SOOTY TERN BEHAVIOR

By

JAMES JAY DINSMORE

A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF

THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA
1970

ACKNOWLEDGMENTS

I have benefited greatly from the counsel arid interest of many
individuals. The advice and encouragement of Dr. Oliver L. Austin, Jr.
have aided ne throughout this work, both in the field and in
preparing a written report. Dr. William B. Robertson, Jr. continually
provided ne with information on Sooty Terns and also arranged for my
stay on the Dry Tcrtugas. The comments and criticisms of Drs. George
W. Cornwell, Frank G. Nordlie, Thomas J. Walker, and S. David Webb
have pointed cut many weaknesses in my work and thereby have been
of great help to me. I also nave profited from discussions on Sooty
Terns with Brian Harrington, Robert W. McFarlane, John C. O^ien.
Dr. Glenn E. Woolfenden, and others, especially man,/ members of
banding parties to the Tortugas. Personnel at Fort Jefferson
National Monument and Everglades National Park i>sre especially
helpful in providing a place for me to live at Fort Jefferson and
then making that living enjoyable. Dr. Gary D. Schnell generously
gave me access to his unpublished data on Sooty Tern flight speeds.
The Florida State Museum provided a boat to use at Fort Jefferson.
Two grants from the Frank M. Chapman Fund, American Museum of Natural
History, and a Louis Agassiz Fuertes Research grant from the Wilson
Ornithological Society helped finance the field work. A University
of Florida Graduate School Fellowship made it possible for cie to spend
an extend? I riod at the Dry Tortugas. My wife, Pat, encouraged me
thr . .... it the sti ;, - nd typed the final drafts of! this dissertation.
To all these individuals and organizations, I am grateful.

TABLE OF CONTENTS

ACKNOWLEDGMENTS •

LIST OF TABLES • •

LIST OF FIGURES

ABSTRACT • • •

INTRODUCTION . . .

THE DRY TORTUGAS •

Physical Features . - •

Fauna •

Clinate •

Previous Jtfork . . • • «

METHODS. . . . .

breeding activities.

Formation of the Colony • -1-0

Ac- rial Flock
First Land.

• * *

• •

■ ■•••••••**

Page

11

v

vi

vii

1
?

2

5
6
6

10
12

Pre-laying Activities j-

15
18

c

Aerial Display

Ground Activities

Establishing Territories .....
Incubation Period ....... .....

Egg-laying . .

Behavior During incubation .........•••

Attentiveness.

lapping

Temperature Regulation ...

Nest Relief

Length of Incubation Shifts

Influence of Weather .
Fledging Period

Hatching

Attentiveness

Feeding

Frequency of Feeding .....

Individual Recognition

Chick Behavior •

in

41

42
43
43
45
48

9

Page

Other Aerial Activities ...... 61

Thermal Scaring 6l

Fly-ups. . . 62

Panics 63

ECOLOGY 65

Interactions with Other Species 65

Enemies 65

Other Terns 69

Nesting Habitat , . 71

Site Tenacity 75

DISCUSSION 77

Behavioral Comparisons 77

Sooties as a Pelagic Species 80

LITERATURE CITED 90

BIOGRAPHICAL SKETCH Q6

IV

LIST OF TABLES

Table Page

1 Activities other than incubating or shading by Sooty

Terns caring for an egg Jl

2 Summary of 113 dipping flights by Sooty Terns . Jk

3 Time of occurrence of flights away from the egg by

adult Sooty Terns 3&

4 Tine of nest relief during incubation 39

5 Time of feeding of young Sooty Terns 50

6 Rate of foraging by adult Sooty Terns 53

7 Nest density and success in five 5-yard— square plots

on Bush Key in 1968 72

8 Some information on the breeding biology of several
species of terns 83

LIST OF FIGURES

Figure Page

1 Map of Dry Tortugas Islands 3

2 Posture of Sooty Tern in the parade display 19

3 Activities by adult Sooty Terns during incubation 29

b Activities by adult Sooty Terns during the first

2 weeks of caring for the chick *r?

VI

Abstract cf Dissertation Presented to the
Graduate Council of the University of Florida in Partial Fulfillment
of the Requirements for the Degree of Doctor of Philosophy

SOOTY TERN BEHAVIOR

By
Janes Jay Dinsmore

August, 1970

Chairman: Dr. Oliver L. Austin, Jr.
Major Department: Zoology

A 3-year study of the breeding behavior of Sooty Terns (Sterna
fuscata) was made at Bush Key, Dry Tortugas in the southeastern Gulf
of Mexico. The results are compared with the behavior "of other terns
and the differences discussed, particularly in regard to the pelagic
environment the Sooty Tern inhabits.

Sooty Terns arrive at the Dry Tortugas some 2 months before eggs
are first laid. At first they circle Bush Key at night only, but
eventually land and extend the time they spend on the ground, arriving
earlier in the evening and leaving later in the morning. After the
first eggs are laid, flocking subsides. Aerial display by Scoty Terns
consists mainly of the high flight in which two birds ascend, usually
by jerk-flying, circle, and then descend together in a coordinated
glide. Scoty Terns may have a poorly developed low flight display.
On the ground the major display is the parade, similar to that of other
Sterna terns. Courtship feeding is rare. Terns often interfere with
pairs attempting to copulate.

The single egg is incubated about 29.5 days with the male car_

vii

for it somewhat more than the female. The birds are attentive over
95 per cent of the tine, incubating when it is cool, shading when it
is hot, and engaging in ether activities such as preening, fighting, and
loafing .cost commonly when they change between incubating and shading.
Nest relief usually occurs in the evening after 24 or 48 hours of care.
Dipping, in trhich adults dip their feet, bill and/or breast feathers in
the ocean, is- common during incubation aid may cool the egg or provide
it with needed moisture.

Chicks are closely brooded for 4 or 5 days, after which adults seen
to recognize them individually and spend progressively less tine with
them. By the end of the 3"d week, chicks are alone much of the tine
except when fed. Adults feed the chick by regurgitation, spend about
3.5 hours per foraging trip, and often feed a chick several tines after
one trip. Males feed the chick somewhat mere than females do. Vocal
exchanges between parent and chick seem most important in individual
recognition. Occasionally adults feed a chick ether than their own.
Chicks first fly when about 9 weeks old and leave the colon;/ scon
after that.

The ground and aerial displays of Sooty Terns are similar to those
of other .Sterna, terns, especially the Common Tern. The rarity of the
low flight and courtship feeding, both of which are common in other
terns, may be due to the different way in which Socties carry food and
the distance they travel to forage.

Sooty Terns have a lower clutch size, longer period cf development
of the chick, and first breed when older than most other terns, nany of
which feed in raarshes and coastal waters. These characteristics of
.Sooty Tern breeding biology are similar to those of many other pelagic

V1X3.

birds. A distant food supply and high adult survivorship apparently
have contributed to these differences from other terns.

IX

INTRODUCTION

Sooty Terns (St, em a fugcata) occur worldwide in tropical and
subtropical waters, breeding mainly on low isolated islands (Ashnole,
1963). Scne colonies number in the millions (Ridley and Percy, 1958)
and the species could well be one of the most numerous cf the world* s
birds. This tern ranges ever the open ocean to Teed, thus differing
from, the many terns that feed in coastal waters. Sooties apparently
return to land only during the breeding season. Other pecularities
of Sooty Tern behavior include a nonannual breeding cycle at some
localities (Chapin, 195^; Ashmole, 1963), and a long migration by
juveniles (Robertson, 196?).

Although the ethology of several species of terns is well known,
no pelagic species cf Sterna are adequately studied. The basic studies
of Sooty Tern behavior were made $0 years ago (Watson, 1908 ; Watson
ana Lashley, 1915; Lashley, 1915) and certainly need to be updated.
From 1963 through 1970 1 studied the behavior of Sooty Terns at the Dry
Tortugas Islands, watching a group of terns throughout the breeding
season and recording their behavior at all accessible stages. This
paper presents basic information on the behavior of a tropical Sterna
and attempts to relate this behavior to the species' pelagic habits
and thus show how fuscata ciffers from its coastal— feeding relatives.

THE DRY TORTUGAS

Physical Features
The Dry Tortugas presently consist of seven snail low islands at
approximately 2^°38'N, 82°52'V, about 70 miles west of Fey West,
Florida (Figure 1). In the past, century at least four other islands,
once a part of the group, have disappeared (see Robertson, l?6k, for
history of all the islands). The islands are located on a large
shallow bank, much of it less than 5 fathoms deep, in the southeastern
Gulf of Mexico. Here, near the northern limit of their breeding range,
some 80,000 fecty Terns have nested in recent years. The earliest

definite record. of Sooty Terns nesting en the Dry Tortugas is

•• -^ +>io +

at c:

Audubon (1844) who found them nesting with Brown Noddies (Anous
jitolidus) on Bird Key in I832. Both species continued to nest at Bird
Key until the early l?3Cfs ^hen it finally washed away. The birds then
moved to nearby Bush Key, which has been the major breeding site ever
since, although occasionally some have nested on Garden, Long, and
hospital Keys.

All the islands are low sand bars or piles of coral rubble. The
largest, loggerhead Key (ca. 30 acres), has a small Coast Guard
lighthouse and station and a good stand of large trees, mainly
Casuarina. Garden Key is dominated by Fort Jefferson, a former army
outpost and later a prison. Starting with a National Audubon Society
warden stationed en the Tortugas during the 1903 nesting season, the
terns have had son- ■■ ection from eggers and ether disturbances

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most subsequent years. Fort Jefferson was designated a National
Monument in 1935, and its snail permanent, human population provides
some protection for the terns in the nesting seascn. Middle aid
Hospital Keys are presently just small bare sand bars. East Key has
some low beach strand vegetation. Long Key is a bar of rough coral
rubble with a small stand of mangroves.

I studied Sooty Terns en Bush Key, currently about 20 acres in
extent and separated from Garden Key by a 5^0— foot channel.. It is
composed primarily of coarse light-colored sand with coral rabble on
the east end where a long narrow spit joins it to Long Key at low tide.
Maximum elevation is about k feet above mean high tide, Bush Key
apparently was present in the mid-lSOO's, disappeared, and reappeared
in the early 1900* s (Robertson, 19c4). In this century it probably
first appeared as a series oH sand bars that eventually connected,
gained some vegetation, and gradually grew and stabilized.

Three snail brackish ponds in the center of the island are rimmed
by mangroves (Rhizoohora man-le and la -unoulr-ria racemosa) and
buttonwood (Conocarous ereotns). Around them a thicket of bay cedar
(jforiana maritima; 5 to o feet tali covers much cf the island. Outside
the bay cedar thicket and extending from it to the tide line, the
vegetation is relatively low and open. Most Sooty Terns nest on these
flats, although seme nest in openings in and under the mangroves or the
bay cedar. The major plants on the periphery of Bush Key are sea
rocket (Cakile 1-noeolata) , sea oats (Unioia oaniculata) , prickly
pear (Oountia sp. ) , a grass ( Snorobolus sp. ) and sea purslane C Sesuvium
lortulacastruia) wi th some sea lavender (Toumefcrtia gr.aphalodes)
around the edges. The density of these plants varies from year to

year, primarily depending on the amount of rainfall.

i'auna

Besides Sooty Terns, at least three other species of terns have
nested on the Dry Tortugas in recent times. Host numerous today are
the several thousand Brown Noddies that build a simple platform nest
of sticks and dead vegetation in the bay cedar and other low vegetation
on Bash Key. The Black Noddy (A.neus tenui re stria) has csen reported on
the Dry Tortugas almost yearly since 1959 (Robertson, 1964) but as yet
has not been found nesting.

Least Terns ( Sterna albifrer.s) formerly nested on Bush, long, and
Loggerhead Keys but have not done so since about 1950, while several
hundred Roseate Terns (S. dougallii) still nest yearly on the Dry
Tortugas (Robertson, 1964). Royal Terns (Thalasseus maxirvjs) and
Sandwich Terns (T. sandvicensis) nested en the Dry Tortugas in the
1890* s, but other than a single naximus egg found in 195- » no recent
nesting records exist (Robertson, 1964).

Of some 240 species of birds reported from the Dry Tortugas, th •
only others known to have bred there are some species of booby (Sola)
reported by Audubon (1844) and the Mourning Dove (Zenaidura nacroura).

The only mammals reported from the islands are the introduced rat
(Rattus rattus) that at times has killed young birds and destroyed
eggs (Russell, 1933) and the West Indian Seal ('.-lonachus tro cicalis ) ,
currently faced with extinction.

Several land reptiles have been collected on the islands (Duellman
and .Schwartz, 1958) but apparently none has permanently colonized the
islands and they pose no threat to the terns. A _• the name of the
islands suggests, sea turtles formerly nested there in great numbers.

A few, probably Loggerhead Turtles (Caretta caretta) , still lay their
eggs en the Tortugas and occasionally destroy tern nests when they dig
their own nests.

Clinate
The climate at the Dry Tortugas is best described as hot and dry.
Daily temperatures often reach the low 90s (°F) and seldom drop below
the low 60s at night (Vaughan, 1918). The intense radiation of heat
from both water and the light— colored sand creates a severe
microclimate in the layer of air just above the ground, the micro-
habitat in which Scoty Terns nest. Rainfall probably averages less
than 40 inches per year, much of it coming in sudden squalls. Several
hurricanes have stru.de the Tortugas in recent years. On 6 June 1566
the 123 raph winds of Hurricane Alna drove water over Bush Key and
buried many young terns in the sand. Many died, but overall mortality
was surprisingly low (Mason and Steffee, 1966). On 3 June i960 the
center of Hurricane Abby passed about 60 niles west of the islands and
heavy winds, rain, and waves belted the islands for several days. The
high waves washed away some eggs laid on higher parts of the beaches
but did not swamp Bush Key, and mortality was very low. Adults with
eggs or chicks sat tight on the scrape throughout the storm, and some
eggs hatched during the storm without noticeable detriment.

Previous Work
The earliest scientific studies of Sooty Terns on the Dry
Tortugas are those by Watson (1908), Lashley (1915), and Watson and
Lashley (1915). They investigated its breeding behavior and made
some preliminary tests of its honing ability. From 1936 to 1941 some

13,300 terns, mainly juveniles, were banded at the Dry Tortugas.
During the 1940* s and 1950' s National Park Service personnel and others
visited the colon;* almost yearly and made estimates of its size in
most years. Summaries of much of this work appear in Sprunt (19^)
and Robertson {196k).

In 1959 the National Park Service, the Florida State Museum, and
the Florida Audubon Society began a cooperative banding program on
the Dry Tortugas terns. Robertson (1964-) summarizes the history of
the terns on the Dry Tortugas and later (1969) documents the
transatlantic migration by juveniles from this colony. In the 12 years
of banding seme 153,700 juvenile and 81,100 adult Sooty Terns have
been banded on Bush Key.

METHODS

I lived on Garden Key from 29 March to 10 July 1968 and worked
almost daily on nearby Bush Y.ey. I erected a blind near a snail plot
(25 x 30 feet) from which I cleared nearly all vegetation. Terns
were captured in mist nets and 182 were color-marked, each with a
unique pattern of three colored leg bands and a Fish and Wildlife
Service (FWS) band for individual recognition. The terns gradually
acclimated to ray presence in the blind, and their behavior appeared
normal once I was out of sight.

I determined the sax of the terns by noting their position in
copulation, keening in mind that nale-niale mountings may occur. Once
the birds were color-narked, I recorded their activities and the roles
of nal.es and females in the care of their eggs and chicks. As Sooty
Terns spend much of their time flying, I had to limit ry studies to
their activities at Bush Key and its immediate, vicinity. After the
eggs hatched, I tethered some chicks to prevent them from hiding
throughout the day. This altered their behavior, but it was the only
way I could keep more than a few chicks in sight after they were about
3 weeks old.

In 1969 and 19?0 I spent shorter periods on the island after the
young hatched.

BBEEDING ACTIVITIES

The yearly activities of Sooty Terns at the Dry Tortugas can be
summarized as follows: Starting in late January or February, Sooty
Tems appear near the islands in numbers at night only, circling,
calling, and sometimes landing, but generally are absent during the
day (Robertson, 1964). Gradually the terns start, arriving at the
Tortugas earlier each evening and also in greater numbers. Finally
in late March or April they remain in daylight and land, display,
copulate, dig a scrape, and lay a single egg that is incubated 29 to
30 days. The chicks are closely attended for the first week or two
and then are alone much of the tine except v:hen fed. Chicks first flv
when about 9 weeks old and apparently leave 3ush Key scon afterward.
Some juveniles are flying by late June and nest terns have left the
Tortugas by ]ate August or early September.

Little is known of the activities of Sooty Terns away from the
colony. Out of some 153,700 chicks banded on Bush Key, about 30 have
been recovered away from the colony and provide some information en
the movement of juveniles. After leaving Bush Key, juveniles drift
west, in the Gulf of Mexico and then south along the eastern coast of
Central America, finally moving east along northern South America and
out across the Atlantic in the vicinity of the equator. They spend
about 2 to k years in tb<^ Gulf of Guinea off West Africa, and gradually
drifi 1 xjk ac -oss / . Atlantic. A few 3— year—old birds occur ax, the
lortugas late in the breeding season, cut almost cert .wily do not

10

breed. At least a few 6-year-old birds breed but sone Sooties may not

breed until older (Harrington, pers. comm. ).

Among those banded as adults the recovery rate is lover, with
only about 25 recoveries away i^-cn the colony out of 81,100 banded
through 1970. These recoveries suggest that the range of most adults
is the Gulf of Mexico throughout the year ( Robertson, 1969).

Formation of the Colony
As the terns assemble and reform the colony, they are very
skittish and difficult to follow. Their activities can be divided
into periods of aerial flocking and first landing.

Aerial Flocking

Robertson (19&0 describes a period of night flocking, starting
some 2 months before eggs are laid and during which large flocks of
terns gather near or over the breeding grounds to call, circle, and
land, only to leave around dawn. Ashmole (1963) describes "night
clubs" of terns that land on the breeding grounds in groups at nigh:,
and leave before daylight. Tracks in the sand on Bush Key indicate
that night groups for;; there too, although I never saw then.

When 1 arrived at the Dry Tortugas on 29 March 1963, Sooty Terns
had not yet laid any eggs, but they were flocking in well-developed
patterns every evening. Few terns were present during the day. By
about 03:00 (all times are EST) most Sooties left the island and ell
was quiet except for a few Noddies. Occasionally at various times 01
day, small flocks of up to 10 Sooty Terns circled rapidly over 3ush
Key, giving loud "wide— a-wake" calls or sharp "yip yip" notes as one
bine, ens ed another. The birds passed over the island once or

11

sometimes repeatedly, flying low and often in pairs. They flew slowly
with deep wing beats. Their flight resembled the low (fish) flight
display described for other terns (Palmer, 19^1; Cullen, 1960a), except
tha.t they did not carry a fish in their bill.

In early evening, between 16-.J0 and 1?:00, flocking began. The
terns approached the Dry Tortugas from the northwest, much as Robertson
(196*0 describes and gathered northwest of Bush 'iley, circling and
calling. As more birds joined the flocks, their calls grew louder and
the circling more pronounced. Usually they started in a loose flock
of 50 to 200 birds circling 50 to 100 feet above the water. They flew
with a slow, seemingly exaggerated wing beat, often gliding between
beats and drifting along in a circle 100 to 200 feet across. Eventually
a few dropped low over the water and, assuming a more rapid wing beat,
made a fast circuit over the edge of Bush Key and returned 00 the flock
over the water. Gradually more circled over the island. A few
started landing on the open sand beach on the north side of Bush Key.
The calling got still louder. Occasionally all the birds on the beach
flew rapidly and noisily back to the flock. There they circled and
again started moving back toward the island. Around 18 :30 the number
of birds gathered on the beach increased. Fewer flew when disturbed,
and they returned sooner when disturbed. All this time more birds
continued to join the circling flocks.

Out over the water the flocking pattern changed slowly. At first
all the terns were in one staging or circling flock, from which they
flew to the island. As more birds arrived near Bush Key, more staging
flocks formed in a line strung out to the northwest. Presumably birds
entered the farthest one first and then moved from flock to flock as

12

they approached the island.

Cn 5 April at 17 OO I cculd see three such flocks, one about 1,000
feet north of Bush Key and the other two beyond. Between the flocks
I could see lines of birds flying with sic.;, exaggerated wing beats
about 5^ feet above the water. Beyond the last staging flock a line
of terns, again about 5° feet above the water, was stretched cut for
several miles. Perhaps some birds approach Bush Key low- ever the
water as they normally do later in the season, but most drifted in
with the flocks described above. The largest flocks 1 saw were north
of Bush Key; occasionally smaller flecks formed south of Garden Key.

After sundown more terns landed and gathered in groups. On
10 April, a night with a full noon, most Scoties were sitting quietly
along the beaches of Bush Key at 21:30, although a few still circled
and called from the air.

At sunrise no staging flecks were present, although many birds
circled and called over Bush Key or stayed on the ground. By 08:00
most birds had left the island, flying to the northwest. The exodus
was not so well defined as the approach.

Flocking by Booties is apparently a significant social activity.
Perhaps its function is to delay breeding by early arrivals at the
colony until some critical number of terns is present. With adult
Sooties apparently spread throughout the Gulf of Mexico in the
nonbreeding season, such flocking might synchronize the reproductive
cycles of individual birds and insure that most Sooties breed at about
the same (most favorable?) tine each year.

First Landin

In 19c3 the first daylight landings of Sooty Terns appeared to be

13

gradual extensions of time spent, on the ground by night groups. No
massive influx of terns with almost immediate egg-laying was observed,
although that has been implied fcr this colony (Thompson, 1903;
Sprunt, 19^8). Similarly on Ascension Island, after a period during
which Sooties are present only at night, they suddenly start to return
earlier in the evening arid stay later in the morning, and egg-laying
soon follows (Ashraole, 1963).

When I arrived on the Dry Tortugas in late 'larch, 1963, the terns
had already started staying later in the day so that some were present
until around 08:00. On 1 April I found three Sooty eggs en the west
end of Bush Key, the first of the 196S season. Within a few days
nore were laid near these three, and also several hundred feet east of
them near my blind. Egg— : laying then seemed to spread out from these
two foci and by 21 April eggs were present virtually throughout Bush Key.

On the plot I watched from my blind, egg— laying started later
(9 April) and peaked around 15 April. Hence I followed the behavior of
these birds before they laid their eggs. At first they appeared
nervous and were easily frightened. They hovered over open ground,
landed briefly, and then flew in a panic (see p. 63) almost immediately.
Gradually more birds landed and began to fill the available open
ground, only to fly at the slightest disturbance and then return and
go through the whole process again.

As soon as the birds landed and settled down, at least two postures
became common. One of these is the parade that wd.ll be described
later. In the other the bird points its bill down, sometimes almost
burying it in its breast feathers. This posture is common in Larids.
Harrison (1965) calls it the "stare-down" and says it seems to

14

"cut off" or end a lew intensity agonistic or conflict situation.
Sooties often give a stare— down almost immediately after landing and
then preen their back and wings. An almost feverish amount of
displaying occurred during these early morning hours in the few days
from the time they first stayed on the island until the female laid
her egg.

Little displaying occurs in the night clubs and most of the birds
just rest (Ashmole, 1963). I made only one night visit to Bush Key
during this period, and I found that most terns were resting on the
beaches or in open parts of the island with a few in the air. One
evening when eggs were present, I remained in my blind until well
after dark. Even then the birds were highly excitable, panicking
whenever I made the slightest disturbance. To prevent undue
disturbance of the colony, I avoided night work and have little
information on the bird's nocturnal activities.

Pre— laving; Activities

All of the displays and activities leading up to egg— laying may
occur within a few days after the birds first land in daylight,
Eecause I was marking birds at this time, there were few I could
follow through the entire behavioral sequence. I also found that birds
were still moving around within the colony, since many color-marked
birds immediately left my plot upon release and never returned. Hence
the sequence of events has been pieced together from the few pairs
that I could follow throughout this period and also by noting when
various activities were most common on my plot.

15

A e ri 3.1 J> srla*.~

Early in the nesting season and to a lesser extent later, aerial
displays are a characteristic activity of Sooty Terns. Culler. (lyoCa)
studied aerial activities of the Arctic Tern (Sterna paradisaea) in
detail and summarized much of the information available on other terns.
I follow his terminology for activities he describes that appear
homologous to those of the Sooty Tern.

Except for the chases ever the colony prior to egg-laying, which
nay be homologous to the low (fish) flight that is common among Sterna
terns, aerial display by Sooties seems zo consist solely of a high
flight similar ^o that of the Arctic Tern (Cullen, 19oGa).

In Sooties, the high flight is a slow steady ascent by two or
sometimes three terns to a height of several hundred feet and then a
rapid gliding descent. 1 noted two means of ascent, one often occurring
after the other. In one the two birds ascended almost vertically in
snail circles, flapping their wings rapidly as the lower bird chased
and occasionally tried, to grasp the tail of the upper bird. This seems
comparable to the upward flutter, a hostile behavior that Cullen (19o0a)
describes. I saw the upward flutter at the start of only about 3°
per cent of all aerial displays, but usually it was given only briefly
and I may have missed seeing it in other cases. From the upward
flutter, the birds changed to seemingly exaggerated deep wing beats
that Cullen calls jerk—flying. In many instances the first sign of
aerial display was this type of flight, and it was the initial pattern
in many high flights. Flying thusly, the birds continued to climb,
though not so steeply as with the upward flutter. Again they stayed
close together as if ons was chasing the other. In these chases -

16

lead bird gave a rattling "ka ka ka" call while the pursuer gave
upward-inflected "wek wek" notes. As they climbed, they flew in long
looping circles above the colony and adjacent water, often reaching a
height of several hundred feet. They sometimes continued to climb
for 2 or 3 minutes, although more commonly they climbed for about a
minute.

At the peak of the flight they often chased briefly and then
maneuvered close to each other by flapping their wings in slow, almost
half beats with the wings barely moving to bole:-; the level of the body.
Then they began a rapid gliding dive. The angle of descent was
usually fairly shallow at first and became steeper as they descended.
Several times I noted that the pursuer passed the leading bird and
took the lead at the start of the glide, and this may be usual, much
as Cullen (19oUa) reports. In the dive one bird was above and just
behind the other, the two about a foot apart. Both held their wings
rigid; the lower bird's wings were usually bent slightly at the carpals
and the upper one's -wings were held almost straight out from the body.
They held the long outer rectrices "scissored" together and in those
instances where I could see their bills, they pointed straight forward.
In this position they dove together, their movements coordinated so
that the second bird closely followed the sometimes shifting and
dodging flight cf the first. Sometimes they started the glide several
times, leveled, off .after descending a short way, and then dove again.
At ether times they separated and each flew away alone. In a complete
display they glided together down to just above water level, swooped
up a short ways end then separated, each flying away in a normal
flight. Several times I followed both birds after the glide, but I

17

never saw then stay together to repeat the high flight.

I saw numerous variations of the high flight. Of 50 high flights
in which I saw all of the display, 18 started with the upward flutter
and 32 with jerk— flying. Thirteen times the birds changed from the
upward flutter to jerk— flying. Thus jerk— flying was part cf the
ascent in ^5 of the y0 high flights. The other 5 tines the entire
ascent was with upward flutter. In h-G high flights the two birds
circled after this ascent, mainly by jerk— flying. The birds started a
descending glide in 47 of the 50 high flights but continued to glide
down to about sea level in only 28. Thus ascending by jerk— flying
and descending in a glide are the two most conspicuous parts of the
high flight, although they do not occur in all of them.

The birds often change positions, first one leading and then the
other. These changes occurred both during the ascent and in the glide.
In the glide the change was made by the upper bird moving beneath the
lower one. I was not able to determine the sex of the birds during
the aerial display.

Except that Arctic Terns sometimes carry a fish in the high flight
(Culien, 19o0a), the high flight of Sooties seems very similar to that
described by Culien for the Arctic Tern.

Aerial displays were most common from 15:00 to 19:00, but I also
saw some in early morning. Most occurred early in the breeding
season in April but I also saw some in May, June, and July when they
may have oe.cn given by late arriving adults or renesters. Unfortunately
I did not see the events on the ground, that preceded the high flight.

The function of the high flight is not known, but Culien (19o0a)
suggests that it may be part of pair formation. If so, perhaps the

18

high flights late in the nesting season involve young birds. As Sooties
as young as 3 years old visit Bush Key late in the nesting season but
probably do not breed for several years, perhaps they pair a year or
more earlier. It is not known how long Sooty Tern pair bonds last
but it would not surprise r.e if the sane birds pair in successive
years. Such an extended "engagement" period is known for another
long-lived pelagic species, trie Laysan Albatross (Dionedsa irvy.'.tabilis)
(Fisher and Fisher, 1969). Moynihan (1962) also saw many aerial displays
by Brown Noddies and the Inca Tern (Larostema inca) late in the
nesting season, and suggests that at least in the Inca Tern these may
have been young birds pairing. Brown Noddies at Bush Key also commonly
give aerial displays late in the nesting season.

Ground Activities

Parade. — Once the birds landed and stopped the almost continuous
panicking, displays on the ground began. The most conspicuous ground
display of Sooty Terns is the parade in which it assur.es a distinct
posture and displays to other birds by prancing rapidly with short,
quick steps. A bird may prance toward, in front of, or around another
tern; or two may prance together, moving parallel to each other.
Palmer (19^1) describes a similar display in the Common Tern (Steri a
hirundo) and says it takes two different forms, each with a different
motivation. Although I saw these two forms in Sooties, I saw much
variation in posture between the two extremes.

In the parade posture, a Sooty extends its head and neck far
forward, often sleekin t . feathers. _ The wings are usually held well
away from the body, particularly at the carpals, and are scnrttir.es
lowered until they drag on the ground (Figure 2). Occasionally the

19

Figure 2. Fosture of Sooty Tern in the parade display (taken
from Iredale, 1914).

20

wings are held tight against the bird's sides. Usually the tail is
tilted upward and often the tips of the primaries cross under the
base of the tail.

If the bill is pointed upward, it is called the erect posture and
Pal ner (19^1) says the display shows submission. Sooties nay point
the bill horizontally or slightly above horizontal but seldom point
it straight up as some other tems do. Sooties also tilt their head
to the side, especially when displaying with or to another bird.
Then they tip their head awaj iron the other individual but seen to
lean the body even more toward it so that the near wing often drags
on the ground.

I saw Sooties give this display almost immediately after landing
on the ground. Often it preceded copulation but other tines two
birds separated after parading. Both saxes gave the erect form of
the parade, but when one bird circled and obviously displayed to
another, whenever I could determine the sexes, it was always the male
that did so. As the erect form of the parade was common almost
immediately after birds landed ana before they established territory,
it certainly is important in pair formation.

In the other form of the parade, the tern points the till down,
but Sooties seldom point it directly at the ground. Palmer (19^1)
calls this the bent position and says it is given when a tern tries
to intimidate another tern. Sooties do not take so extreme a posture
here, usually not holding the winga so far away from the body or
stretching the neck so far forward. Several times I sav; a parade start
with the bill pointing upward and end with it poin rare
ground. Thus the position of the bill varies considerably, although

21

the basic wing and body postures are similar in the bent and erect
forms of the parade.

Both Fairer (19^1) and Cullen (1960a) note these two bill positions
occurring in aerial displays of terns. Although I watched for this,
in all cases that I could see the bill was pointed straight forward,
the aerial position apparently comparable to the erect on the ground.
In the high flight Sooties usually fly away from the island, so I may
just have been unable to see the birds well enough.

The only calls I heard associated directly with the parade were
occasional low "wuk wuk" notes, but usually the birds were silent.
Once I noted the throat of a parading male vibrating rapidly, but
although I was only 10 feet away I heard no sound. Notes too low
for ne to hear may accompany the display.

Several tines I saw a bird en the ground raise its head and give
a loud ,:ke-v;at— ic" or "wan— dick" call that was answered by a second
bird in the air. After the two exchanged several calls, the second
bird landed and then the two. paraded. In one exchange the male called
to the female, and after parading the two copulated. Apparently the
male called to attract a female and then paraded to her. As the
erect form of the parade was most common when the birds first landed,
I think it functions along with the high flight in pair formation.

Courtship feeding;. — I saw courtship feeding only I? times, mainly
from 13 to 20 April but once as late as 1? Jvne. Not all of these
terminated with the actual transfer of food. V/here the sex was known,
the female always begged. Generally she crouched, turned her head
toward the male, 2nd gave a rapid series of low chuckling "ka ka"
or "yip yip" notes, raising her open bill toward him, and biting at

22

his bill, much as a young tern begs for food. A few tir.es I actually
saw the male transfer food to the female, sometimes reoeatedly in one
bout of courtship feeding.

Occasionally the parade preceded courtship feeding, but the latter
is relatively rare and certainly not a regular component o± the oarade.
Courtship feeding is a typical precopulatory behavior in most Larids
(Culler, and Ashmole, 1963). In Sooty Terns courtship feeding seemed
to be relatively rare and only occasionally preceded copulation.

Copulation. — The parade commonly precedes mounting and copulation.
The male parades in front of and around the female, stops beside her,
and then mounts. The female crouches, holds her wings out from her
sides, raises her tail, and stretches her head forward. The male then
moves back on the female, crouches, and copulates. After coitus the
male dismounts, and the two usually fluff tneir body feathers and then
preen. A single pair may copulate repeatedly.

Copulation was commonest on my plot from 6 to 23 April. Of 208
times I noted a male trying to mount a female, in only 56 (26.9 per cent)
did they seem to copulate. In 68 (32.7 per cent) attempts the male
fell off the female, the female did not crouch, or he simply
dismounted without trying to copulate. More commonly (8^4- times,
kO.k per cent), another bird interfered and either knocked the male
off the female or the male dismounted and fought with the intruder.
Interference from other birds became commoner as the breeding season
progressed and eventually few pairs could copulate without other
birds interfering. Several times I saw two to six males try to mount
one female.

23

Establishing Territories

Sooty Terns apparently do not establish, territories until after
copulation. Even then the territory is not permanent until they
finally dig a scrape and the female lays an egg. Normally Sooties
claim an area spanning little mere than what they can reach while
sitting on their egg. The scrape itself is a shallow hollow some
3 to 4- inches in diameter and about an inch deep.

Scrane^-bui Iding. — By 10 April scrape— building was common en my
plot and from then until 1c April it was the bird's mos^ conspicuous
activity. Although scrape— building obviously provides a site for the
egg, the incipient scrape— bui] ding behavior associated with it often
occurs before copulation and seems to be part of courtship in Sooties,
much as it is in Common Terns (Palmer, 19^1).

Pairs of Sooties usually spend some time selecting a site and
nay make several false starts. While selecting a site they walk
together, poke at the ground, picK. up bits of gravel, shell, or
vegetation and then drop them; start work at one site only to abandon
it shortly, and continuously give low "puck puck" calls. Typically
two birds stand close together, point their bills toward the ground,
then lower their body, and kick dirt back with their feet. They
often give low "puck puck" notes as they poke at the ground.

Nearly always a pair works together to dig the scrape. Usually
the male does most of the digging but the female helps at least
occasionally. As they work at the scrape, they pick up small pebbles,
shells, twigs, or other small objects and sometimes drop them into z] e
scrape.

Scrape— bi - ; generally stops once the female lays her egg, but

24

the birds occasionally work on the scraps after it contains an egg.
Several pairs that lost their chick shortly after it hatched began
work on new scrapes, but I did nc'^ see any bird lay again.

Fighting. — Conspicuous fighting started around 11 April when
scrapes were being dug. In fighting, two birds face each other,
some tines approaching each other with their heads and bodies held low
and forward or else upright, the head erect and the crown feathers
raised. The two hold their wings out from the body and usually hold
the tail up. Usually each jabs viciously at the head and bill of the
other bird or grips the other bird's bill and then both shake their
beads vigorously. Occasionally they beat their opponent with a wing,
but normally the wings are used only for balance. Sometimes they give
lev; rasping growls, Finally the two release each other and each
retreats to its own scrape.

While some fights ended with one bird clearly supplanting the
other, I often saw them end differently. The two birds stopped
scuffling, faced each other with wings against the body, bills forward;
and first, one and. then the other lowered its bill into the bent posture
and turned its head slowly away and then broke off the altercation.
Occasionally they gaped, at each other before lowering the bill. Gaping
apparently is an aggressive signal (Moynihan, 1962) as is the bent
position. Thus the aggressive display apparently ended the fight.

The stare-down posture described earlier also occurs commonly
after fights. After a paired bird drove off an intruder, it commonly
gave the stare-down upon returning to its mate. Again the aggressiveness
seemed to end with the stare-down and the bird turned its attention
back to its mate.

25

Thus the sequence of events preceding egg— laying seems tc be as
follows : First the terns flock and land on Bush Key every evening in
a mass social activity, perhaps synchronizing the breeding cycles.
Gradually they extend the time they spend on the ground and as flecking
decreases, they begin individual sexual activity. Males seen to land,
I arhaps near where they bred in previous years ( see p. 75 ) £^-d call
to try to attract females to them. When a female lands, the parade
and high flight that follow probably contribute to pair formation.
They continue courting with parading and incipient scrape— building
until they copulate. The two then dig a scrape and begin to defend
the area immediately around it.

Incubation Period
ESpy— laying

After copulating and then working on a scrape for several days,
the female Sooty Tern lays a single egg. All 14 eggs whose time of
laying I was able to determine accurately were laid in the afternoon;
five between 12:00 and 14:00, five between 14:00 and l6:C0, and four
from 16:00 to 16:00. Ridley and Percy (1953) and Ashmole (1963)
also note that Sooty Terns usually lay their egg in the afternoon.

After the first eggs were laid on 1 April 1968, egg— laying
gradually spread through the island. The first eggs were laid on my
plot on 9 April and the peak of laying there was IS April.

Usually both members of a pair are present when the egg is laid.
The female crouches low in the scrape while the male stands nearby,
occasionally walking around her or poking at the ground. In the first
few minutes after :. ; is laid the two usually sit on it alternately
several times. The female leaves the egg, walks around it, and the

26

male takes over. He pokes at the egg, rolls it into the scrape if it
was laid on the edge, and in general worries over it before finally
settling down to shade or incubate it. Usually the female flies within
about 10 minutes, apparently to drink and dip as she often returns
with her breast feathers wet. The two then stay near the egg, first
one and then the other shading or incubating it. Exchanges now, as
later in incubation, consist of one bird forcing the other off the
egg and then taking over care of it. Exchanges are frequent during the
first few hours after the egg is laid, but by early the following
morning the male assumes care of the egg and the female is absent,
presumably feeding.

During incubation and to some extent after hatching, besides
noting which adult cared for the egg and the adult's general behavior,
I maintained accurate records of activity patterns of 15 pairs in the
following manner. I made a complete catalog of the bird's activities,
described below, i knew the date of laying and I could distinguish the
sexes of each of these pairs, In the activity records, every 30
seconds I made a 1— second "spot" observation and record of the bird
caring for the egg, assigning the bird's activity to one of several
readily identifiable categories (e. g., incubating, shading the egg, off
the egg and preening, etc. ). This method of making 120 observations
per hour per nest was used for 1— hour periods throughout incubation.

One fault of this method is that by cataloging activities,
slightly different activities must be grouped in a single category.
The categories with the most variety are those that occur when the adult
is off the egg, and as the adults either incubate or shade the egg
more than 90 per cent of the time, this variation is only a snail

27

fraction of ail the birds' activities. The ether alternative is to
watch a few nests continuously and record all the activities. Even in
a Sooty Tern colon;/, where nests are close together, it is difficult
to follow all activities at a few nests, especially when several
suddenly erupt into activity with fights or other interactions.
Watching a few nests involves the risk that some or all of them may
be destroyed, deserted, or otherwise disturbed.

On the practical side, continuous watching of a few nests is very
tiring. By viewing 15 nests, I kept my eyes moving and thus avoided
some of the monotony and fatigue of continuous watching.

I analyzed these activity records by sex, hour of day, and date
in the incubation cycle, excluding all 1— hour watches in which
exchanges occurred. Curing incubation, I have records for 1,632
bird— hours, each including 120 spot observations for a total of
195 > 840 soot observations. These include at least one hour of watching
for each sex for each of the 12 daylight hours for most days curing
incubation, and for some I have many more. These were used to prepare
Figure 3 and Table 1.

Behavior During .Incubation

During incubation Sooty Terns primarily tend the egg and perform
a few maintenance activities during brief spells away from it. I
categorized their activities into the following types.

Incubation, in which the adult bird places cne of its two brood
patches directly against the egg, is obviously one of the most important
activities. Sooties incubate mainly at night, in the early morning,
and in late afternoon (Figure 3).

As the air temperature increases in the morning, adults gradually

Figure 3. Activities by adult Sooty Terns during incubation.

29

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30

rise up off the egg so that most days by around 10:00 they are shading
the egg rather than incubating it. They usually continue to shade the
egg until around 17:00, when they gradually change back to incubating
again (Figure 3). To shade the egg the adult merely stands over it and
keeps it in its shadow. As the sun changes position during the day
the birds turn to keep their backs toward the sun. This keeps the
maximum surface area of the body exposed to the sun and, as the upper
surface of a Sooty Tern is black, would seen to present problems of
heat load for the bird. However, keeping their back toward the sun
best insures shading of the egg. By turning one side of its body
toward the sun, the adult could probably lower its heat load, but it
also would be more likely to expose the egg to che sun. The adults
probably reduce the heat load several other ways. As it becomes hotter,
they erect their backfeathers and hold their wings out from thsir
sides, increasing insulation and the area for heat less. They also
gape widely and pant, apparently dissipating more heat than in normal
breathing. Another common activity is dipping as described below.

The change from incubating to shading and back again is very
gradual and proved my most difficult choice in recording activity.
Figure 3 shows clearly when this change is made. Both incubating and
shading involve direct care of the egg. All other activities, in
which the adult is not actually caring for the egg, are shown as the
bottom line in Figure J.

Among these other activities, the most frequent is standing to
the side of the egg and preening (Table 1). Incubating adults commonly
rise up off the egg, poke at it, and apparently turn it. Other times
the adult walks around the scraoe or to the ed^e of its territ< ry

31

able 1. Activities other than incubating or shading by Sooty Terns

caring for an egg

iber cf
spot observations
Activity of each activity

Preening 4258

Poking at egg 15^0

Standing beside egg 1171

Walking 886

Fighting 673

Off away from scrape 2?6

Total 8804

Per

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such activi

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48.

4

17.

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13.

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6

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100.

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32

and stands. Occasionally a bird leaves its egg to spar briefly with
another tern, either a neighbor or one that ha.? cone too close.
Generally Sooty Terns are much less bellicose daring incubation than
they are earlier in the nesting cycle or after eggs hatch. Perhaps
hone ground is permanently established and recognized, and the absence
of chicks to intrude on others' territories enhances stability.

Adult Sooties commonly defecate near their egg. They walk to the
edge of their territory and turn to face the egg before defecating.
Sometimes a stretching motion accompanies defecation. The bird
stretches both wings vertically above the body, leans forward with
head outstretched, and lets fly, thus stretching and defecating in
almost the same motion.

Another activity of adults during incubation is suddenly flying
away leaving the egg unattended. On some occasions the birds fly
away when panicked and return almost immediately. Other times this
seems to be associated with the aerial activity called dipping.

Attentiveness

Sooty Terns are closely attentive during incubation. The
percentage of time adults spend incubating or shading differs only
slightly between sexes. Females spend a little more of their time
incubating and males a little more shading, but with incubating and
shading combined, the total attentiveness, analyzed by hour, differs
by less than 1.2 per cent between sexes. In all, the bird caring for
the egg spends over 95 per cent of the daylight hours either
incubating or shading and only about k,5 per cent in a variety of
other activities (Table 1). These other activities prevail from
0?:00 to 10:00 and from 17:00 to 18:00, the tines when birds shift

33

between incubating and shading (Figure 3). Presumably at these tines
egg and air temperatures are much the same and care by the adult is
not so critical. Sooties are most attentive from 12:00 to 15:00 and
spend over 98 per cent of their tine directly caring for the egg,
mostly by shading. This is certainly the time when the egg experiences
the highest environmental temperatures, and presumably when adult
care is vital to the embryo's survival.

An activity of Sooty Terns at Bush Key that is especially noticeable
during hot midday hours, but occurs occasionally throughout the day,
entails their flying rapidly from the island, briefly dipping their
bill, feet, or breast feathers into the water, and returning to the
island. When only the bill is skimmed along the water's surface, it
seems obvious that the birds are drinking, but dipping the breast
feathers requires another explanation,

Usually each bird flies directly and rapidly from the colon,;/- at
a height of about 10 feet. It then drops down, dips the bill in the
water, drags the feet in the water, and sometimes almost lands
momentarily, wetting the feathers in the process. About 100 to 300
feet offshore the bird turns abruptly and flies back to the island.
An individual may dip from one to as many as ten times on one flight.
In II3 flights between 13 May and 7 June in which I recorded from a
boat all dipping movements, and also whether the bird landed back in
the colony or with groups cf terns sunning or. the beach, ever 80 per
cent of all dipping occurred on the flight out. In about 95 per cent
of the flights the birds dip] :i the bill at least once (Table 2), and
in only about 17 per cent . dipped their breast fea hers. As one

34

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dipping- motion night result in more than one part of the body being
dipped, I recorded the number of tines each part cf the body" actually
was dipped (Table 2). The number of times a bird dropped down to dip
is somewhat less.

None of the flights that ended with the bird landing on the beach
among birds sunning or resting included dipping the breast feathers,
while about 25 per cent of the flights that ended with the bird going
into the colony where eggs and young were present included dipping the
breast feathers (Table 2). This suggests that dipping the breast
feathers possibly has some relevance to nesting, something also
suggested by behavior I noted at the scrape itself.

Often during the hot midday hours, I saw a single bird fly from
its egg, not in a panic, and head toward the water (Table 3)« After
about, a minute it returned, flying low and rapidly toward the scrape.
As the bird settled back on the egg, 1 often could see that its bill,
feet, and sometimes the breast feathers were wet. Obviously those
direct flights from the egg are the start of the dipping flights that
I watched over the water. As dipping is most common when it is hottest,
it seems possible that this behavior is thermoregulatory, both for
the auult and the egg. Both sexes dip with equal frequency (139 to
137) so I have combined their records. Dipping by incubating adults
is most frequent during the first 15 days of incubation (Table 3).

Watson (1908) mentions an activity similar to dipping but says the
birds were bathing. Tne Ashmoles (196?: 6z) note that incubating
Sooties sometimes fly off to drink. Both Tompkins (19^2) and Hardy
(1957) have seen Least Terns dip their breast feathers during
incubation and suggest that it provides water necessary for the eggs.

Table 3, Tine of occurrence of flights away from the egg by adult
Sooty Terns

36

Number seer, during:

Time

First 15 days
of incubation

■Second 15 days
of incubation

Total

06:00-07:00

4a(^)b

1 (1)

5 (5)

07 :CG-0S :00

6 (3)

0 (0)

6 (3)

OS :00-09 :00

8 (7)

5 (4)

13(H)

09 :00-10 :C0

3K24)

4 (3)

35(27)

10 :00-ll ;00

3K25)

11 (8)

42(33)

11:00-12:00

24(18)

5 (5)

29(23)

12:00-13:00

21(19)

17(15)

38(3^)

13:00-14:00

24(20)

14(11)

33(3D

14:00-15:00

23(20)

4 (4)

27(24)

15:00-16:00

18(12)

3 (3)

21(15)

16 :0C-i? :C0

20(14)

1 (1)

21(15)

17 :00-l8 :C0

0 (0)

1 (1)

1 (1)

Totals

210(166)

66(56)

276(222)

a. Number of tiraes in activity records bird- were away frcn egg.

b. Number of ac a fl: its represented, i.e., one flight night
involve two ?:• more soot observations in activity records.

37

Tenre^atu-e Regulation

Howell and Bartholomew (1962), working on Midway Island in the

Pacific, show the delicate role that parental care plays in preventing
Sooty Tern eggs from approaching the high and probably lethal
temperatures they would reach in open sunlight. On Midway the adult
maintained the egg temperature above that of the surrounding air and
below that of the surrounding sand. Unfortunately they do not say
whether the adult is incubating or shading the egg.

I attached themisters to several eggs and monitored surface
temperature of the egg and air temperature at about 4- inches above the
ground for parts of several days. Unfortunately the eggs I worked
with were close to ry blind, and the adults caring for them were easily
disturbed by my isovements. Also the wire attached to the egg hindered
the tern in turning it and affected the bird's behavior.

I obtained useful information from a 24-day-old chick on 6 June
and conflicting information from the sane egg the next day. On 6 June,
in 23 temperature readings taken between 10:20 and 13:05, the egg
averaged 101. 5°F (range 99 to 105°F) and the air at k inches averaged
105. 5°F (range 103 to 108. 5°F). The highest egg temperatures occurred
when the adult left it to dip or flew off in a panic. In both cases
the adult dipped the breast feathers before returning to the e^. The
next day in 35 readings the egg temperature averaged 105. 2°F,
somewhat above the air temperature that day (105°F) and 3.7°F above
the egg's temperature on the 8th. Although the adult repeatedly left
the egg to dip, it seemed unable to lower the egg' s temperature. As
the egg cracked later that day and proved infertile, perhaps the heat
of decay foiled the adult's attempts to lover the egg temperature.

33

On the 6th the adult had been able to maintain the egg's
temperature some k°F below the air temperature. When the adult flew
off to dip, the exposed egg's surface temperature rose 1 to 2°F while
the bird was absent but then dropped 2 to 5°F within a few minutes
after the adult returned. The adult did not hold the wet breast
feathers against the egg, but held them right above the egg.

Although 1 have United information, I suggest that dipping the
breast feathers by incubating terns helps regulate either the egg's
temperature or humidity. Some adults dipped often during incubation
while others did so only rarely. If dipping the breast feathers is
thermoregulatory , it is puzzling that the tern does not hold the wet,
cool feathers directly against the egg. Possibly the water dripping
off the feathers onto the egg provides moisture needed by the egg.

Late in the breeding season adults continue to dip but seldom dip
their breast feathers. On 2? June 1970 when most chicks were 6 to 8
weeks old and few eggs were present, on Bush Key, nearly all of the
hundreds of adults flying out dipped only their bill or occasionally
their feet.

Nest Relief

Most of the 61 nest reliefs observed during incubation occurred
during early morning or late afternoon (Table k). Generally the adult
lands near its scrape and walks up to its mate on the egg; The bird
on the e?:g either steps off the egg so the second bird can incubate,
or else the newcomer pushes the incubating bird off the egg and then
takes over care of it. No elaborate ceremony occurs. The relieved
bird generally flies away within a few minutes, presumably to drink,
but often returns to the scrape and lingers for several hours or more

39

Table 4. Time of nest relief during incubation

Number of Hours of Exchanges/hour
Time exchanges Observation Observation

0.60
0.14

0.13

0.12

0.25

0.0?
0.09
0.29
0.09
0.20
0.70

0.35
0.6?
1.70
0.27

06:00-: 7

:00

13

21.52

07

: 00-O3

:00

4

27.92

03

:C0-09

:00

4

29.03

09

:00-10

:00

3

23.50

10

:0O-ll

:

5

19.33

11

:0O-12

z U J

l

13.33

12

: 00-13

:00

1

10.33

13

:00-14

:00

3

10.25

14 :00-15

:00

l

10.9?

15

:00-l6

:00

3

14. - -

16

:G0~17

:C0

12

16.93

17

:00-l8

:

6

17. 00

18

:00-19

:

3

4.50

19

:00-20

2

1.17

To-

tal

61

220.46

40

before leaving Bush Key. Sometimes this bird forces its way back
onto the egg so several exchanges occur before the new bird finally
takes over arid the other leaves the island. I never saw an adult
feed another one at a nest exchange during incubation.

Although I saw some exchanges in midday (Table 4), most birds
return to the colony in the evening and relieve their mate. then.
Instead of circling near Bush Key as they do earlier in the nesting
cycle, returning adults fly directly to the island and presumably
to their scrape. Some circle over the island itself, calling loudly
so that gradually the colony becomes noisier. The number of hours I
watched in early evening is much smaller than for other times.
Otherwise 1 certainly would have seen more exchanges then.

At Bush Key few adults regurgitate when mist— netted in the
morning or early afternoon, but many do so in the late afternoon,
indicating they just returned from feeding. The exchanges I saw
around sunrise may have been pairs making a final exchange after
several exchanges during the night, and the relieved bird was at last
leaving to feed. Much as when the colony is forming, the noise
gradually diminishes and by about 08:00 few birds are in the air.
Those that remain at the colony incubate quietly from about 08 :00 to
16:00.

Ashmole (1963) found nest relief most abundant from 21:00 to
06:00. He reasons that arriving at that time means that they fed during
daylight and then flew some 5 to 8 hours to arrive at the colony in
the middle of the night. I have no records of changeovers from 21 :00
to 06 :00, but the massive influx of terns early in the evening suggests
that changeovers are commoner then than late at night as they are on

41

Ascension island. If Ashmole's reasoning holds, then Bush Key Sooties
feed within a few hours flight of Bush Key at most and can easily
return vhen finished feeding.

Length of Incubation Shifts

To determine the length of incubation shifts (the length of time
one adult cares for the egg without relief), I recorded which adult
incubated each day for most of the nests I kept activity records on.
Usually I found that the bird present in the morning remained there
until at least late afternoon. Because most adults seem to return
and exchange in the early evening, I assumed this was true unless I
had evidence tc the contrary. I have computed incubation shifts as
multiples of 1— day periods. I may have missed a few exchanges but
the pattern I found is generally true of incubation shifts at Bush Key.
Watson (1908) also notes that most Dry Tortugas Sooties return in the
evening and that incubation shifts generally are 24 or 48 hours long.

Of 23I incubation shifts measured in this manner, 148 (64.1 per
cent) were 1 day in length, 70 (3O.3 per cent) were 2 days long, and
only 13 (5.6 per cent) were 3 days long. Figured differently, Sooties
did 45.3 Fer cent of their incubating in 1— day shifts, 42.8 per cent
in 2-day shifts, and 11. y per cent in 3— day shifts. Even though 1— day
shifts are more than twice as coercion as 2--day shifts, the two
contributed about equally to incubation.

Some pairs alternated daily throughout incubation, while others
alternated regularly in 2— day or, in a few cases, 3— day shifts. Others
seemed to follow no set pattern of relief. Overall on the basis of
these records, males spent son iiat more time (175 to 1^2 day's) caring
for the egg than did females, but the difference is not significant

kl

2
(X=l.o2, P>0.20). In the hourly activity records, naies again

predominated (880 to 752 hours), the difference being statistically
significant fc2=0.0. 04-, P<0.005). As the female left the egg shortly
after laying and the nale cared for it on the first shift, some of
this difference occurred then, but even excluding the records for the
first day of incubation, the nale still cared for the egg more than
the female.

Watson (1903) shows that incubation shifts on the Tortugas average
somewhat over a day long with the longest a little over 3 days, well
within the range I found in 19 68. On Ascension Island incubation
shifts average I32 hours (Ashmole, 19 63), over twice as long as those
on the Dry Tortugas. On Christmas Island in the Pacific, incubation
shifts are about 7 days long (Ashmole and Ashmole, I967). Incubation
shifts vary from 2 hours to 3 days on the Seychelles (Ridley and Percy,
ic<8l

The striking differences in incubation shifts between Bush Key
Sooties and those at two other colonies are probably related to food
availability and distance from the island. Ashmole (1963) reports
many young at Ascension dying of starvation one year, apparently
because of a failure in their food supply. On the Dry Tortugas Sooty
Tern chicks have never been known to experience heavy mortality from
food shortage (Robertson, 19c4). No evidence exists to tie the almost
total nesting failure at Bush Key in 1969 to food. The shorter
incubation shifts seem to indicate that the terns have an adequate
food supply near the island.

Influence of Weather

The most obvious effects of weather on incubation patterns of Sooty

43

Terns are the shifts from incubating to shading and back tc incubating,
depending on the environmental temperature (Figure 3). Sudden changes
in weather provided natural experiments on weather effects. On hot
days when clouds rapidly lower the air temperature, shading birds
quickly change to incubating or move away from the egg to preen or
perform other maintenance activities. Once the clouds pass and open
sunshine returns, the birds again shade the egg. On normal, clear hot
days an adult virtually never leaves its egg between 11:00 and 15:00
except, to dip. The few records I nave of terns performing other
maintenance activities during those hours nearly always occurred en
cloud;/' days.

Rain also changes activity patterns. At the start of a rainstorm,
swarms of Sooty Terns rise ana circle over the colony, calling noisily.
As nearly all birds that are caring for an egg incubate throughout the
storn, it is primarily free birds that circle ever the colony.

Apparently Sooty Tern feathers are not water repellent and are
easily soaked by rain. After rain Sooties preen their body and flap
their wings vigorously. This apparently helps them dry the feathers
to permit flight. Birds have great difficulty taking off in early
morning after a heavy dew has soaked their feathers. The flocks cf
terns rising at the start of rainstorms are probably birds getting
airborne before they become too waterlogged. Once in the air, many
remain there until the storn is ever, the motion of their wings
preventing them from getting soaked.

Fledging Period

Sooty Terns incubate their eggs 29 cr 30 aays (mean 29 days 12.3 —

2.k hours, range 28 days 22 hours to yo days for 16 eggs). This agrees
well with Ashmole (1963) who found that usually the egg is incubated
28.5 to 30 days. Watson's (1908) figure of 26 days seem somewhat
short., though Ridley and Percy (1958) say Sooties incubate for 26 to
29 days on the Seychelles.

The chick may pip the egg as much as 3° hours prior to hatching,
but usually does so only the day before hatching. For eggs that I
could determine the tine of hatching exactly, most hatched around
sunrise or around neon.

Adults attend pipped eggs very closely and seem reluctant to leave
them. The only time that Sooty Terns ever actually mobbed or struck me
in the colony was when eggs ver-e hatching. Howell and Bartholomew (1962)
show that such an egg is particularly sensitive to heat stress and has
no effective means of dissipating excess heat. Thus care by the adult
is especially important to insure the chick's survival.

Adults appear indifferent to the presence of the empty egg shell
in the scrape and sometimes continue to sit on it after the chick has
emerged. About 20 times 1 saw an adult pick up and carry off a piece
of egg shell, often one from a neighboring scrape. Usually adults
allow the empty shell to roll around until it is finally broken and
crushed.

Sooty Tern eggs (like those of most terns) are speckled and
camouflaged on the outside, whereas the white inside lining contrasts
sharply with the sand background of the nesting colony. Tinbergen ez al.
(l9o2) show that rapid removal of the empty shell has definite survival
value for Black— headed Gull (Larus ridibundus) chicks, in a colonial
species like fuscata. removing the egg shell probably makes little

45

difference to chick survival. Any predator that reached the colony
would have little trouble finding a chick, whether the white inside
lining of an empty egg marked the scrape or net.

Attentiveness

My data en parental care of chicks are less complete than these
for care of the egg. Every time 1 entered my blind, the chicks more
than a few days old scattered and hid under the nearest cover. It
sometimes took several hours for then to return to their scrapes where
I could watch then again, and some never returned.

The records I do have, summarized for the first 2 weeks of the
chick's life, indicate that the adults are most attentive during the
midday hours, much as they were when caring for the egg (Figure k). As
in incubation, adults rise up off the chick to shade it in hotter
hours of the day and brood it when it is ccol. Fenales care for the
chick somewhat r.zre than males (157 bird—hours to 132 bird-hours), but
statistically these are net significantly different (X?-2. 16, P>0.10).
I found little difference between males and females in care of chicks
and I have combined the records in Figure 4. .

Generally adults are less attentive when caring for the chick than
when caring for the egg. They often stand to the side of the chick
for an hour or more, and their attentiveness decreases as the chick
grows older. After the chick is about 3 weeks old, the adult spends
very little time actually caring for it other than feeding it, although
they may stand near it during the day. From then on cricks even seem
to spend the night unbrooded cy an adult.

£* ; ■« ,- j

Figure k. Activities by adult Sooty Terns during the
2 weeks of caring for the chick.

h-7

w

1'

•r-1

tr>

-P

c

tn

•H

•H

c

>

T3

■r-4

u

■ri

0

'u

o

Jj

0

fO

^

a

u

rC

-U

(0

ffi

CO

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o
o

co

V-.

\

I

c
o

o
o

o
o

o

Q

o
a>

-H

E-<

■»-..

o
o

o

o

o

O

O

o

CM

o
o

co
c

o
o

o

^uao.iaj

k8

Feeding

Chick care centers around the provision of food. Sooty Terns
apparently feed almost exclusively on fish and squid (Ashmole, 19&3;
Ashnole and Ashmole, 1967). Much of this food is caught \<hen schools
of tuna, mackerel, or other large predacious fish drive smaller fish
to the ocean surface. There the Sooties dip down to seize food from
the surface or in the air above it, but seldom dive headlong into the
water for their prey as most other terns do (see Ashnole and Ashmole,
1967).

In the detailed study by the Ashmoles (19o7) on Christmas Island
in the Pacific, fish make up about 60 per cent of the food items and
squid ^0 per cent, with the figures reversed when volume is measured.
Flying fish (Exocetidae) and tuna (Scombridae) are two of the most
important fish along with snake mackerel (Gempylidae) and
bonnetmouths (Eramelichthyidae).

On the Dry Tortugas Sooties have been reported eating fish of the
families Carangidae and Clupeidae (Watson, 1906). A variety of
regurgitated fish have been found in the tern colony (Longley, 1929;
Longley and Hildebrani, 19^1; see Erdman, I9&7) but none of these lists
separates what was taken specifically by Sooties. In recent years
Robertson has collected many fish and squid that Sooties and Noddies
regurgitated when mist— netted. At present only the Scombrids have
been analyzed (Potthoff aid Richards, 1970).

I saw one chick fed about k- hours after hatching, and they may be
fed even earlier. The adult stands near the chick and points its bill
down. The chick then either grips and bites the adult's bill or else
pecks at it. The chick may also beg with some rapid "cheep" calls.

^9

This action seems to release regurgitation by the adult. The adult.
stretches its neck and head upward, tips the bill down, gags, and then
brings up a fish. Tae adult nay hold the fish in its bill before
feeding the chick, or it nay slide the fish directly down the bill and
into the chick's mouth. Sooty Terns were never sesn returning to the
colony with fish in their bill, but always carried the fish in their
esophagus and regurgitated it for the chicks.

Older chicks seemed to beg more by opening their bills and giving
rapid "cheep" calls than by pecking at the adult's bill. They also
grabbed at the fish while the adult was still holding it, sometimes
leading to tugs— of— war between the two.

In nearly every case where I could see clearly the exchange of
food from adult to chick, the fish was regurgitated and passed to the
chick tail— first, Many of the fish appeared to be scaleless, and in
some cases they were partly digested or fragmented, especially the
last ones passed to the chick (presumably the first ones the adult
caught). The regurgitated food is often coated with mucous. This may
retard digestion of the food as the adult carries it back to the colony
(Ashmole and Ashmole, 196?).

A chick might get six or more fish in one feeding, depending on
the size and degree of digestion of the food. Several times I saw an
adult feed a chick six times in 3 to 4 minutes, each time passing one
fish. Other tines a whole meal might be one large bolus containing
several fish.

Of 34-9 occasions when I definitely saw a chick fed, the majority
occurred from 06:00 to 10:00 and from 16:00 to 18:00 with many of the
rest occurring just before or after these periods (Table 5). Fassing

50

to

CD-
En

>

o

c
s
o

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hi)

c

CD
«H

<H
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<U
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•H

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■a

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5?

5
o

b

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3

W

bfl

B fn

•H 3

"d o

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0)

fc

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CO to

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3 C

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ON

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rH H CNJ

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ON 0O NO NO -^ -H;

CNJ ON

O- ON UN CO -3" ON

CNJ H r-\ CN rH

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CNJ

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o

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CNJ

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|H

NO

rH

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H

51

several fish to a chick in the space of a few ninutes is counted as
one feeding. As the nunber of observation hours varied during the
day, the nunber of feedings per hour of observation (Table 5, last
column) is a mere valid comparison. This shows that peak chick feeding
occurred in late afternoon and early evening with a smaller peak early
in the morning. These observations were made from 16 May to 10 July,
the tine when chicks were present on r>y plot.

In about 5 per cent of the feedings, both adults fed. the chick; far
more commonly two adults were present but only one actually fed it.
Of 240 cases in which I knew the sex of the bird feeding the chick, the
nale did so no re often than the female (133 to 102) and the difference
is statistically significant (7-~=5.4C, P<0.025).

In addition I recorded sone 240 occasions where a chick begged
and an adult tried to feed it without actually doing so. Sometimes the
adult regurgitated and held food in its bill and then reswallov;ed it,
even when a chick begged loudly. The adult night do this several tines,
but usually eventually fed the chick. Other tines an adult tried to
regurgitate but was unable to bring up any food.

Tne adult places all food directly in the nouth cf very snail
chicks. Any that drops on the ground renains there unless an adult.
picks it up to eat. or feed to the chick. The youngest chick to pick
up food from the ground was about a nonth old, but they nay do so when
younger. These older chicks pick up food fron the ground on scrapes
other than their own. Once as an adult held a fish in front of a chick,
I saw another adult seize tne fish and swallow it.

52

Freauency of Feeding

Sooty Terns on 3ash Key feed their young infrequently enough to
make it difficult to obtain good information on the rate of feeding.
Although I watched many chicks continuously for long periods, I have
few good records of an adult feeding it, leaving it ^o forage, and
returning to feed it again. A major complication is that rather than
immediately giving a chick all the food it has in its esophagus, an
adult may feed the chick several times over a period of several hours.
I saw cne adult regurgitate and feed its chick some 5.5 hours after it
returned to the colony. Thus if an adult fed a chick, flew away and
returned in k or 5 hours and fed it again, I could not be sure if the
adult had left the island to forage in that time or if it had just
loafed elsewhere on Bush Key before returning to the chick. Consequently

1 have determined the rate of feeding two ways.

I have five gocd records of an adult leaving the chick and
returning later to feed it. These were all made en 26 May with chicks

2 to 8 days old. The length of time the adult was absent (foraging
time) averaged 3-87 hours and ranged from 2.63 to 5. 3^ hours.

I also tabulated the number of times chicks were fed during long
periods of continuous observation, separating these into chicks up to
15 days old and ones 15 to 27 days old (Table 6). After listing the
total number of times the chick was fed, I subtracted those extra
feeding" that occurred when an adult fed a chick more than once after
returning to Bash Key. This gives the probable number of foraging
trips made per bird— hour of observation and gives a rough estimate of
the time an adult spent foraj;ir;^, about 3.^ hours. Apparently
foraging trips are about an 'nour longer when the chicks are older,

53

Table 6. Rate of foraging by adult Sooty Terns

o~- *o

Number Probable Number of

cf number of lira— hours hours per
Age of chick feedings foraging trips of observation foraging trip

1-15 days

144

113

.6-27 days

20

19

Totals

164

132

369. 42

3.2?

84. 58

4.45

454.00

3.44

5^

but the sample size is small and the adults nay just loaf part of the
tine. Note that here tine is in bird— hours of observation whereas in
Table 5 it is hours of observation, during each of which I watched
about 15 chicks.

The two estimates are close enough to indicate that Sooty Terns
at Bush Key probably forage 2 to 5 hours when they are feeding their
chicks. Adults seen to pass food to older chicks more rapidly after
they return to Bush Key. That is, rather than standing with the chick
and feeding it several times in a few hours, they seen to feed it all
the food they have and then leave it.

Often I saw exchanges that appeared to be after foraging trips of
h to 8 hours but the birds may have exchanged in short intervals when
I was not watching. Thus my records of foraging time are biased for
shorter periods. Watson (1908) gives k to 7 hours as the interval
between feedings by Dry Tortugas Sooties, consistent with my findings.

Sooty Tern chicks on Bush Key are fed much more frequently than
those in other colonies that have been studied. On Ascension Island
chicks are probably fed about once a day (Ashmole, 1963). On Christmas
Island chicks are probably fed every 2 or 3 days (Ashmole and Ashmole,
1967) but good information is lacking.

As the chicks grow, the adults become less attentive and often
gather by the hundreds en the Bush Key beaches and sun nuch of the day.
Other adults soar over the island during the hotter tines of day (see
p. 6l ). Perhaps adults away from their chicks spend part of their time
in these groups. They must spend some of their tine foraging but
exactly how much is difficult to determine.

At Bush Key more adults are present and the colony is noisier in

55

the evening than at any other time of the day. Hence I think that both
adults are probably at the colony at night, though both nay not be
with the chick at all times. Elsewhere Sooty Terns feed at night at
least occasionally (Bruyns and Voous, 1965; Gould, 1?6?) , bub at Bush
Key, with the noticeable influx of birds in the evening and exodus in
the morning, I doubt that many adults feed at night. As during
incubation, mist-netted birds regurgitate far more frequently in the
evening than at other tines.

The average flight speed of Sooty Terns is about 27 mph (Schnell,
unpublished). Thus on longer trips they nay forage as much as 1G0
miles from Bush Key, but the average foraging trip of about 3.5 hours
gives them a range of about k7 miles. Booties from Bush Key usually
fly toward the west when they leave the colony and return from that
direction. How far they go is unknown but they easily could fly to
the nearby Florida Current and feed there.

At about 3 weeks of age, the chicks begin to wander from their
scrape. They may spend much of the day alone, being accompanied by an
adult mainly when being fed. Some adults seen to check on their chick
periodically during the day, landing beside it, possibly feeding it,
and then flying off only to return in an hour or so„ These adults
probably spend much of their time loafing on the beaches or soaring.

I was not able to visit Bush Key during the height of Hurricane
Abby (3 June) but on h June it was obvious that the tern's normal
schedule had been disrupted as few adults were present until that
evening when they started to stream into Bush Key and continued to do
so until the evening of the 5th. They probably had been unable to
feed during the storm and had left as soon as possible to obtain food

56

for themselves and their chick. Mason and Steffee (i960) noted a
similar disruption after Hurricane Alma.

Individual Recognition

As the chick grows, its physical appearance changes and it may
wander farther from the scrape. As the colony has thousands of chicks,
adults undoubtedly have problems locating and feeding their own (or
adopted) offspring. Sooty Tern chicks and adults evidently learn to
recognize each other so the chick can be located and fed.

Much as Lashley (1915) and Burckhalter (1969) report, I found
that parents apparently do not recognize their chicks individually at
first. In the first 4 or 5 days chicks often return to the wrong scrape
and are readily accepted and reared by foster parents. After that they
are pecked savagely if they intrude near another scrape, and some are
killed. Thus adults seem to recognize their chicks when they are about
4 to 5 days old. As Davies and Carrick (19o2) and Hutchison et al. (1968)
suspect for other terns, this recognition probably is based on calls
between the adults and chicks.

After a disturbance, typically the adult tries to attract its
chick back to the home scrape with a combination of bill movements and
low calls. The call is a low two syllable "kraa-unk" note accompanied
by bowing movements of the bill and head, ending with the bill
pointing down toward the breast feathers. Often an adult used such
behavior to lure the chick back to the scrape, backing away from the
chick and toward the scrape until the two were finally back at their
own scrape. Once I watched an adult hold a fish in its bill and back
away from its chick, apparently using the fish t,o lure it back to "ohe
scrape.

57

By 3 weeks of age nost of the chicks en my plot spent much of
their tine hiding under the nearby bay cedar. By removing most of
the vegetation from the nesting plot I nay have forceo them to move
to this cover. Typically these older chicks emerged fron the bay
cedar late in the afternoon and stood at its edge or ran oat to their
scrape. The chicks ran swiftly to a spot and stopped, seemingly
knowing where they could stop and not be attacked by other chicks
and adults. Once on the scrape, they stood and waited for an adult to
cone and feed them. Whenever 1 caught one of these chicks, it was
always at the scrape where it had been reared and thus had "homed"
correctly. Also when an adult fed the chick, normally the adult was
one of the pair that had used that scrape. Hence the chicks apparently
knew their heme scrapes and returned there to be fed by a parent.

Sonetines the adult landed at the scrape before the chick arrived.
Then the adult typically circled 10 to 15 feet over the scrape and
delivered loud "wid— ik" or "ka-wiri-ik" notes from the air until it
was answered by a loud piercing "che— up" call from a chick hidden under
vegetation. The two birds exchanged calls several times and the adult
landed. The chick then ran out to the adult, begged, and was fed. As
the chick approached the adult, the adult often gave what appeared to
be a greeting, flying straight up 2 to 5 feet in the air, giving a
loud "wide— a— woke" call at the peak, and dropping back down to the
ground to feed the chick. These "fly-ups" occurred in other situations
and are discussed later. Occasionally the adult pecked the begging
chick sharply. I was not able to determine the relationship of the two
in these cases, but I suspect that the twe had made a mistake in
recognition and the adu] t lid not realise it until the chick came close.

53

Although chicks are usually fed by their parents (or by adults that
adopt then), at least eight tines I saw a tern feed a chick other than
its own. In five of these instances an adult that had lost its chick
or whose egg had failed to hatch fed a chick, usually at an adjacent
scrape. Three tines an unmarked adult fed a chick whose parents were
both marked and thus recognizable. .

I also saw seme chicks that seemed to approach and beg to any
adult that came near. Although these chicks were not narked, 1 am
fairly sure that some were fed by adults other than their parents.
Often the adult pecked the chick and drove it auay, but often the adult
tried to regurgitate. Occasionally it was successful in bringing up
some food that it then fed the chick. Possibly these chicks had lost
their parents in the shuffle of chicks in the first few days of life,
and no adult recognized them as its own. The adults that fed then or
tried to feed them nay have been their lost parents, or other adults
that had lost their chick or egg.

Thus older chicks seen to find their parent both by knowing where
the home scrape is and by recognizing the adult's voice. Although I
think Sooty Tern adults and chicks do recognize each other
individually, I believe that much of this recognition is done by the
adult, while the chick may try to get food from almost any adult that
comes near.

Chick Behavior

Sooty Tern chicks can walk almost immediately after hatching and
stand and beg for feed within 4 hours. For the first few days they are
closely brooded by one of the parents, the chick resting either
between the parent's feet or crossways in front of then. At first

c

9

they walk by half crouching with the body low and almost on the ground,
but they soon walk upright like adults. After a disturbance these
small chicks frequently end up in the wrong scrape. I often saw two
small chicks under one adult that seemed to accept both of then. The
extra chick eventually returned to its own scrape, usually when
called oj an adult. A common alarm response of these small chicks was
to lie flat on the ground with the head and bill extended forward.
Chicks apparently did this to avoid being pecked by adults, as adults
pecked viciously and sometimes killed strange chicks that came close.
Chicks 5 days old preened and voided with movements typical of adults.

For the first 2 weeks, one adult nearly always stays at the
scrape with the chick. Ehring the 3**d week this attention gradually
diminishes, and by the end of the 3^d week the chick is often alone.
Chicks defend the scrape, pecking at and driving off other chicks and
Brown Noddies that intrude.

Most chicks stay close to their scrape until they can fly, but
older chicks that wander or are moved can find their way back to the
scrape when displaced several hundred feet (Burckhalter, 1969). Chicks
from scrapes close to the beach may sun and rest with adults in large
flocks en the beaches during the day, and then disperse in late
afternoon. Generally the chicks congregate just above water level and
the adults assemble higher on the beach. Perhaps the sand is cooler
there than, higher on the beach. I did not see adults feed chicks
in such flocks. Apparently chicks return to thexr home scrape or
some other nearby place to be fed.

1 observed 6-week— old chicks jumping into the air and flapping
their wings vigorously. I lack exact records of when they begin to

6o

fly, but one chick flew several hundred feet on 4 July and others
were doing so within a week. If that chick had hatched from one of the
eggs laid on 1 April (it was in that part of the colony), it would
have been about 9 weeks old, the best estimate I have for the age
Sooty Terns begin flying at Bush Key. As noted by-Ashmole (1963),
chick development varies with the food supply. One year he found
chicks at Ascension flying when about 8 weeks old and the next year,
when food apparently i/as scarce, birds around 9 weeks old still could
not fly. Burckhalter (1969) saw 8-*;eek— old chicks flying on the
Hawaiian Islands, but he says they stayed at the island another 2 to
3 weeks..

Most of the chick's activities before it starts flying seen to
be related to feeding and temperature maintenance. Obviously the adults
help with the latter when the chicks are snail, but 3~ to ^—week-old
chicks start assuming their juvenal plumage and are largely independent.
When placed in open sunlight, their body temperatures rise to the
lower range of black bulb temperature and then level off (Howell and
Bartholomew, 1962). Chicks may help stabilize body temperature by
panting,, facing away from the sun, drooping the wings, and erecting
the back feathers, much as adults do when it is hot. A few of ny
tethered chicks died in open sun when about this age, apparently from
heat stress. Thus body temperature must closely approach lethality
when c'nicks are exposed to open sun.

Juveniles seem to leave the colony soon after they learn to fly,
as there seldom are many flying juveniles at Bush Key at any given
time. As they do not feed near Bush Kev, little is known of their
activities. In all probability, the adults stay with the young and

61

continue to feed it until :_;. c?j\ capture food by itself (Robertson,
1964; Burckhalter, 196?) as do adult Royal Terns (Ashraole and Tcvar,
1965), Caspian Terns iB-crc.rc-ne cassia) (Jozefik, 1969), and Elegant
Terns (Thalasssus elexans) (Monroe, 1956). Bush Key ad-alts cannot ~
feed flying Sooty Tern young more than about 2 months though, as the
juveniles leave the range of adults by about October (Robertson, 1969).
This certainly nust be a difficult time for young Sooties as they must
learn to capture food while en the wing, a skill requiring good
eyesight, coordination, and timing.

Other Aerial Activities
Thermal Soaring

Sooty Terns at Bush Key commonly scar in dense circling columns
during midday hours en hot days. Possibly at these times updrafts or
thermal s develop near Bush Key. The columns approach 200 feet in
diameter and sometimes go from just above sea level to several hundred
feet in altitude, although most birds are usually below 200 feet.
Within the columns the birds circle as they climb and then soar off to
join another column. Most common late in the breeding season, these
columns of scaring birds are taller than the circling flocks seen
earlier, and the birds also appear to fly faster than in the early
season flocks. Usually the terns are silent as they soar but
occasionally one gives a "wid— ik" call.

Soaring often continues for several hours although the composition
of the flock changes as birds join or leave it. The columns often
move laterally and two nay merge into a single column. Magnificent
Frigatebirds (Fregata magnificens) and occasionally Brown Pelicans
(Pelecanus occide l . las) and Laughing Gulls (Larus atricilla)

62

wn

(Harrington, pers. conn.) join these flocks, but I did not see Bro
Noddies do so.

Fly— uos.

A rather common activity in Sooty Tern colonies, especially when
chicks are present, is for one or more adults to fly straight uP 1 to
5 or more feet, give a loud "wideawake" call at the peak, and drop
back to the ground. I call these flights "fly-ups," and 1 believe they
are a type of social behavior, often used in greeting. Besides the
birds that actually fly, other birds around then often raise their
wings vertically over the body in a flight intention movement, but do
not fly. These fly-ups usually involve a snail group of terns. During
a fly-up, other nearby birds becons very active and noisy for 15 to
60 seconds and then gradually quiet down.

During incubation, 1 saw few fly-ups. They usually occurred
when an adult arrived at the colony to assume care of the egg. Other
adults near the arriving tern's scrape might fly up, seemingly in
excitement over the arrival of another bird at the colony. Burckhalter
(1969) also saw this group activity when an adult arrived at the colony.

Once the chicks were present, I often saw fly-ups when an adult
lanced to feed a chick or had started feeding it. Again several adults
flew up in the air in a sudden burst of activity and then quieted

down.

Fly-ups were rather common when the chicks got fairly large. Then
I often saw adults fly up when a chick ran out from cover and approached
then. As the chick approached, the adult flew up, called and then
landed to feed the chick. In these instances the chick seened to
stimulate the n.^P- Several tines I saw a chick run past several

63

adults, each one flying up in turn as the chick approached, until the
chick finally came up to an adult that fed it. Thus the adults
seemed to react as if the chick v/as their own while the chick went by
and approached another adult, presumably its parent, to be fed.

Panics

Sooty Terns exhibit two distinct types of disturbance flights,
commonly called panics. Of these, dreads apparently include the flights
Palner (194A) calls dreads and panics for Common Terns while those
termed alarms are similar in both species.

Alarm. — When an intruder openly approaches the colony, the birds
stand upright with the neck and head stretched vertically. As the
intruder comes closer they fly, calling as they do sot and then circle
and hover over the intruder until it leaves the vicinity of the
scrape. Unlike many other terns, Sooties seldom actually strike an
intruder, but rather hover near it or dive at it without striking it.
I did see Scoties attack Cattle Egrets (Bubuicus ibi s ) and once a
Purple Gallinule ( Porphyrula martinica) that had broken a tern egg.
Then the Sooty landed on the gallinule and jabbed it. The few -times
a Sooty Tern struck me occurred about the time the eggs were hatching.

A leud, long alarm call, usually a downward inflected "kee aa"
or "kerr aa" often precedes the alarms. Most alarms are quite local,
involving relatively few birds in the colony. Occasionally though,
they spread throughout the colony and virtually ail of the terns
fly. Alarms seem to start from an intrusion that is not sudden,
but anticipated for a short time. The circling and calling by Sooties
during rain are probably just a form of the alarm. Other disturbances
are caused by less tangible factors such as a loud sound (e.g., sonic

64

boon-., boat whistle) cr a. sudden movement. These elicited a different
response — the dread.

Dread. — In dreads the terns suddenly become silent and fly rapidly
from the colony to the water, darting and swooping as a unit silently
down low over the water. At the end of the swoop, they rise up,
start calling loudly, and gradually drift back to the colony. Thus if
a bird is on the ground when the dread starts, it flies rapidly out
over the water, and those birds that are already in the air suddenly
swoop out over the water. This seems to be a high intensity form of
panic,

One final, zoorly defined alarm reaction is that in which virtually
the entire colony flies up from the ground, either at once or, more
often, in a gradually spreading group from one end of the island to
the other. In the air the birds call loudly and drift out over the
water and then gradually move back to the colony. This delayed alarm
seems to start as an alarm in one part of the colony, and as those
birds fly, zhey scare up birds near them and so on until the whole
colony is in the air.

ECOLOGY

Interactions wi th Other Scecies
Enemies

As they nest on isolated islands, Sooty Terns cone in contact with
relatively few vertebrate predators. Those predators that reach the
nesting colony find eggs and chicks plentiful and vulnerable.

Adult Sooty Terns are fast and shifty in flight, and probably
few other birds are capable of capturing a flying adult. Certainly the
most serious natural predator on Bush Key is the Peregrine Falcon
(Falco peregrinus). One and occasionally two peregrines were present
at the Dry Tortugas frcn S to 11 April 1963. I saw a peregrine sT-.cop
at Sooty Terns 23 tines without capturing a bird, but I did find the
decapitated remains of four Sooties that the falcon had almost
certainly killed. 1 have seen peregrines capture incubating Sooty
Terns on Little Tobago in the southern West Indies.

On 15 May 1968 a Purple Gallinule cracked an egg and ate the
contents until a Sooty attacked and drove it away. In June, 19&9»
Ruddy Turnstones Qr-^ra.ria interpres) broke open and ate the contents
of several unattended eggs from which I had frightened the adult Sooty
Tern. Normally the adult terns sit tight if a tarr.stone approaches,
but turnstcnes can cause considerable mortality anong unattended eggs.

The Magnificent Frigatebird is perhaps the greatest threat to
Sooty Tern chicks. Frigatebird predation must vary considerably from
year to year as in 19 6? I saw one capture a Sooty chick only once

65

66

(3 June) while other years they have taken many chicks (Beard, 1939;
Sprunt, 1948). When frigatebirds do prey on tern chicks, it seens to
be a prey preference of only a few individuals rather than of all the
frigatebirds present (Robertson, pers. comm. ). On 24 May 1968 a
frigatebird chased a Sooty Tern adult. They may occasionally steal
food from the terns. Ashmole (1963) reports Fregata acuila talcing many
young Sooty Terrs on Ascension Island and F. miner does the same on
Christinas Island (Ashmole and Ashmole, 196?).

Frigatebirds usually take only snail chicks, mainly in open parts
of the island where they can not find escape cover. Thus the relatively
thick vegetation on Bush Key in 1968 probably discouraged frigatebird

predation.

Predation by rats (Rattus rattus) also must vary from year to
year. I saw no evidence of predation by them in 1963 but in other
years they have been a serious predator (Russell, 1933) on Bush Key.

Several times an Osprey (Pan si 0.1 haliaetus) circled over Bush Key
but usually the terns ignored it. Once several terns mobbed an Osprey.
A few other times the birds panicked, but it nay not have been due to
the Osprey1 s presence. The only other hawk I have seen cause any alarm
was a Broad-winged Hawk (Buteo nlatynterus) that circled Bush Key on
lo June 1969 and apparently started several panics. Other hawks
migrate through the Dry Tortugas and occasionally may disturb the terns.

Cattle Egrets occasionally take a Sooty Tern egg or click on
Bush Key (Robertson, pers. comm.) but more commonly just disturb them.
Ridley "and Percy (195S) report that Cattle Egrets often take Sooty Tern
eggs and chicks en the Seychelles. Of the Cattle Egrets that migrate
through the Dry Tortugas, those that linger generally are unable to

67

find enough food to maintain their flying strength and eventually
starve to death. This shortage of food apparently forces then to
feed on almost anything they can find as they often take snail migrant
passerines (Cunningham, 1965; pers. obs. ). They also feed en fish the
terns drop and on insects, disturbing the terns as they feed.

Typically as an egret approaches a tern with an egg or chick, the
tern faces the egret and gives a series of low, hoarse "wuk wuk" notes.
As the egret ccmes closer the tern raises up off its egg or chick,
erects the feathers on the top of the head and continues to direct
"wuk" calls at the egret, the calls becoming leuder and none rapid and
often changing tc a rapid "ka ka ka" call. If the egret ccrec within
a few feet of the tern, the tern flies from the scrape and circles
over the egret until it departs. A few tines a tern stretched its
head and bill forward, raised the wings almost vertically ever the
back, and ran at the egret. Sooties sometimes dive at an egret but
they seldom actually strike it.

Cattle Egrets often fly low over nesting terns, eliciting a long,
drawn out "kaaa" note from the terns and sometimes starting a panic.
The Sooties often chase egrets in the air and dive at then, mobbing
them more commonly ir flight than when the egret is on the ground.
Brown Noddies frequently join the Sooties in these attacks on Cattle
Egrets and the Noddies are much more aggressive, striking them on the
ground and in the air, and chasing them farther than the Sooties do.

Cattle Egrets harm Sooty Terns mainly by disturbing then and
keeping the adults away from the egg and chick, which can be fatal to
the young tern. The Great White Heron (Ardea Occident, alls) has been
reported preying on Sooty Terns on the Tortugas (Robertson, l?c2).

68

A few Herring (Laras argentatus) , Ring— billed (L. delawarensis) ,
and Laughing Gulls frequently linger around Bush Key. Gulls commonly
prey on terns (Hatch, 1970), but I never saw one enter the colony at
Bush Key to prey on tern chicks or eggs, although they could do so
easily. Watson and Lashley (1915) saw Laughing Gulls prey on Sooties
on Bird Key.

Adult Sooties nay peck and kill chicks that wander away from
their scrape and can be a serious cause of mortality.

Ghost crabs (Ocypode quadrata) occasionally prey on tern chicks.
A few times a tern gave a "wuk" note as a crab scuttled by or pecked at
it but usually they ignored the crab. They also seemed to ignore the
hermit crabs (Coenobita clypeatus) that probably feed only on dead
chicks and broken eggs.

Although most Sooty Tern colonies are on relatively isolated
islands, man reaches many colonies at least occasionally and adversely
affects nesting success, Tern eggs are still collected for human
consumption at some colonies (Cott, 195^; Ridley and Percy, 195°), but
National Park Service protection has curtailed egging at Bush Key.

It is much more difficult to measure mortality caused by human
activity in the colony, but at times such activity must be very
harmful. Every time I entered or left my blind, I disturbed the birds.
Although they soon habituated to this and rapidly returned to the scrape
once I was out of sight, I still caused mortality, both by stepping
on eggs and by separating chicks from their parents. Such mortal! ty
is inevitable whenever a human works extensively in a Sooty Tern colon ',

Other disturbances such as sonic booms, boat whistles, and low
flying airplanes may cause the birds suddenly to leave the island in a

&9

dread. However I did not see any of these disturbances keep the birds
away iron their scrape for more than a few minutes unless they were
repeated.

Other Terns

The interrelations of Sooty Terns and Brown Noddies on Bush Key
are interesting. These two species are abundant and widespread in the
tropics and nest together on many islands. On Bush Key the Brown
Noddies nest primarily along the edge of the bay cedar, but also in
other low vegetation and occasionally on the ground. Although some
Sooties nest under the bay cedar, they always nest on the ground and
hence are usually vertically segregated from the Noddy nests. Most
Sooties nest in the open fiat parts of the island outside the bay
cedar, again separating then from the Ilcddies. Some Noddies nest in
low vegetation within inches of Sooty scrapes. I often saw young of
both species in such situations, so apparently they can breed
successfully in close proximity.

Early in the nesting season Noddies often searched for nest
material en my study plot. In all 35 instances of direct supplanting
between these species that I saw en r^- study plot, the Sooty drove off
the :''oddy. In two of these a Sooty chick drove an adult Noddy off the
plot. Generally the Sooty stretched its head and neck forward, held the
wings either tight against its sides or slightly away from the body,
and advanced toward the Noddy until the Noddy retreated or flew. I
never saw a Noddy fight back or withstand the attack long. Once I
heard a Sooty give a low growling "urr" note as it advanced at a Noddy
but usually both were silent. The few tines I watched for the reverse
encounters, Z saw Noddies drive off Sooties that had cone too close

70

to Noddy nests.

Noddies arrive at Bush Key at about the same time as Sooties,
but begin work on their nest almost at. once rather than having a
pronounced period of flocking as Sooties- do. In 1968 Noddies started
laying somewhat after Sooties (10 April) and as their incubation
period is longer than the Sooties' (35 to 36 days, Thompson, 1903) *
Noddy eggs hatched later than the Sooties' eggs. Young Noddies
mature faster than Sooties and many were flying by late June, well
before most Sooty chicks.

Both species feed in a similar manner, dipping down to capture
food at the ocean's surface, although Noddies occasionally plunge
to the surface or rest on the water to capture food. The Ashmoles
(I967) show that Sooties and Noddies have very similar diets on
Christmas Island, but the Noddies seem to feed much closer to the
island. They also note that the wing of the Noddy, broader than
that of Sooties, perhaps gives it greater maneuverability and allows
it to forage more efficiently near the island, whereas the narrower
wing of Sooties enables them to forage farther from their colonies.

At the Tortugas Noddies often feed within sight of Garden Key,
dipping down and capturing food at the ocean's surface. Noddies
have incubation shifts of 3° minutes to 5 hours, much shorter than
Sooties. They feed their chick every 2 to 4 hours (Watson, 1908),
slightly less often than Sooties. The fact that Sooties virtually
never feed within sight of Bush Key and the somewhat different foraging
times, especially during incubation, suggests that the two feed in
different areas, much as they do on Christmas Island. The two species
apparently feed in different zones at other colonies in the Pacific

71

and Indian Oceans (Ashmcle and Ashmole, 1967: 65—66).

The few hundred Roseate Terns at the Dry Tortugas generally nest
in habitat quite different from, that used by the Sooties. In recent
years they have nested in the coral rubble on Long Key and on the o-cen
sand on sone of the other islands. When they have nested on Bush Key,
it usually has been at the far east end away fron most Sooties.
Roseates usually arrive later than Sooties and in 19 68 did not start
nesting until nidr-May when Sooty eggs were hatching. The incubation
and fledging periods of Roseates are much shorter than those of Sooties,
so young Roseate Terns were flying well before anv Sooty Tern chicks.
Roseate Terns often forage within sight of Fort Jefferson end probably
avoid competition with .Sooties by foraging in different areas. They
dive from the air and plunge into the water, thus capturing food that
is unavailable to the Sooties.

Nesting Habita'

Although Sooty Terns nest throughout Bush Key, they nest in
greatest concentration outside the bay cedar thickets and mangroves
of the central part of the island. I set up five plots outside the
bay cedar, each 5 yards sajiare, and at 2— to 6—day intervals marked
all the eggs laid on each. I usually visited the plots in. late
afternoon. As the eggs hatched, I banded the chicks in each. To
avoid mortality, I did not fence xn the plots, and thus I missed many
chicks that fled as I approached. Hurricane Abby caused no appreciable
mortality on these plots.

The density of eggs en each plot is summarized in Table 7.
Plot 1 was close to the beach and had a thick cover of Gtuntia, Uniola,
and Cakile. Plot 2 was mainly low grass with some Opuntia on one

72

Table 7. rlest density and success in five 5-yard-square pi its on
3ush Key in 1968

Plot

no.

o &&

1

75

2

16.1

3

62

4

23

5

131

To^

,als

457

No. eggs Minimum no.
No. eggs/ broken or chicks known Dead chicks
sq yd infertile to hatch found

3. 00 13 29 i

6.^ 21 91 5

2.^3 13 33

1

1.12 2 13 2

5.24 13 90 ?-0

3. 66 72 256

?-'

73

side and about 30 per cent bare ground. The first eggs on Bush Key
in i960 were laid on this plot. Plot 3 had a thick cover of Soorobolus
and spurge (E'u-horbia buxl folia) 6 to 12 inches tall. The main cover
on plot 4, 4— to 6— foot tall Uniola, probably made it difficult for
terns to drop to the scattered openings en the ground. Plot 5 had
scattered Euphorbia and Cakil i and numerous bare spots. The highest
nest densities were on plots 2 and 5» both of which had scattered bare
areas. This seems to be a major factor in determining nest sites for
Sooty Terns, as care sand is avoided. The terns apparently require
some features such as plants or rocks as reference points for locating
the scrape and as cover for the shick. Ashmole (19&3) noted the same
thing on Ascension Island, The other three plots all had a very thick
ground cover in which adults probably could not dig an adequate scrape.

>y data or, density of eggs are somewhat higher than those used by
others for determining Sooty population densities. For instance,
Sprunt (19^8) used densities ranging from 0.5 to 5 per square yard
depending on the thickness of the vegetation, but he counted birds
only once. Soccies arrive over an extended period and fill in space
as some eggs are broken or deserted. Ky counts, made over an extended
period, included these additional eggs and thus increased the estimate.
Ashmole (1963) also used 5— yard squares and counted over an extended
period. His maximum density of 132 eggs in a plot is somewhat less
than my maximum o£ l6l.

I'-fy" data, provide a rough estimate of the nesting success of Sooties
at Bush Key. At. least 256 of 45? eggs hatched, and probably nearly
385 hatched (4-5/ minus 72 known broken or infertile). Thus somewhere
between 56. 9 anc? 34.2 per cent of the eggs laid hatched. As scr.o pairs

74

undoubtedly relaid (I often found a fresh egg next to one of the
deserted ones), perhaps 85 to 90 per cent of the breeding pairs on
Bush Key produced a chick. Ridley and Percy (1958) report that
82.5 per cent of the eggs at the Seychelles hatch.

Chick mortality is much harder to measure. Some chicks ran into
nearby thick dunes of Cpuntia and Suriana and I was unable to band
many of them. Cf the 256 that I know hatched, I found 2? dead, a
known mortality of 11. 3 per cent. Chick mortality was undoubtedly
higher as I could not follow chicks until they were flying and some
older, more mobile chicks certainly died without ny knowing it. As
a rough estimate of total mortality, if 385 of the k$7 eggs hatched
and, of these, 11.3 per cent died, Jkl out of V?7 would be the maximum
number surviving, a survivorship of about 75 per cent. This of course
assumes that all eggs that I did not find broken or deserted did
hatch and also underestimates mortality of chicks. This is not far
from the 70 per cent survivorship Robertson (1964) gives as typical
for Bush Key for summers without hurricanes.

My presence on the plots undoubtedly increased mortality of eggs
and of chicks, and I see no way that it can be avoided if one works
with the birds intensively. Some chicks that ran from me probably did
not find their way back to their parent and starved or were pecked
severely by adults and died from the blows. Repeated disturbance
also caused desertion of some eggs.

For example, I have good records of the number of eggs laid on
ray study plot and the number of these that hatched. Of 122 eggs,
93 (76.2 per cent) hatched, somewhat less titan the maximum figure
arrived at on the other five less frequently visited plots, but

75

certainly not inconsistent with it in consideration of the variables
in that information. Fully half the eggs that did not hatch, were those
closest to my blind, the ones that I disturbed repeatedly.

Site Tenacity

Site tenacity, the tendency for a bird to return to the sane nest
site year after year, occurs in the Connon Tern (Austin, 19^9). As I
color— banded 152 Sooties at one location on Bush Key in 1968, later
sightings of these provide some information on site tenacity. I later
saw 106 of the 182 (53.2 per cent) in the genera], area of banding,
and most of then probably nested in that general part of the colon,/-.

Fron 23 April to 2 May 1970, 0. L. Austin, Jr. sat for several
hours each day near the plot where the terns had been banded to watch
for and record color— banded birds. He saw at least ICO birds with
color bands on the plot or immediately adjacent to it. I sar.% three
more in June and July, and three others were caught in other parts of
Bush Key. Of the 103 birds on the plot, 76 could be identified
individually; the others had lost some of the bands and were not
caught to read the FWS band. Of the 76 individually recognizable
Scoties, 53 (69.7 per cent) were anong the 106 that had been seen en
the plot in 1963 after banding. Thus, of 106 color-marked birds that
probably nested on or near the study plot in 1968, at least half of
then (53 of 106) were on the same plot 2 years later. The 30 birds
that could not be identified individually nay well have included birds
that nested on the plot in 1968. Thus 50 per cent is a minimum
figure and perhaps 70 per cent is a more meaningful estimate of site
tenacity. These data suggest that individual birds had a strong
tendency to nest, in the same parts of Bush Key in 1965 and 1970. As

76

my plot and the ground immediately around it covered much less than
1 per cent of the habitat suitable for Sooties on Bush Key, it seems
unlikely that by chance alone so nan;." color—banded birds would return
to this same area in 1970.

Site tenacity implies individual attachment to a specific locale
in the colony. Croup adherence implies that subgroups exist within the
colony and these subgroups stay together and individually recognize
other members of the group. Austin (1951) describes such subgroups in
Common Terns and they may well exist in Sooty Tern colonies. Young
Sooties, if not allowed to return to their scrape, establish a spatial
arrangement with respect to other chicks similar to the pattern that
existed when they were at the home scrape (Burckhalter, 1969). This
implies that they recognise other chicks around them arid act as a group.

The fly— ups already described also suggest that subgroups exist in
Sooty colonies. In these, adjacent birds react to the activities of
one bird, seemingly as if they recognized one another.

Subgroups that move together within the colony could also explain
the clustered nesting of color— banded birds seen at the plot in 1970,
but this implies a fairly sizeable group of perhaps several hundred
birds functioning as a subgroup. Although I definitely think subgroups
exist In Sooty colonies, site tenacity remains the more likely
explanation for birds nesting in the same general part of the colony in
two different years. If individuals repeatedly nested in the sane
part of the colony, they wculd of necessity contact the same birds
year after year and then possibly form subgroups.

liscusoio::

Two broader aspects of Sooty Tern behavior are particularly
important and warrant further discussion. First is the comparison of
the behavioral repertoire of fuscata to that of ether species of terns,
especially those within the genus sterna. This should clarify the
affinities Sooties have with other terns. Fortunately, several, other
terns have been studied carefully enough to provide a basis for
comparison.

Equally important are the adaptations of 1 uscata that have allowed
it to survive and thrive in a pelagic environment. 3y feeding in
pelagic waters, Sooties differ considerably from most other terns and
in many ways closely resemble pelagic species of birds in other orders
such as the Frcceilariifomes.

Behavioral 'emoarisens

Behavioral comparisons can be made with several ether temperate
Sterna terns, the White Tern (Gy-is alba), trie two £nous, several
species of Thil-.~?eus, and the Black Tern (Chlidcnias niger).

Trie high flights of the Common, Arctic, Roseate, and Sandwich
Terns are all quite similar (Cullen, 1960a). The high flights of the
Caspian Tern and Black Tern resemble those of the others, although in
niger many birds may ascend together. The circling ascent and gliding
descent of the Sooty Tern high flight are almost identical to those of
the Common and Arctic Tern described by Cullen (39oCa). In the high

77

73

flight of the Brown "Acddj- the ascent is somewhat like the upward
flutter cf Scoties but the birds descend in a circling glide rather
than a long, essentially straight glide as the Sterna terns do
(Moynihan, I9c2 ; pers. obs. ). The high flight of Gygis is similar
to that of the dark .loddies (Moynihan, 1962).

The low flight is poorly developed in fuscata and not nearly so
common as in other terns except Gygis (Borward, 1963). Cullen (1960a)
notes that the low flight has fewer special features and may have
developed independently in each species. The rarity of the low flight
by fuscata, may be due to its different manner of carrying food.
Cullen (I9o0a) notes that food also has little importance in
advertising by unmated birds of both species of Anous, both of which
carry food in the esophagus as Booties do.

Perhaps carrying fcod in the esophagus allows Sooti.es to transport
more food (Ashmole and Ashmole, 196?) than if it were carried in the
bill. Also it may prevent food desiccation during the often lengthy
return flight to the colony. The food is the major water source for
the chicks, so moist food may be necessary for the chick's survival.
Moist food should be easier to handle and swallow than dried— out food.
Besides Sooties, J3. a,naethetus and both Anous carry fish in their
esophagus.

The ground displays of Sooties seem similar to those reported for
other Sterna terns, all having some form of a parade display. In
particular the parade cf Sooties seems to resemble that of the Common
Tern described hy Palmer (1941 j. Incipient scrape-building activities
are part of courtship in Scoties, just as they are in Common Terns
(Palmer, 1941).

79

The most conspicuous ground display of the dark Noddies is nodding,
and generally their behavior is gull— like (Moynihan, 1962). Nodding
emphasizes head movements, whereas the parade emphasises the position
of the wings and head, providing a clear contrast between the Noddies
and the Sterna terns. The displays of S-ygi s are similar to those of
the dark Noddies, but are very simplified (Moynihan, ±962).

Comparisons of behavior help clarify the relationships of various
terns. Moynihan (1959) used comparative behavior as a basis Tor
classifying the Laridae and put all terns into three genera. Lara sterna
remains monctypic, Anous includes Gygis aid Frocelsx.erna, and the rest
are placed in Sterna. I prefer the older terminology, retaining
numerous genera, but nonetheless agree with the group limits he has
established on the basis of behavior. In Moynihan' s classification, the
typical black— capped terns include fuscata, hirundo. caradisaea, and
others. He puts the large Thalasseus terns and the small Sterna terns
like albifrons in separate groups within his large genus Sterna and
also keeps the marsh terns (Chlidonias) together as a separate group
within Sterna.

This grouping agrees well with my observations. 'Trie high flight
of fuscata is almost identical to that of hirundo and oaradisaea. The
ground displays of fuscata are very similar to those of hirundo and on
behavioral grounds the two species clearly are closely related. In
contrast the displays of Gyi'is and both species of Anous, three species
that share the tropical oceans with Scoties, are clearly different from
those of Scoties.

Moynihan (1959) points out, and I agree, that a most profitable
area for future work is with the many old world and tropical forms that

80

are only slightly known. Two other tropical terns, Sterna lunata and
anaethetus, resemble fuscata in their feeding habits .and probably their
breeding biology, although they nay feed more offshore than pelagically.
As yet these two are poorly known and further work on them certainly
would be of interest.

Sooties as a Pelagic Species.

Of the kO species of terns (Sterninae), most feed either on.
freshwater marshes, lakes, and streams, or along marine coasts and
estuaries. A few venture into offshore waters (on continental shelves
or near islands), especially in the nonbreeding season. Sooty Terns
seem to be the most pelagic (beyond the continental shelf) of terns,
although several other Sterna, both species of Anous, and Gy^is feed
in offshore and pelagic waters. Apparently by exploiting pelagic
waters and breeding on isolated islands, Sooty Terns utilize a niche
available to few other species. Sooties are highly adapted to exploit
this niche, particularly in regard to flying ability, population
structure, and breeding biology.

Probably foremost among their adaptations for exploiting pelagic
waters is flying ability. Sooty Terns rapidly become waterlogged when
placed on water (Watson and Lashley, 1915) and apparently seldom rest
on water. As Sooties inhabit the open oceans during the nonbreeding
season, birds from some colonies must fly continuously for 6 months
or more. Virtually nothing is known of the physiological adaptations
involved in flying for such long periods. For example, do they sleep
on the wing as the Swift (Apus apus) apparently does (Lack, 1956), or
do they go without any sleep at all?

Their flying ability allows Sooties to exploit food in a large

31

area far from land, reached by few other birds. Several factors
restrict Sooty Tern food availability. Except for some areas where
upwellings or convergences occur, tropical pelagic waters are poor
in nutrients and plankton that form the base of their food chain
(Raymont, 1963). Sooties apparently are restricted to feeding on food
items that occur at the very surface of the ocean, or in the air above
it. This food is driven to the surface mainly by large predacious
fish (Ashncle and Ashmole, 19o7 : 53) and thus is not evenly distributed
throughout the ocean.

The length of incubation shifts and brooding spells varies
considerably between different Sooty Tem colonies. Adults usually
shift at 1 or 2 day intervals on Bush Key whereas they vary from
2 hours to 3 days in the Seychelles (Ridley and Percy, 1958), and are
as long as 6,5 days on Ascension Island (Ashmole, 1963), and 7 days on
Christmas Island (Ashmole and Ashmole, 196?). The duration of the
shift must depend on the time required to fly to the feeding ground,
find enough food to last until the adult can feed again, and then return
to the colony. Unfortunately feeding rates and exact locations of
feeding grounds for various colonies are not known. The most
reasonable explanation for the great variation between colonies in
time scen'o foraging is that food is farther away or scarcer for some
colonies than others.

More puzzling is the change in length of spells once the egg
hatches and the chick must be fed. At Ascension Island adults change
from. 5. 5-day— incubation shifts to feeding their chick about once a
day (Ashmole, 1963). As the breeding cycle at that colony is nonannual
(9.5 months), it seems unlikely that a regular change in abundance of

32

food could occur and thus account for the differences in foraging
between incubation and fledging periods. On Bush Key adults change
from 1 or 2 day incubation shifts to feeding the chick several times
a day. As the breeding cycle there is annual, perhaps food is more
abundant, closer to the island, or the adults forage more diligently
when chicks are present. The meager evidence available indicates that
terns do not tend to capture smaller fish when they are feeding chicks
as might be expected (Potthoff and Richards, 1970) but little is known
about the food supply. As the parent forages not only for itself
but also for the chick in a shorter period of time, it probably works
harder when feeding the chick than during incubation. Put another
way, after sitting quietly on the egg for several days, adults probably
loaf at least part of the time during breaks from incubation.

Table 8 gives some information on the breeding biology of several
representative tern species. I have selected one species each from the
genera Chlidonias , Thalasseus, and Anous and only a few from Sterna,
picking species for which information is available and that seem to be
representative of other similar species. This table is similar to one
in Lack (1968: 2o2). The patterns in Table 8 vary both between various
colonies of a species and from year to year within a colony, but this
does not negate the comparisons to be made here. The purpose of the
table is to try to compare some general features of the breeding
biology of marsh and coastal feeding terns with those that feed
offshore or pelagically.

On Bush Key as at other Sooty Tern colonies, occasionally a scrape
contains two eggs, but by far the usual clutch is one egg. Single egg
clutches are common among marine birds (Lack, 1968) and seem to imply

83

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that something about the oceanic habitat Units their clutch size.

In contrast, coastal and freshwater feeding terns commonly lay
more than one egg (Table 8). Besides fusc=ta. only Angus, Gygis, and
a few other Sterna tems (e.g. S. anaethetus and lunata) have a. normal
clutch of one. A clutch of two or more presumably is primitive in
Sooties and Brown Noddies, as they both have two brood patches. Gygis
lays one egg and has only a single brood patch, but its unusual
nesting habits may explain this (Dorward, 19^3 ).

With the smaller clutch size, pelagic terns must have either a
high nesting success, a long life span, cr some combination of the two
to maintain their populations. With a clutch cf one, partial nesting
success does not exist. Either they fledge a young or they fail.
Among Sooties, a maximum of 50 per cent renest if the first attempt
fails and the percentage declines as the breeding season advances
(Ridley and Percy, 1956; Ashmole, 1963).

The single egg of fuscata is incubated longer and the chick is
cared for longer than are those of inshore feeding terns (Table 8).
As Sooty Tern chicks may not be fed at regular intervals, a lengthened
period cf development probably allows them to withstand the occasional
short periods of fasting (Lack, 1968). Chicks grow rapidly when food
is available, but can survive long periods without food if the adults
cannot find any for several days (Ashmole, 1963). Storms and a distant
food supply guarantee that, short fasts occur regularly at some colonies.

Feeding rates also differ between Sooties and coastal feeding
terns. The latter generally forage close to their colony and, although
they may raise more than one chick, they feed their young frequently.
On the Fame Islands, Common, Arctic, and Sandwich Terns all feed each

35

of their young about once an hour (Pearson, 19 68) and Black Terns
feed young as often as 15 tines in an hour (Goodwin, I960). As a
result their young fledge much faster than do young Sooties. The
differences between Sooties and noddies, both of which raise one chick
per pair, have already been discussed. Noddies forage closer to the
colony, feed their young somewhat no re frequently, and their young
mature faster than the farther ranging Sooties (Table 8).

An important factor favoring the slow development of young Sooties
is the scarcity of predators on islands where they breed. They can
"afford" long incubation and fledging periods without drastically
raising the risk of predation. Common and Least Terns nesting on or
close to the mainland are constantly exposed to predation (Austin, 1948 ;
Hardy, 195?) so it is probably advantageous for them to raise their
young as quickly as possible. In turn rapid growth of their young is
made possible by a food source close to the colony that permits more
feeding trips in a day. Cat predation on Ascension Island shows
vividly what can happen to seabirds, including Sooties, when a land
predator is introduced (Stonehouse, 1962; Ashnole, 1963).

Sooty Terns defecate near their egg, do not always remove the empty
egg shell, and nest in dense colonies. These habits seem related to
the isolated and usually land-mammal— free islands where they nest.
Sandwich Terns have similar habits, but frequently nest with gulls that
help drive off intruders (Cullen, 19o0b). In contrast, Arctic and
Common Terns carry off the empty egg shell, defecate away from the
nest, space their nests farther apart, and vigorously attack intruders,
all of which should help reduce predation. Common and Arctic Terns
also have much shorter incubation and fledging periods than Sooties,

86

again reducing the time they are vulnerable to predation.

With all the problems that Sooties encounter in .feeding far from
their nesting colony, they still are very plentiful. Obviously the
number of islands, suitable for nesting, uithin their range is finite.
On some islands man has greatly reduced tern numbers, either by
egging (Ridley and Percy, 1958), or by introducing predators (Ashrxle,

1963).

Sooty Terns probably do not first breed until 6 years old (Robertson,
1969) whereas most. Common Terns first breed v'aen 3 or A- years old
(Austin and Austin, 1956). Delayed maturity occurs in many other
seabirds, including many Larids (Lack, 1965). Robertson (19°9)
suggests that the transatlantic migration by juvenile Sooties from
Bush Key, a colony relatively stable in numbers, keeps them from
competing with older and presumably more efficient breeders for several
years. The 3-year-^>ld birds that visit Bush Key late in the breeding
season have little chance of finding a suitable nest site and breeding
successfully. By returning to the colony earlier in successive years,
eventually they should be able to arrive early enough bo find a
suitable nest site and breed successfully. Thus delayed maturity
keeps younger birds from competing with older, established breeders
for nest sites, and also keeps the younger birds from undergoing the
rigors of reproduction until they have a fairly high probability of
success. The Fishers (1969) note a staggered arrival of age groups
of Laysan Albatrosses, with the young birds not breeding for several
years.

For delayed maturity to persist, individuals with such maturity
must rai.se more offspring in the long run than if they first bred

87

when younger. As sone 30-Jrear— old Sooty Terns are presently on Bush
Key, the adults there clearly have many breeding opportunities. Still
Sooty Terns, by delaying breeding for several years and then by
raising at most a single young per breeding season, have a lower
maxintni natality than other similar terns that feed close to their
colony.

lack (1968) proposes that birds raise as many offspring as possible
with the usual clutch size being that that results in the most
breeding adults in the next generation. According to his ideas, Sooty
Terns with a clutch size of one canr.ct raise more than one young per
breeding season. Presumably the limiting factor is their distant
and sometimes unreliable food supply.

Certainly the hypothesis fits the information from Ascension
Island where an apparent failure of the food supply one season resulted
in few chicks surviving (Ashnole, 1963). This and the long spells
that Ascension adults spend foraging suggest that at most a pair could

'-o ca

raise one chick. Other colonies that have been studied have not had
such failures.

Except for the cats en Ascension Island, adult Sooty Terns suffer
little predation and have a lew annual adult nortality, perhaps
around 12 to 18 per cent at Bush Key (Austin, pers. comm. ), versus
25 per cent in adult Common Terns (Austin and. Austin, 195.6).. Although
the food supply may limit the numbers of young they can raise and hence
detemine the usual clutch size of Sooty Terns, perhaps delayed
maturity is a function of the low predation on and high survivorship of
the adults. High survivorship of adults could result in many birds
cc.npeting for a United number of nest sites, but delayed maturity

88

could reduce this competition as has already been discussed.

Some pelagic species can raise more than one young (see Harris,
1970), indicating that food is not entirely limiting, but so far the
problem has been following the survival of these "twins" until they
return to breed.

Tims Sooty Terns differ fron most other terns. Many of these
differences derive from their adaptations to a pelagic environment.
The basic behavioral patterns in pair formation and courtship show
clearly that Sooties are closely related to typical Sterna terns such
as hirundo.

I believe several behavioral differences between Sooties and most
other species of Sterna are due to their different way of carrying
food. Sooties do not have a well-developed low flight display, a
display in which other terns often prominently carry a fish in their
bill. Other terns also nay carry a fish openly in the bill during the
high flight while Sooties do not, although they do have a well-
developed high flight.

The high cost of obtaining food, compared to that of coastal
feeding terns, perhaps explains why courtship feeding is relatively
rare in Sooty Terns. A coastal feeding tern can easily replace the
food it feeds to another bird in courtship. For a Sooty Tern,
replacement would require a long flight.

Comparison cf the breeding biology of Sooties and coastal feeding
terns shows other differences. The development of the Sooty Tern egg
and chick is much slower than that cf most other terns, probably sc
their young can survive lengthy periods when the parents are unable
to find food. The high survivorship of adults and the distant food

69

supply have led to their delayed maturity and small clutch size.
Thus in their breeding biology, with a long incubation and
fledging period, delayed maturity, and a one-egg— clutch, Sooty Terns
resemble other pelagic birds such as the albatrosses, shearwaters,
and tropicbirds more closely than most other terns.

LITERATURE CIIBD

Ashmole, H.P. If 63. The biology of the Wideawake or Sooty Terr.

Star:.: fjtscata en Ascension Island. Ibis, 103b: 297-364.
Ashmole, 3. P« 19c8. Breeding and molt in the White Tern (Oyj/is alba)

on Chrxsteas Island, Pacific Ocean. Condcr, 70: 35—55.
Ashmole, M.?. , and L.J. Ashmole. 1967. Comparative feeding ecology

of sea birds of a tropical oceanic island. Bull, Peabcdy Mus.

Nat. Bgjst. , 24: I-I3I.
Ashmole* M.P-, and H. Tcvar 3. 1965. Prolonged parental care in

Royal Barns and ether birds. Auk, 3.5: 90-100.
Audubon, J.J. 1844. The birds of America, vol. 7. 1967 reprint.

Hew York, Dover Publ. , Inc.
Austin, O.L. 1948. Predation by the common rat (JtaUW nqrvegicus.)

in the Cape Cod colonies of nesting terns. Bird-Banding,

19: 60-^5.
Austin, O.u. 1949. Site tenacity, a behaviour trai L. of the Common

Tern (Sterna hirando Linn.). Bird— Banding, 20: 1-39.
Austin, Q.L„ 1951. Group adherence in the Common Tern.

Bird- Baading, 22: 1—15.
Austin, 0.1*, and C. L. Austin, Jr. 195°. Some demographic aspects

of the Cape Cod population of Common Terns (Sterna. hirund£).

Bird-Banding, 27: 55—66.
Beard, D.B. 1939. Kan-o'-^ar-birds prep on eastern Sooty lerns.

Auk, 56= 327-329.

90

91

Bruyns, W. F. J. 'A. , and K.H. Voous. 1965. Night—feeding by Sooty

Terns ( Sterna f..; scat a) . Ardea, 53 : 79.
Burckhalter, D. L, 19&9. Orientation and communication of Sooty Tern

chicks. M. S. thesis. Tuscon, Arizona, Univ. of Arizona.
Chapin, J. F. 1952*-. The calendar of wideawake fair. Auk, ?1: 1—15.
Cott, H.B. 195^. Ihe exploitation of '.-did birds for their eggs.

Ibis, 96: I29-I4.9.
Culler., J.M. 1960a. The aerial, display of the Arctic Tern and other

species. Ardea, 48: 1-37.
Cullen, J.M. .1960b. Some adaptations in the nesting behaviour of

terns. Froc. 12th Intern. Ornithol, Congr. , 153—157.
Cullen, J.M. , and M. ?. Ashiaole. 19&3. The Black Noddy Anous

tenul ro s tri-S on Ascension Island. Part 2. Behaviour.

Ibis, i03b: 4-23—446.
Cunningham, R.L. 19&5. Predation on birds by the Cattle Egret.

Auk, 52: 5C2-503.
Davies, 3. J.J.?., and R. Carrick. 19o2. On the ability of Crested

Terns, Sterna ■ bersii . to recognize their own chicks. Australian

J. Zcol. , 10: 171-177.
Dor.-/ard, D. ?. I963. The Fair;- Tern Cryqis alba en Ascension Island.

Ibis, 103b: 365-372.
Dorward, D.F. , and N. P. Ashnole. 1963. Motes on the biology of the

Brown Noddy Anous stolidus on Ascension Island. Ibis, 103b:

447-^57.
Duellman, W.S., and A. Schwartz. 1958. Amphibians and reptiles of

Southern Florida. Pull. Florida State Mus. , 3= 179-324,
Srdiuan, U.S. 19&7. Sea birds in relation to game fish schools off

92

Puerto RLco and the Virgin Islands. Caribbean J. Sci. , 7: 79—35.
Fisher, H.I., and M.L. Fisher. 1969. The visits of Laysan

Albatrosses to the breeding colony. Micronesica, 5 "• 173-221.
Goodwin, R. E. I960. A study of the ethology of the Black Tern,

Chli< onia ririnariensis (Gmelin). Ph.D. thesis. Ithaca,

New York, Cornell Univ.
Gould, P.J. 1967. Kocturnal feeding of Sterna fuscata and Puffir.uo

racificus. Condor, 69 : 529.
Hardy, J.W. 1957. The Least Tem in the Mississippi Valley. Publ.

Mus. Michigan State Univ. Biol. Series, 1: 1— 60.
Harris, M.P. 1970. Breeding ecology of the Swallow-tailed Gull,

C rearms furcatus. Auk, 87: 215-243.
Harrison, C.J.O. 1965. "Stare-down" in birds, and its apparent

function. Ardea, 53 : 57—72.
Hatch, J.J. 197C. Predation and piracy by gulls at a temery in

Maine. Auk, 87: 244-254.
Howell, I.H., and G.A. Bartholonew. 1?62. Temperature regulation in

the Sooty Tern Sterna fuscata. Ibis, 104: 98—105.
Hutchison, R.2., J.G. Stevenson, andW.H. Thorpe. 1965. The basis

for individual recognition by voice in the Sandwich Tern (Sterna

s and vi c ens i s ) . Behaviour, 32: 150—157.
Iredale, T. 1914. On Sterna fuscata Linne'. Ibis, 56: 436—937.
Jozefik, K. 1969. Caspian Tern, Evdroprogne cassia Pall, in Poland -

the biology c£ migration period. Acta Ornithol. , 11: 381-443.
Lack, D. 1956. Swifts in a tower. London, Methuen and Co. Ltd.
Lack, D. 1965. Ecological adai tations f'^r breeding in birds.
London, Methuen and Co. Lid.

93

Lashley, K..3. 1915. Notes on the nesting activities of the Noddy

and Sooty Terr?. Carnegie Inst. Washington, Papers from the

Dept. Marine Biol., 7: 6I-83.
Longley, W.H. 1929. Observations on Tortugas fishes, especially

those en which the Toddy and Sooty Terns of the Bird Key rookery

feed. Carnegie Inst.. Washington Yearbook, 23: 286—290.
Longley, W.H», and S. F. Hildebrand, 1941. Systematic catalogue of

the fishes of Tortugas, Florida, with observations or. color,

habits and local distribution. Carnegie Inst. Washington,

Fapers fron Tortugas Laboratories, 34: 1— 33L
Mason, C.R., and N.D. Steffee. 1966. Banding party waits out Alma. .

Florida Naturalist, 39: 147-148.
Monroe, 3. L. , Jr. 1956. Observations of Elegant Tern at San Diego,

California. Wilson Bull. , 66 : 239-244.
Moynihan, M. 1959. A revision of the family Laridae (Aves). Amer.

Mus. Xovitates, 1926: 1-42.
Moynihan, M. 19&2. Hostile and sexual behavior patterns of South

American and Pacific laridae. Behav. Suppl. , 3 : 1— 365.
Palmer, R. S. 1941. A behavior study of the Common Tern ( Sterna

hirundo hirundo L. ). Proc. Boston Soc. Nat. Hist., 42: 2—119.
Pearson, T. ?;. 1968. The feeding biology of sea-bird species breeding

on the Fame Islands, Northumberland. J. Anim. Scol. , 37: 521—552.
Potthoff , T. ,. and W. R. Ri.chards. 1970. Juvenile bluefin tuna,

Thunnos thynnus (Linnaeus), and other scombrids taken by terns

in the Dry Tortugas, Florida, Bull. Marine Sci. , 20: 389-413.
Raymont, J.E.G. 1963. Plankton and productivity in the oceans.

New York, Mac : I 1 an Cc.

94

Ridley, M.W. , and R. Percy. 1958. The exploitation of sea birds in

Seychelles. Colonial Research Studies, 25: 1-78.
Robertson, W. B. , Jr. 1962. Florida region. Audubon Field Motes,

16: 468^73.
Robertson, W.B. , Jr. 1964. The tsms of the Dry Tortugas. Bull.

Florida State Mus. , 3: 1-94,
Robertson, W. 3. , Jr. 1969. Transatlantic migration of juvenile

Sooty Terrs. Mature, 222: 632-634.
Russell, J.C. 1938. Narrative report on wildlife of Fort Jefferson

National Monument. Unpublished report, National Park Service

files.
Sprunt, A. , Jr. 1948. The tern colonies of the Dry Tortugas Keys.

Auk, 65: 1-19.
Stonehouse, B. 1962. Ascension Island and the British Ornithologists'

Union Centenary Expedition 1957-59. Ibis, 103b: 107-123.
Thonpson, J. 1?03. The Tortugas tern colony. Bird— Lore, 5= 77— 84.
Tinbergen, N. , G.J. Broekhuysen, F. Feekes, J.C.W. Houghton, K. Kruuk,

and E. Szulc. 1962. Egg shell removal by the Black— headed

Gull, Lams ridibundus L. ; A behaviour component of camouflage.

Behaviour, 19 : 74-117.
Tompkins, I. R. 1942. Least Tern watering eggs: Gideon Mabbett* s

query. Auk, 59: 308.
Vaughan, T.W. 1918. The temperature of the Florida coral-reef

tract. Carnegie Inst. Washington, Papers from the Dept. Marine

Biol., 9: 319-339.
Watson, J.B. 1908. The behavior of Noddy and Sooty Terns. Carnegie

Inst. Washington, Papers from the Tortugas Laboratory, 2: 187-255.

95

Watson, J. 3. , and K. S. Lashley, 1915- An historical and

experimental study of homing. Carnegie Inst. Washington, Papers

from the Dept. Marine Biol., 7: 7-6Q.
Witherby, K.F. , F.C.R. Jourdain, N.F. Ticehurst, and B.W. Tucker.

1941. The handbook of British birds, vol. 5. London,

H. F. and G. Witherby Ltd.

BIOGRAPHICAL SKETCH

Janes Jay Dinsmore was born 25 February 19^2 at Owatonna,
Minnesota. In June, i960, he graduated from Owatonna High School. He
enrolled at Iowa State University and in Hay, 19&I-, received the
Bachelor of Science degree with a major in Fish and Wildlife Management.
He then enrolled at the University of Wisconsin, Madison. From
19b5 to 1966 he lived in Tobago, West Indies doing field work and in
August, 196?, received the Master cf Science degree with a major in
Zoology. In September , 196? , he enrolled at the University of Florida
and began work on the Doctor of Philosophy degree. At Florida he
held a Graduate School Fellowship and later worked as a graduate
teaching assistant;, first in the Department of Zoology and then in
the Department of Biological Sciences. From 1969 to 1970 he held an
appointment as Interim Instructor in Biological Sciences.

James is married to the former Patricia Ann Hoops, and they have
two sons. He is a member of the American Ornithologists' Union,
Cooper Ornithological Society, Wilson Ornithological Society, Sigma
Xi , and Phi Kappa Phi.

96

This dissertation was prepared under the direction of the chair-
man of the candidate's supervisory committee and has beer, approved
by all members of that committee. It was submitted to the Dear; of the
College of Arts and Sciences and to the Graduate Council, and was
approved as partial fulfillment of the requirements for the degree of
Doctor of Philosophy.

August, 1970

Dean, College of Arts and Sciences

Dean, Graduate School

Supervi so ry Commi ttee

Q 0 V^aJ^U,