, exilis ) . The polymorphism
of caerulea is developmental, immatures being white while the full
adults are dark blue; during the shift from white to dark the changing
birds are strikingly pied. Exilis has a very rare, melanistic phase, for¬
merly known as “Cory’s Least Bittern” ( see Carpenter, 1948, for details ) .
The polymorphism of rufescens is complex, this species having a mini¬
mum of three phases: dark, intermediate, and white (see the treatment
of rufescens in Part II). Quantitative analysis of the large numbers
of rufescens breeding on Green Island, Texas, offers an opportunity to
study the inheritance of color phases. Such inheritance is not known
for any heron species, although Mayr and Amadon ( 1941 ) and Mayr
(1956) have suggested several genetic models.

The available evidence indicates that color phases do not play a role
in choice of mates ( Meyerriecks, 1957b, for herodias); the functions
of such color phases remain obscure. Some species ( albus and thula ,
for example ) have only the white phase. Palmer ( 1909 ) claimed that
the dark-bodied herons obtained their aquatic prey “almost entirely
where the fringe of vegetation, sedges, bushes, or more distant tree
tops, make a background and prevent the shape of the birds from
affecting the skyline as seen by the prey.” In contrast, according to
Palmer, the white-bodied species tend to feed in the open, away from
the fringing vegetation. I cannot agree with the generalization “dark
heron-shore line feeder, white heron-open water feeder,” because I
have seen on countless occasions the reverse situations, that is, dark-
colored herons feeding on fully exposed reefs and shoals, and pure
white herons feeding for long periods among mangrove roots and
similar vegetation. Palmer believed that the white plumage of the
open water feeders harmonized with the sky (as seen by the prey).
If this were significant, one would expect that in the smaller species, a
two-color pattern would have evolved, white below (to “fool the
prey”), and dark above (to protect the heron from aerial predators).
In North America tricolor is an excellent example of such a species:
white below, blue above, and a diurnal feeder. However, tricolor may
feed among the tangled vegetation lining the shore or out on the most
exposed flats. Many of the other small, diurnal feeders are all white
( thula and garzetta ), while other small species are completely dark
( adult caerulea ) . If white plumage did evolve as a cryptic adaptation,
then caerulea , white as an immature, dark as an adult, would have to

144 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

reorganize its entire feeding behavior when its plumage changed. Of
course, this is not the case.

Whiteness or near whiteness is not a unique attribute of egrets,
because this characteristic is shared by a number of birds (gulls, for
example). The coloration of gulls — cryptic or conspicuous? — has been
the subject of a recent controversy. Craik (1944) believes that the
white color of many gulls is cryptic, while Darwin (1890) and Arm¬
strong (1946), among others, believe that such coloration is conspic¬
uous. For a more detailed discussion of this subject, see Tinbergen
(1953). I agree with Tinbergen that only a thorough experimental
analysis will resolve the situation.

Finn (1919) linked whiteness in herons with a tropical distribution
and conspicuousness, but he gives no details. Several all-white species,
such as albus and thula, range far north of the tropics, but it is true
that the white herons tend to be much more common in the warmer
regions of the world. Wells, Huxley, and Wells ( 1929 ) link whiteness
with colonial nesting and relative invulnerability to predation, the
advantage being that the white birds are much more conspicuous to
conspecific individuals. Koenig (1952) is very definite about the
significance of whiteness in albus. He states, “Bedeutung des weissen
Gefieders zeigt sich deutlich, wenn andere Reiher mittags den Schatten
aufsuchen, wahrend die Silberreiher [albus] unbekiimmert stehen-
bleiben.” However, I have seen white herodias, albus, and thula, to¬
gether with immature caerulea, retire to the shade at high noon on
bright, hot days, while the dark-bodied species, such as adult caerulea
and virescens, remained in the blazing sun for hours. I made observa¬
tions such as these so many times that I feel that the primary function
of whiteness in herons is not correlated with temperature.

The matter of conspicuousness may also be considered in relation
to warning colors and edibility, but Cott ( 1947 ) has clearly shown that
the Cattle Egret is a striking expection to his experimentally based
generalization that conspicuousness implies inedibility, while a cryptic
coloration indicates edibility. Cott rated his experimental birds on
visibility, edibility, and vulnerability to attacks by predators. Ibis
rated the highest ( 10 ) in visibility, 19 in edibility ( 20, most edible ) ,
and 12 in vulnerability (10, relatively immune, 21, highly vulnerable).
Hence, whiteness in herons is not correlated with inedibility. Because
most herons are relatively immune to predation, at least as adults, it
does not matter whether or not their flesh is edible or otherwise.

In a recent description of a new race of Butorides striatus from the
Maidive Archipelago, Phillips and Sims ( 1958 ) express the belief that
the increase in white in the Maidive striatus is probably correlated with
the exposed coral reef habitat of these birds, and these authors assign
a cryptic function to such partial albinism. The Striated Herons of
western Australia, however, are reddish in areas of reddish soil; hence,

SOME EVOLUTIONARY TRENDS IN HERON BEHAVIOR

145

the partial albinism of the Maidive birds (which is common in many
groups) is not related to the general problem of whiteness in other
herons and egrets.

Most of the all-white species of herons, or the white individuals of
those species that show color phases, are primarily social throughout
the year. Even those white species that tend to be solitary outside the
breeding season are frequently observed flying in groups between
feeding areas or to a roost for the night. Furthermore, most of the
white species that tend to be solitary or semisocial outside the breeding
season, become highly social at the start of each breeding season.

Being conspicuously white might be considered a disadvantage,
especially for a small heron species, but we find that the smaller
white species tend to be highly social ( for example, thula and garzetta ) .
By their size the larger all-white species would be relatively immune
from attack. A rule of thumb might be formulated as follows: if you
are a large heron species, it does not matter if you "go white,” but if
you are a small heron species that "goes white,” you had better be
highly social at the same time. The whiteness of the small, highly
social species such as thula might be an adaptation for inter-individual
contact. But why should a large, rather solitary species “go white?”
Again, I believe the answer is conspicuousness. As previously stated,
albus tends to feed alone, but moves in small groups, and during the
breeding season it tends to be a highly social bird. In contrast, a
flock of white herodias is unusual. These feed by themselves, even
though dozens of birds may congregate on favorable feeding grounds.
Roosting flights are usually performed alone. When the males are
dancing and performing the Stretch and Snap Displays, however, they
are very conspicuous, and their white coloration may function as a
social signal. Unfortunately, this does not explain why the mainland
herodias are blue.

In brief, I believe that the primary function of whiteness, at least
in the highly social species, is related to conspicuousness in a variety
of social situations.

SEXUAL DIMORPHISM AND PAIR FORMATION

The bitterns of the genus Ixohrychus are unique among the Ardeidae
in that the sexes are readily separated by plumage. Because no detailed
studies of the behavior of any member of this widespread genus have
been made, the significance of this plumage dimorphism remains
obscure. These small bitterns are typically solitary or semisocial, rela¬
tively silent, and they tend to inhabit Phragmites , Typha, and similar
vegetation. The sexual differences in coloration may act as immediate
visual sex recognition markers in these dense habitats.

Other heron species show noticeable, but not striking, sexual dimor¬
phism during pair formation. Recall that in every species studied

146 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

in detail, the males had much more brilliantly colored soft-parts and
more lustrous plumage than did the females ( see, for example, Table 9 ) .

Not only do the sexes differ in the brilliance of their soft-part colora¬
tion and general plumage luster, but they also differ strikingly in be¬
havior. Table 9 lists a number of behavioral differences between the
sexes of Butorides virescens. There are striking sexual differences as
to the time of appearance, orientation, frequency, and intensity of the
performance of displays, and I believe that the greater brilliance of
the males is correlated with such differences in behavior.

Two trends are evident in the pair formation of the herons studied
so far: (1) a stationary male displays while a mobile female roams
from one male to another; and (2) one male, with a rather mobile
territory (at least during the early stages of pair formation) displays
while a number of conspecific individuals gather to watch. With the
type one species ( cinerea , virescens , for example), one male and one
female are typically involved, but with the type two species (for
example, thula and nycticorax ) , communal displays are characteristic.
However, the composition of the "spectators” with respect to sex in
type two is not as yet clear.

In either type it is the "fixed” male which is on display; hence, the
greater brilliance of his coloration and increased frequency and in¬
tensity of his displays tend to attract a female to his nest or nest site.

COLORATION OF SOFT-PARTS

According to Meiklejohn (1952), the Emperor Frederick II (1194r-
1250), in his book "De Arte Venandi cum Avibus,” was the first to
note the striking changes in color of the soft-parts of Ardea cinerea
during the breeding season. Since Frederick’s time a host of observers
have seen and recorded these unusual changes in many heron species.

As I have said, I believe that the primary function of such color
changes is to emphasize certain movements of the displaying bird.
The color changes occur only at the onset of each breeding season, and
colors fade rapidly during the period of egg-laying. A recrudescence of
such brilliant coloration occurs prior to the laying of a second clutch
(Lorenz, in Allen and Mangels, 1940; Noble and Wurm, 1940), or
before renewal of pairing after the destruction of the nest ( Binsbergen,
1941). Brilliant soft-part colors may function chiefly to emphasize
sexual displays, their role in hostile situations possibly being secondary.
Hostile displays are common outside the breeding season, especially
among the social species (Snowy Egret and Black-crowned Night
Heron, for example), but the soft-parts at this time are dull. Further¬
more, many individuals begin to set up territories when their soft-parts
are still dull or changing, but when they begin to display sexually, the
soft-parts are always brilliant.

Because the majority of displays, especially the Stretch Display, are

SOME EVOLUTIONARY TRENDS IN HERON BEHAVIOR

147

shown more frequently and more intensively by the male, it is reason¬
able to assume that the earlier acquisition and greater brilliance of
the male’s coloration is correlated with his greater activity, especially
during sexual displays. Noble and Wurm (1942) and Lowe feel
that the function of such brilliant colors is to enhance the displays,
but these authors do not assign a primary role to brilliant soft-part
colors in sexual displays and a secondary role in hostile displays as I do.
However, my suggestions are speculative, and final conclusions must
await experimentation.

As to the physiological basis for such changes in the coloration of
the soft-parts, Noble and Wurm demonstrated that an androgen is
responsible. When both sexes of the Back-crowned Night Heron were
treated with testosterone propionate, the experimental birds showed
a sharp increase in black pigment deposition in the lores and buccal
cavity, darkening of the bill, and extreme vascularization of the legs.
Full breeding coloration was induced in gonadectomized birds after
treatment with testosterone propionate.

Noble and Wurm expressed the belief that the darkening of the lores
was due to an “enormous increase in pigment below the epidermis,”
and that autonomic effects on the capillaries of the loral region played
little or no role in such color changes. However, the melanins re¬
sponsible for the black coloration of the lores of nycticorax are not the
only pigments involved in such color changes in other heron species.
For example, the yellow lores of thula and the red bill of cinerea and
herodias are probably due to a combination of carotenoid pigments and
some degree of vascularization. The effects of androgens on the deposi¬
tion of carotenoids in these herons is not known.

With respect to the rich, rosy-red color of the legs and feet of the
breeding Black-crowns, Noble and Wurm concluded that extensive
vascularization, not pigment deposition, was responsible. When the
integuments of the legs of experimental and control birds were com¬
pared, these authors observed that there was an increase in the size
and number of the superficial blood vessels, together with an increased
activity of the epidermal cells of the treated birds, but the number of
pigment cells was the same in both controls and experimentals. Experi¬
mental proof is lacking, but I suspect that the slow changes in the
coloration of the feet of such a species as thula may be a reflection of
some pigment deposition and autonomic effects.

My observations on the sudden changes in coloration of the lores of
Snowy Egrets during hostile clashes seem to indicate a combination of
pigment activity and direct effects (autonomic) on the blood vessels
of the loral region. During an intense hostile clash, the lores of the
threatening egret changed rapidly from yellow to orange, and then
finally to a brilliant scarlet; however, once the clash was ended, the
lores faded rapidly to their original yellow coloration. Such observa-

148 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

tions clearly indicate to me that there is a complex interaction of pig¬
ment deposition, capillary development, and immediate autonomic
effects. For a more detailed discussion of such autonomic effects, see
Morris ( 1956). Noble and Wurm ( 1940:847) state, “It is probable that
the androgen secreted by the night heron during the breeding season
has a multiplicity of effects on the body.” These same authors noted
that rough handling of the Black-crowns frequently resulted in tem¬
porary color changes in the legs and feet of those birds that had been
treated with testosterone propionate. The “blush” of the Little Bittern’s
( Ixobrychus minutus) legs is another example of such rapid color
changes brought about by excitation ( George Edward, in Lowe, 1954 ) .

Because the head of a threatening heron is typically directed frontally
toward the opponent, such rapid changes in lore and bill color may
have a signal function in hostile encounters. Iris color changes that
take place during intense hostile clashes ( Green Heron, for example )
may also act as “short range” social signals.

An additional function of soft-part color changes may be their role
as supplementary isolating mechanisms. Among the heron species
described in this paper no two that breed sympatrically have the same
combination of breeding season colors. Such color differences, acting
as visual signals, may function as one of many reinforcements in a
chain-like series of reproductive isolating mechanisms.

Although too few species have been studied to formulate generaliza¬
tions about the evolution of soft-part color changes, there appears to
be a trend in all-white species toward bright colors in the bill and lores
(for example, albus and thula ), while in dark-bodied forms, the trend
is to develop more subdued bill and loral coloration ( virescens and
nycticorax) . For example, the all-white Cattle Egret has a brilliant
red bill and lores ( Tucker, 1936; Peterson et al., 1954, among others ) .
Adults of North American albus , which are all-white, have a brilliant
orange bill and bright, lime-green lores, while those of the white thula
have scarlet lores and jet black bills. In contrast, such dark-bodied
species as virescens have black bills and black or bluish-black lores. A
trend toward contrast in coloration is evident in a species such as the
Black-crown. This species is dark above (crown and mantle), and it
has dark soft-part colors during the breeding season on the bill and
lores; however, the underparts of breeding nycticorax are pure white,
and the legs and feet turn a brilliant rosy-red or salmon-pink. This
trend toward contrasting colors is also shown by breeding Green Herons,
which have dark bodies but brilliant coral-orange legs and feet.

Failure to take into account the changes in soft-part coloration that
occur at the onset of the breeding season may lead to taxonomic diffi¬
culties, especially with regard to studies on intra-specific variation
(Van Tyne, 1950:5; and Bemis, 1956).

SOME EVOLUTIONARY TRENDS IN HERON BEHAVIOR

149

SIZE AND ACTIVITY

Although generalizations based on the comparison of a limited
number of species are always dangerous, one trend is the correlation
between size and activity.

The smaller North American species, such as virescens, thula , and
rufescens , all have a number of aerial displays, both hostile and sexual.
For example, Flying Around, Pursuit Flights, Circle Flights, and Flap
Flight Displays are very common in the courtship of virescens , while
Aerial Stretch Displays and Jumping Over Displays are common in
rufescens and thula. In contrast, the bulk of the displays, both hostile
and sexual, of the larger species ( herodias and albus ) are mostly non¬
aerial in nature. In fact, many of the displaying males of the larger
species perform mainly from one site — the nest platform or the crown
of the nest tree.

The active hostile behavior of the smaller species (Full Forward
Displays of rufescens and thula ) is in sharp contrast with the relatively
inactive threat behavior of the larger species. Two threatening indi¬
viduals of herodias wade toward one another slowly, but thula and
rufescens rush headlong to the attack. Threatening Black-crowns,
which are stocky birds, typically stalk their opponents methodically,
while Green Herons frequently attack a territory trespasser by flying
directly toward the intruder.

The Stretch Display of herodias is performed simply and slowly, but
the small thula pumps its head up and down vigorously, rufescens
shows violent head-tossing during its Stretch Displays, and virescens
actively sways from side to side. None of the larger herons have aerial
Stretch Displays, but the smaller species show highly ritualized aerial
variants of this sexual display. If a displaying male of one of the larger
species is approached by the female, the male usually threatens while
remaining stationary, but the typical response of one of the smaller
species is active pursuit of the female.

In general, the feeding behavior of the larger species is rather inactive,
Stand and Wait and Wade or Walk Slowly being the principal feeding
methods. The techniques used in feeding by the smaller species ( thula
and rufescens ) are in sharp contrast, such Disturb and Chase methods
as Open Wing, foot-stirring, and Canopy Feeding being most prom¬
inent. However, a noticeable exception to the rule of small-active,
large-inactive, is to be found in the Green Heron. This species is
typically a slow feeder, Stand and Wait being its principal method of
securing food. I believe that the inactive feeding behavior of virescens
is one of several characters that link the Green Heron to the bitterns
(Ixobrychus) . The relatively inactive feeding methods of Ixobrychus
may be prescribed by the dense nature of its typical habitat. The Black-
crowned Night Heron is an inactive feeder, but I place this stocky
species with the larger herons with respect to size.

150 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

After reading the many recent contributions on the comparative
ethology of the gulls and their allies ( family Laridae ) , it is evident to
me that here also the small-bodied species are much more active than
their large-bodied relatives. For example, the gulls of the Hydrocoloeus
group (e.g., Lams ridibundus Linnaeus and L. minutus Pallas) show
more active and more frequent aerial activities than do the larger
species of the typical Lams group ( e.g., L. argentatus and L. marinus
Linnaeus). For details, see especially Moynihan (1955c, 1956, 1958)
and Tinbergen (1953). Rensch (1954) should be consulted for a dis¬
cussion of the relation between body size and central nervous functions.

COMPARATIVE BEHAVIOR CHART

The following chart is a summary of existing knowledge of the dis¬
plays and related activities of ten North American herons. An “X”
denotes the occurrence of a particular display or activity, while a
question mark (?) signifies that information is unavailable or that
descriptions in the literature are not detailed enough for comparative
purposes. The numbers following certain question marks refer to
available published descriptions. The herons are referred to by the
following abbreviations:

GH — Green Heron, Butorides virescens
GB-GW — Great Blue and Great White Herons, Ardea herodias
CoE — Common (American) Egret, Casmerodius albus
RE — Reddish Egret, Dichromanassa rufescens
~ SE — Snowy Egret, Leucophoyx thula
LB — Little Blue Heron, Florida caerulea
LH — Louisiana Heron, Hydranassa tricolor
CaE — Cattle Egret, Bubulcus ibis
BC — Black-crowned Night Heron, Nycticorax nycticorax
YC — Yellow-crowned Night Heron, Nyctanassa violacea

Behavior

GH GB-GW CoE

RE

SE

LB

LH

CaE

BC

YC

Spiral descent flight _

X

X

X

X

X

X

X

?

?

?

Hover over intruder at nest ..

?

?

?

X

?

Do not hover intruder _

X

X

X

X

X

?

?

?

?

Fly directly down to roost _

X

?

Walk- jump down to roost _

X

X

X

X

X

X

?

X

X

Underwing preening _

X

X

X

X

X

X

X

?

X

?

Direct head-scratching _

X

X

X

X

X

X

X

X

X

?

Shaking-out plumage _

X

X

X

X

X

X

X

?

X

X

Sun-bathing _

X

X

X

X

X

?

X

?

?

?

Bittern Stance _

- X

?

?

?

X

?

?1

?

?

?

Alert postures _

- X

X

X

X

X

X

X

?

X

?

Alarm call _

X

X

X

X

X

X

X

X

X

X

Feeding techniques:

Stand and Wait _

X

X

X

X

X

X

X

X

X

X

Wade or Walk Slowly —

- X

X

X

X

X

X

X

X

X

X

Wing-flicking .... _

. ?

X

X

X

X

X

X

?

?

?

Open Wing -

?

?

X

X

X

X

?

?

?

Underwing -

. ?

?

?

X

X

?

X

?

?

?

Canopy - -

?

?

X

?

?

?

?

?

?

Head-tilting _

. ?

?

?

X

?

?

?

?

?

?

Foot-stirring _ _

. ?

?

?

X

X

?

X

?

?

?

Foot-paddling _

?

?

?

?

X

?

?

?

?

?

Hovering-stirring _

_ ?

?

?

X

X

?

?

?

?

?

Diving _

- X

X

?

?

?

?

?

?

X

?

Hovering _

?

X

?

X

X

X

?

?

X

?

Onset of breeding protracted

X

X

X

X

X

?

?

X

X

Onset of breeding on arrival _

- X

( Continued

on p,

. 152)

151

152 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Comparative Behavior Chart (Continued).

Behavior

GH GB-GW CoE

RE

SE

LB

LH

CaE

BC

YC

Non-breeding soft-part colors

X

X

X

X

X

X

X

X

X

X

Breeding soft-part colors _

X

X

X

X

X

X

X

X

X

?

Establishment of Territory _

X

X

X

X

X

?

?

?

X

?

Advertising call _

X

?2

X

X

X

?

?

?

X

?

Flying Around _

X

?

X

X

X

X

X

?

?

?

Pursuit Flight _

X

?

X

X

X

X

X

?

?

?

Supplanting Attack _

X

?

X

X

X

?

?

?

X

?

Nipping in flight _

X

?

X

X

X

?

?

?

?

?

Crest-raising _ _ .

X

X

X

X

X

X

X

?

X

X

Tail-flipping _

X

X

X

X

X

X

X

?

X

X

Withdrawn Crouch _

X

?

?

?

?

?

?

?

?

?

Forward Display _ _

X

X

X

X

X

?

?

?

X

X

Full Forward Display _

X

X

X

—

—

?

?

?

X

?

Upright Display _

—

X

X

X

X

X

X

?

X

?

Aggressive Upright _

—

X

X

X

X

?

?

?

—

?

Attack call _

X

X

X

X

X

X

X

?

X

X

Stiff-necked Upright _

X

?

?

?

?

?

?

?

?

?

Fighting _

X

X

X

X

X

X

X

?

X

X

Bill-snapping _ _ _.

X

X

?

X

X

X

X

X

X

X

Feather-nibbling _

X

X

X

X

X

X

X

X

X

X

Peering Down Display _

?

?

?

X

?

?

?

?

?

?

Typical Snap Display _

X

X

?

?

X

?3

?

?

X

?

Bowing or bobbing Snap _

X

X

?

X

X

?3

?

?

?

?

Extended Stretch _

X

?

?

?

?

?3

?

?

?

?

Stationary Stretch _

X

X

?

X

X

?3

?4

?

X

?5

Aerial Stretch _

?

?

?

X

X

?

?

?

?

?

Circle Stretch _ ..

?

?

?

X

?

?

?

?

?

?

Dancing Ground Display _

?

X

x6

X

?

?

?

?

?

?

Gathering Ground Display ..

?

?

?

?

?

?

?

?

X

?

Circle Flight _

X

X

X

X

X

?

?

?

?

?

Crooked Neck Flight _

X

?

?

?

?

?

?

?

?

?

Flap Flight _

X

?

?

?

?

?

?

?

?

?

Twig-passing _ _

X

X

X

X

X

X

X

X

X

X

Jumping Over .... _ ..

?

?

?

X

X

?

?

?

?

?

Tumbling Flight _ .

?

?

?

?

X

?

?

?

?

?

1 — Palmer, 1909; 2 — Cottrille and

Cottrille,

1958;

3 — Meanley,

1955; 4-

-Huxley

(in

Bent,

1926); 5 — Nice, 1929; 6 — Audubon (in Bent, 1926).

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158 COMPARATIVE BREEDING BEHAVIOR OF NORTH AMERICAN HERONS

Wilson, Alexander, and Charles Lucian Bonaparte. 1878. American Orni¬
thology. One Volume Edition. Philadelphia: Porter & Coates.

W interbottom, J. M. 1957. Some African Herons. African Wild Life, 11:
100-105.

Witherby, H. F., F. C. R. Jourdain, N. F. Ticehurst, and B. W. Tucker. 1949.
The Handbook of British Birds. Vol. III. London: H. F. & G. Witherby
Ltd., 387pp.

Wood, Harold B. 1933. Flight speed of some birds. Auk, 50:452-453.

Green Heron preening its upper neck feathers.

Plate 1.

Plate 2. Green Heron preening the underside of its wing.

Plate 3. Green Heron scratching the side of its head directly.

Plate 4. Green Heron shaking out its plumage

Plate 5. Green Heron sleeping while brooding young in the nest.

Plate 6. The normal perch position of the Green Heron.

Plate 7. Crest-raising by the Green Heron.

Plate 9. Young Great White Heron preening one of its primaries

Plate 10. Young Great White Heron sun-bathing

Plate 11. Young Great White Heron giving the Forward Display in its nest.

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Plate 12.

Reddish Egret showing Head-tilting while feeding.

Plate 13. Canopy Feeding by the Reddish Egret.

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Plate 14. Canopy Feeding by the Reddish Egret.

Plate 15. Snowy Egret preening the underside of its wing.

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QL696.C52 M53

Comparative breeding behavior of fo

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