BLM LIBRARY
88034470
T-N-271
Filing Code $#§*. £,6 Ot>
Date Issued June 1975
TECHNICAL NOTE
U.S. DEPARTMENT OF THE INTERIOR - BUREAU OF LAND MANAGEMENT
HABITAT MANAGEMENT SERIES FOR UNIQUE OR ENDANGERED SPECIES
by Stephen A. Trimble, Biologist
Conservation Library
Denver Public Library
Report No. 15
Merlin
Falco columbarius
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84.2
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no .271
10/ copies of Technical Notes are available from DSC, Federal Center Building 50, Denver, Colo., 80225
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POHEWORD
This Technical Note series on wildlife is designed to provide a
literature review and summary of current knowledge pertaining to
endangered and other wildlife species occurring on public lands.
We in the Bureau of Land Management have recognized the need for
basic wildlife information in order to do an effective job in
land-use planning. Sound planning must identify the negative
aspects as well as the positive benefits of any proposed land
management decision or program. It is our hope, too, that this
series will also prove useful to others --be they land managers,
students, researchers or interested citizens.
M
Director
Bureau of Land Management
Department of the Interior
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TABLE OF CONTENTS
Page
Species Description 1
Distribution 6
Status and Population Trend 10
Life History ..... 12
Daily activity patterns 12
Food habits 13
Hunting behavior 15
Courtship 16
Nest site fidelity and nesting density 17
Egg-laying and incubation 17
Juvenile development 18
Territoriality and interspecific relations .... 20
Vocalizations 22
Migration and wintering behavior 23
Productivity 24
Pesticide relationships 25
Habitat Requirements 27
Breeding requirements 27
Wintering requirements 29
Limiting Factors 29
Habitat destruction 29
Chemical contamination 30
Other mortality factors 30
Species and Habitat Management Recommendations .... 31
1. Research needs: Alaska 31
2. Pesticide contamination 31
3. Habitat protection and improvement 32
4. Falconry 32
Protective Measures Instituted 32
A. Legal or regulatory 32
B. Captive rearing 33
Current Research 33
Authorities 34
Summary 34
Literature Cited 37
Additional References 41
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SPECIES DESCRIPTION
The merlin, or pigeon hawk, occurs throughout much of the
northern hemisphere. Brown and Amadon (1968) name eight races
worldwide. In North America, three distinct geographic races
of this small, swift-flying falcon breed in three ecologically
distinct regions: the taiga, Pacific coastal forest, and
prairie-parkland. Although the American Ornithologist's
Union (A.O.U.) Checklist (1957) recognizes four subspecies,
Temple (1972a) and Brown and Amadon (1968) recognize only three
of these forms, corresponding with the above-mentioned breeding
populations. In this report, I follow Temple's taxonomic
revision.
Falco columbarius columbarius includes all North American taiga-
breeding birds (merging the A.O.U. subspecies columbarius and
bendirei, forms similar in phenotype and ecology). Falco
columbarius suckleyi breeds in the Pacific coastal forest.
And F. c_. richardsonii refers to the prairie-parkland merlins.
Although the A.O.U. does not recognize common names for sub-
species, to avoid constant repetition of Latin subspecifics
I will refer to the three subspecies above as the taiga merlin,
black merlin, and Richardson's merlin, respectively.
Merlins exhibit the typical falcon silhouette--long, pointed
wings and a longish tail. They are much smaller than the large
falcons--about 25 to 34 centimeters long, with a wingspread of
about 60 centimeters (Peterson, 1961). Like most other fal-
coniforms, female merlins surpass males in size. Despite
subspecific size variations, the degree of sexual size dimorphism
remains constant in all populations. Females average larger
than males in most measurements—about 20 millimeters longer in
the wing (averaging 213.1 mm versus 192.2 mm), and about 11 mm
longer in the tail (averaging 128.57 mm versus 117.8 mm) (pooled
data for all North American merlins; Temple, 1972a). Males
have a proportionately wider wing than females (Temple, 1970).
Adult and immature merlins also differ in size: juveniles have
longer flight feathers than adults, thus affording young birds
lighter wing loading while they develop their powers of flight.
(Wing loading is the ratio of weight to wing surface area.)
Five male taiga merlins averaged 162 grams in weight (Brown and
Amadon, 1968) . Two taiga females weighed 200 grams and about
227 grams (Swartz, 1972). In a group of Newfoundland and
Atlantic Coast migrants, 47 males averaged 159 grams, and 69
females averaged 218 grams (Temple, personal communication).
Female Richardson's merlins weigh about 227 grams (Oliphant,
1974). A wintering female black merlin weighed 218 grams
(Slipp, 1942).
Merlins display marked sexual dichromatism, particularly in
adult plumage. Subspecies vary, but in general, adult males
(for a "typical" adult male F. c_. columbarius , see Figure One)
have slaty blue-gray dorsal and crown plumage, varying from
light gray to near-black, and marked by a mottled buff nuchal
collar and the distinct black shaft stripes of individual
feathers. Breast, flanks, and abdomen are buffy, boldly
streaked vertically with cinnamon to black, with the widest
streaks on the flanks. Few streaks mark the clear buff-colored
throat and sides of the head. Both immature and adult merlins,
of both sexes, possess dark tails distinctly banded with gray
or buff. Merlins lack the bold moustachial stripe characteristic
of many other falcons. The bill is bluish, the eye very dark
brown; cere, eyelid, tarsi, and toes are yellow, and the claws
black.
Adult females differ from adult males primarily in the dark
brownish color of their dorsal plumage, contrasting with the
slate-blue of the male. This single distinction suffices for
separation of adults in the field. Streaking below tends
toward brown, on a lighter background than the male.
Immature merlins of both sexes closely resemble adult females in
dorsal coloration. The rump and upper tail coverts of the
adult female, however, are slate-brown in contrast to the dark
brown of the back, whereas the rump and upper tail coverts of
immatures are the same shade of brown as the back. Many immatures
have more greenish soft parts than the rich yellows of most
adults. Immature males and females differ in the color of the
light tail bands, especially in the central part of their tail
feathers. Immature males have light gray bands; immature fe-
males have buffy bands. Merlins acquire definitive adult
plumage with their first prebasic molt, which lasts from April
until September of the calendar year following their hatching
year. These falcons continue to molt once each year, during
these same months, throughout their lives, although the female
may molt before the male when breeding (Temple, 1972a, 1972b,
and pers. coram.; Bent, 1938; Brown and Amadon, 1968; Friedmann,
1950; Peterson, 1961; Laing, 1938; Eyre and Paul, 1973;
Oliphant, 1974).
At hatching, creamy -white and pure white down covers the young,
except for the bare abdomen. A second, coarser smoke-gray down
replaces this first plumage. By the age of 18 days, primaries
and tail rectrices have burst their sheaths. By 25 days the
young birds are fully feathered (Bent, 1938; Brown and Amadon,
1968; Friedmann, 1950; Fox, 1964).
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Merlins vary considerably across their range in both size and
plumage characters. Richardson's merlins have longer wings and
tails, on the average, and lighter wing loading, than other
North American merlins. Presumably, this proves a selective
advantage for frequent long flights in their open orairie-
parkland habitat. Black merlins, inhabiting the densest habi-
tat of the three subspecies, have the heaviest wing loading.
Taiga populations are intermediate (Temple, 1972a).
Plumage coloration provides one of the most obvious means for
separating subspecies. Richardson's merlins (prairie-parkland
breeders) are lightest in color, black merlins (humid coastal
forest breeders) are darkest, and taiga merlins intermediate.
Black merlins derive their common name from the extreme darkness
of their dorsal plumage. Richardson's merlins differ from
other populations in having five light tail bands (these figures
do not include the terminal light band). Black merlins usually
have four light tail bands, but extreme saturation of tail
rectrices by dark pigment often eliminates one of these,
creating an appearance of only three bands. Taiga merlins
nearly always have four light bands. The uppermost light band
in all subspecies may be obscured by tail coverts. Crown
streaking and primary barring also show patterns of geographic
variation (Temple, 1972a).
Merlins earn their second widely-used name--the pigeon hawk--
through similarity of size and flight, rather than from any
preference on the merlin's part for consuming domestic pigeons,
or rock doves (Columba livia). Falconers have flown merlins
for centuries—particularly at small birds such as larks
(Alaudidae) . Merlins suggest miniature peregrine falcons
(Falco peregrinus) in their slate-blue coloration and swift,
direct flight. Even in normal flight they exceed speeds of
20 meters/second (45 miles per hour) . In their ability to
twist and turn while pursuing quarry, they surpass other
falcons, and challenge the skill of accipiters. Like other
falcons, merlins attain peak speeds when stooping at prey--at
perhaps 27 to 40 meters per second (60 to 90 mph) . Merlins
soar infrequently (Williams and Matteson, 1947; Fox, 1964;
Brown and Amadon, 1968). Fox (1964) describes a curious
hovering flight, most characteristic of fledglings begging
food, with wings extended and tail spread, flapping the distal
portions of the wing like a kingbird (Tyrannus sp.).
Observers in the field may confuse merlins with sharp-shinned
hawks (Accipiter striatus), but the falcon's pointed wings and
direct flight with deep, regular wingbeats serve to distinguish
the two. Smaller size and lack of moustachial stripes should
easily separate merlins from larger falcons. Lack of rufous
on tail or back, along with indistinct facial markings, dis-
tinguish merlins from North America's other small falcon, the
American kestrel (Falco sparverius) (see Figure Two). Peterson
(1961) emphasizes dark dorsal coloration and the distinctly
banded tail as helpful field marks.
DISTRIBUTION
Merlins occur at some time of year in every province of Canada
and every state of the continental United States. Although
most wintering birds migrate south, some merlins remain near
their breeding grounds all year. Breeding ranges for the three
North American merlin subspecies are considerably more restricted,
nearly coinciding with the limits of the boreal forest, plus
the northern Pacific coastal forest and northern Great Plains
(see Figure Three).
Falco columbarius columbarius , the taiga merlin, breeds in the
boreal forest from tree-limit in northern Canada, from
Newfoundland west to northwestern Alaska, south to south-
central Alaska through British Columbia east of the Cascade
and Coast Ranges to north-central and eastern Washington and
Oregon, and (perhaps) western Montana (Missoula County). The
boundary then swings north into Canada west of the prairie-
parkland, east across the taiga, then south into the northern
parts of North Dakota, Minnesota, Michigan, and New England
(A.O.U. Checklist, 1957; Ellis, 1974; Brown and Amadon, 1968;
Jewett, et_ al_. , 1953; Friedmann, 1950). Taiga merlins occasion-
ally breed as far south as northern California, Idaho (Bingham
County), southwestern Wyoming (Fort Bridger), Colorado (Summit
County, Grand County, and Fort Lewis—although it seems more
likely that these are actually misidentif ied Richardson's
merlins), and the Wasatch Mountains of Utah (A.O.U. Checklist,
1957; Williams and Matteson, 1947; Bent, 1938; Bailey and
Niedrach, 1965; Wolfe, 1946). Most taiga merlins migrate
south in autumn, wintering across the western states, south to
the southern states, through Mexico to Central America and the
West Indies, and as far as Venezuela, Ecuador, and northern
Peru (A.O.U. Checklist, 1957).
Richardson's merlin breeds in the prairie-parkland of the
northern Great Plains from the fringes of the boreal forest
(northeast of Prince Albert, Saskatchewan; Oliphant, personal
communication) through the Canadian prairie provinces
(Alberta, Saskatchewan, and Manitoba) south to northern Montana
(Lewis and Clark, Valley, Cascade, Toole, and probably
Choteau Counties), northern North Dakota, and South Dakota (A.O.U.
Checklist, 1957; Ellis, 1974; Brown and Amadon, 1968; Hunter,
1967). Richardson's merlins have also nested in eastern Wyo-
ming, in Albany and Weston Counties (Fox, 1964). Falco
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&. Rxlco columbarium hchardsonii
columbarius richardsonii winters south through Wyoming and
Colorado to California (where it is uncommon), northern Mexico,
and west Texas (Friedmann, 1950). Most migrant Richardson's
merlins reportedly winter in Wyoming and Colorado (Fox, 1964) .
The black merlin, F. c_. suckleyi, breeds in the moist coastal
forests of western British Columbia, Vancouver Island, the
Queen Charlotte Islands, and the coastal islands of southeastern
Alaska, inland to the Okanagan Valley and the town of Blue
River, and north to Atlin and (probably) the upper Yukon River
drainage (Bent, 1938; Munro, 1946; Laing, 1935; Sutton, 1935;
Friedmann, 1950; Swarth, 1935). No certain breeding records
exist for the black merlin within the United States. Bent
(1938) mentions a possible nesting at Fort Klamath, Oregon,
another in the Puyallup River valley of Washington, and a set
of eggs (suckleyi or columbarius?) collected at Sitka, Alaska
in 1887. Black merlins may have nested at Bumping Lake,
Yakima County, Washington (Jewett, e_t al_. , 1953). Jewett (1939)
observed what appeared to be a family of recently fledged black
merlins in early August in Deschutes County, Oregon—well east
of the Cascades. Gabrielson and Lincoln (1959) report an
adult female black merlin taken on Sergief Island, north of
Wrangell, Alaska, on August 19. An extremely dark male merlin
was taken as a nestling by Pat Redig (of the University of
Minnesota) in northern Minnesota in 1974 (Oliphant, personal
communication). Wintering black merlins remain chiefly in the
coastal area southward through Washington, Oregon, western
Montana (Ellis, personal communication), and central California
(Unglish, 1934), to southern California (van Rossem, 1934;
Miller, 1941; Spaulding, 1947), but also wander southwestward
as far as New Mexico (Jewett, 1944) and Colorado (Bailey, 1942)
and north to coastal Alaska (Friedmann, 1950; A.O.U. Checklist,
1957). Although forty years have passed since Swarth (1934,
1935) lamented the near-complete lack of nesting data for this
subspecies, even today we know little more of its habits and
range.
STATUS AND POPULATION TREND
As a species, merlins are not endangered. But the three North
American subspecies vary dramatically in status.
Taiga and black merlins are probably not yet in any serious
population trouble. Both subspecies breed in environments not
highly altered by man (Temple, personal communication. ; Fyfe,
personal communication) . Swartz (personal communication) found
taiga merlins breeding in interior Alaska with good productivity
and in normal density, which is to say, not very abundant.
Migration counts in Alaska in 1968 indicated merlin populations
seemed to be holding their own (Adolphson, 1969). Richardson's
merlins, however, have suffered population declines.
10
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All three subspecies carry high levels of chlorinated hydrocarbon
residues. For F. c. columbarius and suckleyi, lowering of
reproductivity due to pesticide poisoning is significant, but
slight. Richardson's merlin, in contrast, has suffered massive
reproductive failure over most of its range—which constitutes
the most contaminated of all North American merlin habitats
(Temple, personal communication). Fox (1971) documented a 43%
decrease in hatching success since 1950 for Richardson's merlins
nesting in the Great Plains grasslands, corresponding with a
marked decrease in eggshell weight.
Not only has productivity declined, but the Richardson's breed-
ing range has contracted rapidly during the past 20 years. Two
factors have reduced the area of suitable breeding habitat.
First, Richardson's merlins depend on grassland birds for food.
As areas of native grassland shrink under the pressures of
intensive cultivation, Richardson's merlin populations likewise
shrink. Second, these merlins nest exclusively in trees. With-
out suitable nest sites, they cannot reproduce. Where native
grassland and nest sites persist, merlins seem to be maintaining
their populations, or even slowly increasing (Fyfe, personal
communication). Oliphant (personal communication) considers
the Richardson's merlin common in the Saskatoon, Saskatchewan
area, with good production of young (4-5 fledged in many nests).
However, on Saskatchewan Christmas counts, wintering merlins
were down 43% during 1964-1967, compared to 1957-1963, and down
55% during the same time period in the number of locations
where the bird was sighted (Fox, 1971). Recent Saskatchewan
Christmas counts have been high, six birds seen in Saskatoon
alone in 1973-1974 (Oliphant, personal communication). Temple
and Fox both regard Richardson's merlin as endangered (Temple,
personal communication; Fyfe, personal communication; Fox,
personal communication) .
Migration counts at Duluth, Minnesota, show that while total
raptors observed increased by 87% during 1960-1967 (over 1935-
1942), merlins decreased by 8370. Increased late-season
observations might bias these data somewhat. At Hawk Mt.,
Pennsylvania, merlin counts for these same years were down 137o
while total raptors increased 9% (Fox, 1971). Duluth birds
probably include migrants from the eastern edge of the Rockies
to central Ontario—central taiga merlins, with perhaps some
Richardson's. Hawk Mt. birds breed along the Atlantic Coast
and in Newfoundland, though Hawk Mt. is not on a major merlin
migration route (Temple, personal communication). Variation
in merlin counts may reflect the health of merlin populations
nesting in the differing source areas.
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Ellis (1974) reported eight positive breeding records for
merlins in Montana. Productivity was high for successful nests,
but three of seven nests with eggs failed—all Richardson's
merlins. Merlins have probably long been established in
Montana, but are so rare as to nearly have gone undetected.
Ellis believes the Montana population may be declining.
Although populations of taiga and black merlins seem to be only
slightly declining on a continent-wide basis, biologists should
watch them closely. As the taiga of northern Canada and Alaska
grows ever more civilized while man develops its resources,
merlins there may fail to reproduce successfully.
One note of optimism may be found in work by Oliphant (1974) in
Saskatchewan. He described eight nesting attempts by merlins
from 1971-1974 within the urban environment of Saskatoon. These
merlins successfully fledged young in the constant presence of
thousands of people, dogs, cats, cars, trucks, etc. Their diet
consisted almost entirely of house sparrows (Passer domesticus) ,
rather than native grassland birds. Fox (personal communication)
reports other consistently successful merlin nestings in
Canadian prairie cities.
LIFE HISTORY
Daily activity patterns
Merlins are strictly diurnal birds. When nesting, the male
leaves his roost to hunt early in the morning, usually returning
quickly with prey for the female and young. Taylor (1914) noted
two periods of hunting and feeding activity during the day for
merlins nesting in Britain: daybreak to 11 A.M. and 4 P.M. to
dusk. Selous (1915) found merlins in Iceland following a
similar activity pattern, with the early and late periods
shortened, and a third hunting and feeding period added from
about 1 P.M. to 4 P.M.
On the Minnesota-Ontario border, Craighead and Craighead (1940)
counted three feedings of young merlins between 9 A.M. and
5 P.M. These feedings usually included an early morning and
late evening hunt. Lawrence (1949) counted 1 to 2 feedings in
3 to 5 hours in Ontario. Breckinridge and Errington (1938),
however, observed a nest of half-grown young in northern
Minnesota to which the adults brought food 10 times during the
day.
Merlins hunt at twilight, at least occasionally. Lawrence
(1949) noted this behavior in a nesting male, while Johnson
and Coble (1967) observed merlins actively hunting as late as
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1
9:30 P.M. in Isle Royale National Park, Michigan. Bats made
up part of these merlins' diet, which presumably accounts for
such late-evening hunting.
In migration, taiga merlins feed early in the day (Bent, 1938),
and fly more in the afternoon than do other hawks migrating
along the east coast (Allen and Peterson, 1936).
Food habits
Merlins survive on a diet of birds almost exclusively. Brown
and Amadon (1968) summarize the diet of the species, worldwide,
as 80% birds, 5% mammals, and 15% insects. Merlins also have
consumed toads, lizards, and snakes (Bent, 1938).
Birds taken by merlins range in size from small sparrows
(Fringillidae) and wood warblers (Parulidae), to teal (Anas sp.),
rock doves, and ptarmigan (Lagopus sp.) (Bent, 1938). Fyfe
(personal communication) has not recorded a single mammal in
extensive food data for Canadian-breeding taiga and Richardson's
merlins. Lawrence (1949) estimated that one breeding pair of
merlins require 450 birds to support themselves and their
young during the 2%-month nesting season. Brown and Amadon
(1968) think this an excessively high estimate, suggesting
that 3 to 5 kills a day may suffice for the female and three
young.
Males evidently prey on smaller species than females, reflecting
the smaller size of the male falcon. At one Newfoundland nest,
the male taiga merlin captured all prey during the first week
after hatching, while the female constantly brooded the nestlings,
Of 19 prey items brought to the nest during this week, 79% were
species of birds weighing less than 50 g. From the end of this
first week until fledging, 27% of 109 food items weighed under
50 g. During this period the female resumed hunting regularly,
and the larger species are presumably her prey (Temple, 1972b).
Alternately, merlins may take larger prey species as food
demands of their fast-growing young increase (Lawrence, 1949).
Of 136 prey remains at nests of Newfoundland-breeding taiga
merlins, three species comprised over half the merlins' diet:
gray jay (Perisoreus canadensis) , American robin (Turdus
migratorius) , and Savannah sparrow (Passerculus sandwichensis)
(Temple, 1972c). Merlins, like other raptors, will concentrate
on a few prey species if these species prove easy to capture
owing to local abundance or behavior. Oliphant (1974) found
that merlins breeding in urban Saskatoon, Saskatchewan, con-
sumed primarily house sparrows, plus a few cedar waxwings
(Bombycilla cedrorum) , tree swallows (Iridoprocne bicolor) , and
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horned larks (Eremophila alpestris) ; house sparrows accounted
for over 90% of prey at one nest. Wintering merlins in
Alberta cities fed largely on Bohemian waxwings (Bombycilla
garrulus) and sparrows (Fyfe, personal communication). A pair
of Richardson's merlins breeding on the Saskatchewan prairie
survived on a diet of 53.5% horned larks, 13.3% brown-headed
cowbirds (Molothrus ater) , 6.7% vesper sparrows (Pooecetes
gramineus) , 6.7% song sparrows (Melospiza melodia) , 6.7% Baird's
sparrows (Ammodramus bairdii) , and 13.6% chestnut-collared
longspurs (Calcarius ornatus). With the exception of Baird's
sparrows, these species all were abundant in the merlins'
hunting territory (Fox, 1964) .
In Isle Royale National Park, a pair of breeding taiga merlins
consumed primarily birds, including warblers, sparrows and
crossbills, red-breasted nuthatch (Sitta canadensis), common
nighthawk (Chordeiles minor) , common flicker (Colaptes auratus) ,
and an unidentified member of the Rallidae. These merlins
also preyed on bats (red bats, Lasiurus borealis , and Myotis sp.),
dragonflies (Odonata), and beetles (Coleoptera) (Johnson and
Coble, 1967). Taiga merlins migrating past Cape May, New
Jersey, also had consumed red bats (Allen and Peterson, 1936).
The few other mammals recorded in merlin diets include pocket
gophers (Geomyidae) , squirrels (Sciuridae), and field mice
(Microtus sp.) (Bent, 1938; Allen and Peterson, 1936; Burleigh,
1972).
During summer and fall, insects increase in importance in the
merlin diet. Young falcons, especially, seem to rely on
insects for food as they develop their predatory skills
(Williams and Matteson, 1947; Eyre and Paul, 1973; Lawrence,
1949).
Jewett (1939) described two immature female black merlins in
Oregon which had eaten nothing but black ground beetles
(Carabidae). Other arthropods in merlin diets include cater-
pillars, butterflies, and moths (Lepidoptera) , grasshoppers
and crickets (Orthoptera) , spiders (Araneida), scorpions
(Scorpionida) , and crayfish (Astacus and Cambarus sp.) (Bent,
1938; Allen and Peterson, 1936; Fox, 1964).
Dragonflies, however, make up the most important insect element
in merlin diets. Again, the young birds in particular seem to
favor these winged insects. Several authors have marveled at
the flying skills demonstrated by young merlins hunting dragon-
flies and other insects (Lawrence, 1949; Street, 1960; Oliphant,
1974). Their twists and swoops almost resemble the flight of
swallows (Hirundinidae) , with the notable difference that merlins
snatch insect prey with their feet.
14
At Cape May, New Jersey, large numbers of migrant merlins and
dragonflies occurred on the same days. Investigation of 41
stomachs showed that these falcons had eaten more dragonflies
than anything else (Allen and Peterson, 1936).
Merlin castings, or pellets, have been noted to contain
disintegrated feathers, beetles, red bat fur, and avian bone
chips. Thirty-three taiga merlin castings averaged 25 X 12 mm,
tapering at one end (Johnson and Coble, 1967; Breckinridge and
Errington, 1938).
Hunting behavior
Forest-nesting merlins occasionally do not hunt in their breed-
ing territory, since they prefer more open hunting areas.
Lawrence (1949) recorded eight passerines known to be merlin
prey that nested safely in the immediate vicinity of a taiga
merlin nest. Other small birds passed through the territory,
sometimes very close to the nest, without incident. The breed-
ing merlins also ignored small mammals, such as squirrels and
chipmunks (Tamias sp.). Brown and Amadon (1968) suggested that
merlins' aggressiveness toward common crows (Corvus brachyrhynchos)
and other raptors near their nest might further benefit
passerines nesting in the same area. In contrast, a male
Richardson's merlin in Saskatoon pursued robins and house
sparrows from his perch near the nest site, though he generally
hunted at some distance from the nest (Oliphant, 1974).
Merlins commonly fly low over the ground when hunting, darting
after prey with rapidly beating wings. These falcons will
glide short distances, frequently rising and falling, but
glide and hover less than many other falcons. On the attack,
merlins fly at avian prey with a direct, very fast dash. If
this first strike fails, they generally try a series of short,
twisting stoops from above, shooting up after each attack for
a few wingbeats and twisting into another stoop. When necessary,
merlins follow prey into dense cover (Brown and Amadon, 1968;
Fox, 1964; Lawrence, 1949). High-speed stoops from very high
above prey, in the manner of the larger falcons, seem less
common. Laing (1938) described a black merlin along the
British Columbia coast which struck a black swift (Cypseloides
niger) in a spectacular stoop. The Craigheads (1940) watched
a taiga merlin take a tree swallow in similar fashion. Merlins
hunting over water often pursue escaping prey high into the air
in a ringing flight and then capture the prey in a high speed
stoop as the victim tries to dive to cover (Temple, personal
communication). McClure (1957) described a merlin in Japan
that took a brown-eared bulbul (Ixos amaurotis) to the ground
in a stoop and killed it with a bite at the base of the
skull.
15
Blackbirds (Icteridae) , starlings (Sturnus vulgaris), meadow-
larks (Sturnella sp.), shrikes (Lanius sp.), and kingbirds all
perch conspicuously in open places where merlins can strike
from behind without warning. In the open, the slow flight of
meadowlarks, as well as woodpeckers and flickers (Picidae),
make them vulnerable to direct attack. In forests, merlins
often concentrate hunting efforts on bright-colored and tree-
nesting passerines. Merlins fly full-speed into flocks of
ground -dwelling birds such as horned larks, but seldom make a
second attempt if the first surprise attack fails. With
enough room to gain top speed, mourning doves (Zenaida macroura) ,
rock doves, and robins can outdistance merlins. Smaller,
slower birds often escape by abrupt and agile dodging (Bond,
1936; Lawrence, 1949; Fox, 1964).
Courtship
Male merlins arrive on the breeding territory about a month
before females. Most males arrive in April, but male Richardson's
merlins may arrive back in Alberta and Saskatchewan as early
as late February or early March. Further north, taiga merlins
may not finish migration until May (Bent, 1938; Fox, 1964;
Brown and Amadon, 1968; Lawrence, 1949; Fyfe, personal communi-
cation; Beer, 1966).
On arrival, males make a series of vocal flights from perch to
perch. After the female's arrival, the male repeats this per-
formance near her with a very distinctive fluttering flight.
The wings beat very rapidly and the male calls while flying
(see page 20 for description of call). Richardson's males have
demonstrated this same flight pattern and call when flying to
the female to copulate (Fyfe, personal communication; Brown
and Amadon, 1968) .
Female Richardson's merlins evidently do not solicit the males'
attention. Males, in contrast, call and display both in and at
the entrance to nests to attract the female to the nest. The
male's display posture seems aimed at best presenting the blue
back, upper wings, and head, and at the same time, the full
tail pattern. Males accomplish this by bowing low and raising
the tail, while at the same time spreading the tail and slightly
raising the wings. In this way they present to the female an
almost solid form of blue combined with the striking contrast
of the tail bars. In a variation, the male presents to the
female at one side or the other — usually the left — instead of
head-on. From the side, the male varies his posture slightly,
tilting to one side and raising the more distant wing above
his body, thereby again presenting the back pattern to the
female (Fyfe, personal communication).
16
Nest site fidelity and nesting density
Merlins use the same general area year after year for breeding,
but not necessarily the same actual site, particularly if
young were fledged the previous year (Brown and Amadon, 1968;
Fox, 1964). Beer (1966) observed defending merlins around a
single Minnesota nest in six out of fifteen years. Merlins
nesting in Montana did not reuse the same nest, or nest grove,
in successive years, although a successful 1974 fledging occurred
150 meters from a 1973 nest site (Ellis, 1974). In Newfoundland,
one ground-nesting site on the side of a cliff was used con-
tinuously for 23 years (Temple, personal communication).
Nesting density is generally low--probably as a result of low
merlin populations. The closest concurrent Richardson's
merlin nestings found in Montana were 17 km. apart (Ellis, 1974),
while three pairs of this subspecies nested in one 4.8-km.
stretch of the North Saskatchewan River (Oliphant, personal
communication). Craighead and Craighead (1940) found a concen-
tration of merlins nesting on lakes on the Minnesota-Ontario
border where nests were at least 3.2 km. apart. Raspberry
Island, an 11. 73-hectare islet off Isle Royale, Michigan,
supported one pair of merlins and their two young; the adults
hunted primarily in a spruce bog and along sheltered coves on
the periphery of the island (Johnson and Coble, 1967). Lawrence
(1949) estimated that a single pair of taiga merlins hunted in
a circular area of 1.6 km. radius, with the nest at the center
of the circle. In Saskatoon, a male hunted up to 3.2 km. from
its probable nest (Oliphant, 1974).
Egg-laying and incubation
Merlins lay clutches of four to five eggs. Taiga merlins
breeding in Newfoundland in 1969 averaged 4.3 eggs/clutch (N=15)
(Temple, 1972c). Richardson's merlins on the Canadian prairie
averaged 4.48 eggs/clutch, the mode being four and the range
3 to 7 (Fox, 1964). Montana prairie merlins laid five-egg
clutches in two out of five nestings, and observers noted five
young at another (Ellis, 1974). Second clutches of Richardson's
merlins generally number only three eggs (Bent, 1938). Females
lay eggs at two-day intervals (Fox, 1964, 1971; Brown and
Amadon, 1968; Williams and Matteson, 1947; Eyre and Paul, 1973).
At the southern fringes of their range, merlins may lay eggs in
early April (Brown and Amadon, 1968; Williams and Matteson,
1947). Egg-laying commences on the Canadian prairies during
the first half of May, and peaks about May 20 (Fox, 1964;
Oliphant, 1974). Taiga merlin eggs may be laid in late May or
June (Brown and Amadon, 1968; Bent, 1938; Craighead and Craig-
head, 1940). Temple (1972c) found that 20 pairs of Newfoundland
taiga merlins were incubating eggs between May 24 and July 9.
17
Merlin eggs vary in shape from short-ovate to oval to nearly
elliptical-ovate. They look much like miniature peregrine
falcon eggs. Egg ground color is cinnamon to light creamy
buff, and is almost obscured by spots and blotches of burnt
umber, chocolate, and red-brown. Occasional clutches are
nearly white. Fifty Richardson's merlin eggs averaged 40.2 mm
X 31.3 mm, with ranges of 37.0 - 44.5 X 30 - 33.5 mm. Sixty
taiga merlin eggs averaged 40.5 mm X 31.4 mm, and ranged from
37 - 44.5 X 30 - 33 mm (Bent, 1938; Wolfe, 1946; Brown and
Amadon, 1968). Prairie-nesting Richardson's merlins lay
significantly larger eggs than other merlins (Temple, 1970).
Partial incubation begins before the female completes the
clutch, and full-time incubation begins with the laying of the
last egg. The female does most of the incubating, and the male
most of the hunting, calling the female off to receive food.
One male Richardson's merlin in Saskatoon assumed incubation
duties whenever he brought food to the female, spending as
long as two hours on the eggs while his mate fed (Oliphant,
1974). During incubation, male taiga merlins in Newfoundland
flew from the nest about one-third of the times birds were
flushed (Temple, 1972b). The Craigheads (1940) noted females
sharing in hunting duties soon after the young had hatched. In
observations of captive Richardson's merlins, only the female
incubated (Fyfe, personal communication). Incubation lasts
from 28-32 days (Fox, 1964; Brown and Amadon, 1968).
During the incubation period, the male supplies the female with
food. She flies off the nest to receive prey, either on a
perch or in the air. Oliphant (1974) noted that the female
took prey in the air only after chicks hatched. Prior to hatch-
ing, she alighted on a perch facing the male and both grasped
the prey in their beaks. After spreading their wings, pulling
and calling softly, the male let go and the female flew to her
favorite plucking perch.
Immatures can and perhaps regularly do establish territories
and acquire mates. Three of 20 pairs of taiga merlins breeding
in Newfoundland in 1969 contained males undergoing their first
prebasic molt and not yet in definitive adult plumage; all
three of these pairs fledged young (Temple, 1972c). Bent (1938)
noted a breeding taiga female in immature plumage which had
laid a full clutch of eggs.
Juvenile development
Young hatch at intervals. The eldest is markedly larger than
the youngest, whose small size and lack of aggressiveness at
feedings usually result in the smallest receiving the least
food. Sixty-two percent of 21 taiga merlin nestlings were
18
female (Craighead and Craighead, 1940). Forty-eight percent
of 30 Richardson's nestlings were female (Fox, 1964). Hatching
occurs in Alberta and Saskatchewan in early June. In northern
forests, taiga merlin eggs generally hatch in late June (Brown
and Amadon, 1968; Lawrence, 1949; Fyfe, personal communication;
Craighead and Craighead, 1940).
Adults may feed young on the first day out of the shell. Food
provision consumes more and more time and energy of adults as
the young quickly grow. For the first week or two, the female
broods the young, receiving prey from the male on brief flights
from the nest. Only the female has been noted brooding. Males
or females may cache surplus prey on the ground or in tree
cavities for future use. The male generally transfers avian
prey to the female intact, who plucks it and often consumes
the head. Females feed young birds small bits of food, and
feedings increase in length from five minutes during the first
week to as long as 25-35 minutes, later in the development of
the young. Occasionally, the male plucks prey before transfer,
and very rarely feeds the young himself--this last occurrence
has been recorded only in taiga merlins in Minnesota by the
Craigheads (1940). Taiga merlin females in Newfoundland often
forced the male aggressively from the nest if he lingered after
delivering food (Temple, 1972b). Even up to the time chicks
are well-feathered, the female broods them at night. Daytime
brooding continues until the chicks reach about two weeks of
age (Lawrence, 1949; Selous, 1915; Breckinridge and Errington,
1938; Oliphant, 1974; Temple, personal communication; Brown
and Amadon, 1968; Fox, 1964).
At hatching, the young are weak and immobile, their eyes closed.
At three days of age they open their eyes. Activity increases
steadily, until at 10 days active yawning and wing-flapping
occur, while the primary and secondary sheaths become visible.
Three 18-day-old young weighed about 165, 162, and 106 grams.
At 18-20 days feathers cover almost all the down, wings are
approaching full development, and the young falcons spend much
time in preening, sometimes mutual preening. At this age,
females may be separated from males by their larger feet and
tarsi and bulkier form. At 25 days young beg food from their
parents, "play" with nest mates, and sleep in the adult
position (Fox, 1964; Brown and Amadon, 1968; Lawrence, 1949;
Taylor, 1914).
Merlins fly at 25-35 days, averaging just about a month.
Although they leave the nest at this time, their powers of
flight are poor, amounting to little more than extended gliding.
They remain dependent on their parents for food for several
weeks as they perfect predatory and flying skills. Within a
week after fledging, young take long training flights, and at
the end of a second week can successfully catch insects. They
remain in the general vicinity of the nest during these two
19
weeks. Dispersal begins soon after, until young birds reach
complete independence at about five weeks after fledging (Fox,
1964; Eyre and Paul, 1973; Oliphant, 1974; Lawrence, 1949;
Fyfe, personal communication; Brown and Amadon, 1968).
Territoriality and interspecific relations
Merlin eggs and nestlings occasionally fall prey to predators,
but the adults' defense of the nest generally discourages
territorial trespassers. Avian predation, however, could reach
considerable proportions if parental behavior is seriously
disrupted by pesticide residues (Fyfe, personal communication).
Merlins usually drive away potential predators as soon as they
penetrate the nesting territory, reacting when the invader is
as much as .8 km. from the nest. Except for domestic dogs,
and foxes in Newfoundland (Temple, personal communication),
observers note merlins attacking only avian predators. Breed-
ing and wintering falcons have chased red-billed tropicbirds
(Phaethon aetherus) , sharpshinned hawks, Cooper's hawks (Accipiter
cooperii) , broad-winged hawks (Buteo platypterus) and other
buteos, golden eagles (Aquila chrysaetos) , prairie falcons (Falco
mexicanus) , peregrine falcons, other merlins, American kestrels,
great blue herons (Ardea herodias) , gulls (Larus sp.), great
horned owls (Bubo virginianus) , crows, Clark's nutcrackers
(Nucifraga columbiana) , and common grackles (Quiscalus quiscula) .
Antagonism between merlins and any larger falcon seems to
intensify if the larger bird took quarry; merlins seem particu-
larly intolerant of accipiters (Fox, 1964; Lawrence, 1949;
Bent, 1938; Oliphant, 1974; Fyfe, personal communication; Ellis,
1974; Allen and Peterson, 1936; Taylor, 1914). Migrating
merlins frequently steal prey from flying kestrels and sharp-
shinned hawks by stooping at them and snatching the prey as
the bird tries to defend itself (Temple, personal communication).
Merlins, in turn, may be harassed by other species. Bent (1938)
notes an attack on an Alaskan merlin by seven black-billed
magpies (Pica pica) . Fox (1964) watched his captive male mobbed
by a party of one robin, a pair of northern orioles (Icterus
galbula) , one common grackle, and several house sparrows. Other
observers have seen robins attack merlins very aggressively
(Fox, 1964; Lawrence, 1949). Hummingbirds (Trochilidae) con-
sistently harassed Bond's (1936) trained female taiga bird,
sometimes as many as 6 or 7 at once. On the breeding grounds,
taiga merlins are often attacked by lesser yellowlegs (Totanus
flavipes) if they approach the shorebird's nesting territory
(Temple, personal communication).
Throughout their breeding range, merlins are notoriously bold,
fearless, noisy, and aggressive in defending their nest from
human intruders. All three subspecies consistently advertise
20
nest locations through aerial and highly vocal defense (Bent,
1938; Lawrence, 1949; Laing, 1938; Wolfe, 1946; Williams and'
Matteson, 1947; Craighead and Craighead, 1940; Ellis, 1974).
Generally the male merlin reacts to intruders first, flying out
to scold them as far away as 1.5 km., and frequently at .8 km.,
from the nest (Ellis, 1974; Fox, 1964; Bent, 1938). Incubating
or brooding females may remain silent on the nest under very
close approach--less than 100 meters, or even until the observer
begins to ascend the nest tree (Ellis, 1974; Bent, 1938). Once
flushed, female merlins defend the nest more aggressively than
males, in most cases (Fox, 1964; Lawrence, 1949; Craighead and
Craighead, 1940).
Richardson's merlins defend their territory from the date of
the male's arrival, peaking in intensity when the young are
about two weeks old. Defensive behavior then diminishes rapidly
until it disappears about one month after fledging (Fox, 1964).
One notable exception exists in this pattern of nest defense.
Merlins nesting in densely populated areas of Saskatoon did not
defend their nesting territory, but tolerated humans, dogs,
cats, passing traffic, and large birds (including crows and
gulls) without apparent concern or vocalization. Only when the
nest tree was climbed to band the young, and when crows and
grackles came within 6 meters of the nest, did the adults act
defensively. These merlins did vocalize frequently when in-
specting possible nest sites. At least one, and probably two,
pairs of crows successfully reared young within 61 meters of
one nest with very little interaction with the merlins.
An estimated 5,000 to 10,000 people passed within 15 meters of
two of these nests while the merlins occupied them. The merlins
may have adapted to their urban environment by limiting nest
defense to the nest site itself to avoid spending literally all
day in territorial defense—which would attract still more
predators to the nest site (Oliphant, 1974). The contrast with
parental behavior in all other merlin populations also suggests
pesticide residues as a possible cause of the Saskatoon breeders'
abnormal behavior (White, personal communication), though Fox
(personal communication) reports that these birds lay eggs with
thicker shells and lower residues than nearby rural populations.
Oliphant (personal communication) has observed some successfully
breeding pairs in Saskatchewan outside the city that have shown
little aggression toward territorial intruders.
21
Vocalizations
The merlin's most frequently heard call is given during nest
defense. Observers have variously described this as a harsh,
loud, piercing, high-pitched "ki-ki-ki-ki-keeee" (Craighead and
Craighead, 1940; Brown and Amadon, 1968; Oliphant, 1974);
a "kla-kla-kla-kla-kla-kla," similar to the call of the American
kestrel (Bent, 1938); or a "keeeyick-kyick-kyick-kyick-kyick"
(Lawrence, 1949). Brown and Amadon (1968) attribute this
"ki-ki-kiee" call to the male, and a lower-pitched "kek-ek-ek-
ek-ek" to the female. Several other authors have described a
"kek-ek" call, while not attributing it solely to the female
(Craighead and Craighead, 1940; Bent, 1938). Oliphant (1974)
noted that Richardson's merlins used this primary call during
most male/female interactions, especially when the male brought
food to the nest. A softer version of the call occurred when
adults alternated incubatory duties or transferred prey while
perched on a limb (see page 16 of this report). Female merlins
have used the primary call while calling in fledged young to
feed (Street, 1950).
In courtship flight, the male gives a "chrrrrr" call while
flying. During copulation the call may be either this same
"chrrrrr" or a slightly different and higher note. In display-
ing to the female at the nest, the male gives a high "chip,
chip, chip" note which the female may answer (Fyfe, personal
communication) (see also page 14). This "chip" note may be the
same call described in males by Brown and Amadon (1968) as a
thin, chirruping "chic-chic-chic" uttered more slowly than the
rapidly repeated primary call, and by Bent (1938) in black
merlins in their nest tree with fledged young: a plaintive
whimpering not unlike the "kik-kik-kik" call of the flicker,
but much more subdued.
In calling females off the nest to receive food, males may use
a series of single, soft "chups" (Oliphant, 1974). In the same
situation, the Craigheads (1940) noted a male to emit a call
very similar to the long drawn out food cry of the peregrine
falcon, but higher pitched. Females solicit food with an extended
plaintive "eeep-eeep-eeep" (Brown and Amadon, 1968).
When the female feeds the young during the first week or two
after hatching, and while she herself feeds, she emits a sharp
"tick-tick-tick" or a series of "clicks," touching a chick's
bill with a morsel of food at the same moment (Lawrence, 1949).
The young, in turn, give a "keeyep-keeyep-keeyep" begging call
(Lawrence, 1949; Brown and Amadon, 1968). Just prior to and
after fledging, young merlins may emit a series of single rising
notes, or "chees," especially at feeding times. Within a week
22
-■^■.in^.JM^rwra^
of fledging, the young birds space these single calls more
closely, and begin to group them in calls more like those of
adults (Oliphant, 1974).
Migration and wintering behavior
Richardson's merlins migrate a relatively short distance into
the southern Great Plains; a smaller segment of the population
winters on their northern plains breeding grounds. The black
merlin resides semipermanently in the climatically stable
coastal forests of the Northwest, and only a small portion of
the population migrates south of their breeding range. Most
taiga merlins, in contrast, are highly migratory. Western and
central taiga-breeding birds migrate past Richardson's and
black merlin populations in leapfrog fashion to winter in
Central*and South America. Eastern taiga birds also migrate
far to the south, many individuals crossing the Gulf of Mexico
to winter on the Caribbean Islands and in South America. A
few taiga birds remain in the boreal forest all year (Temple,
1972a; Beer, 1966).
During fall migration, merlins tend to follow sea coasts,
borders of streams and large lakes, and open country, preying
on small birds also making their way south—although in
Minnesota merlins seem to scatter, rather than follow well-
defined migration routes (Brown and Amadon, 1968; Bent, 1938;
Beer, 1966). Fall migration is much more conspicuous than the
return trip northward in spring (Brown and Amadon, 1968; Bent,
1938). Along the New Jersey coast, taiga merlins (along with
kestrels and peregrines) wander southward over salt meadows in
preference to wooded areas favored by migrating accipiters
(Allen and Peterson, 1936). At Fishers Island, New York, they
favor flying on a southwest wind (Bent, 1938). Merlins fly
rather high, sometimes in large straggling flocks (Brown and
Amadon, 1968). In Washington, migrant black merlins prefer
open country and tidal flats along Puget Sound in both fall and
early spring (Bent, 1938). Some merlins in Alaska migrate along
the Alcan Highway, perhaps to take advantage of the abundant
food in the open pathway through the forest, for thousands of
Lapland longspurs (Calcarius lapponicus) migrate along this
same route (White, 1969).
Immature merlins generally migrate before adults, leaving breed-
ing grounds about one month to six weeks after leaving the
nest (Bent, 1938; Brown and Amadon, 1968). Young Richardson's
merlins drift southward from Saskatchewan in mid-August (Fox,
1964), about as early as any merlins in North America. Migra-
tion peaks between mid-September and mid-October in most areas.
Nearly all merlins have left the northern fringes of their
range by September 15; migration peaks in Minnesota during the
second and third weeks of September, in South Carolina in
October (Brown and Amadon, 1968; Beer, 1966; Bent, 1938). Most
23
Richardson's merlins arrive on their eastern Colorado wintering
grounds by the last two weeks of October; immatures migrate
farther south than adults--as far as Arizona and New Mexico,
rather often. More adults remain on the Canadian prairies
through the winter (Fox, 1964; Bailey and Niedrach, 1965).
Some wintering birds seem to wander at random; others stay for
some time at a location (such as an urban area) that provides
abundant prey (Fox, 1964; Oliphant, personal communication).
The three races may mingle in winter quarters (Eyre and Paul,
1973; Brown and Amadon, 1968).
In spring, most merlins pass northward through central North
America in March (Brown and Amadon, 1968; Fox, 1964; Williams
and Matteson, 1947). Most taiga-bound birds pass through
northern Minnesota in April (Beer, 1966). The very last to
leave the more southern winter ranges head north in late March
and early April (Bent, 1938).
Productivity
Productivity in North American merlin populations has declined
significantly in the past 20 years. Prior to about 1950 (and
prior to the widespread use of persistent organochlorine pesti-
cides), merlins across the continent hatched more young per
nest than they have since that date.
The mean clutch of merlins has varied little since the first
data were recorded. The number of young hatched per nest in
the northern Great Plains prior to 1950 averaged 4.3 (N=3) ,
from a mean of 4.7 eggs/nest (N=10) (Fox, 1971). In eastern
Canada, taiga merlins laid 4.2 egg clutches (N=18), and
hatched 3.8 young per nest with young (N=15) prior to 1947
(Temple, 1972c).
Eastern taiga birds have suffered lower productivity since
those years. From 1950-1969, these merlins laid average
clutches of 4.5 (N=2), from which they hatched only 2.2 young
per nest with young (N=6) ; hatching success totaled 48%, down
from almost 91% prior to 1947 (Fox, 1971; Temple, 1972c).
Newfoundland merlins in 1969 laid clutches averaging 4.3 eggs
(N=15), and hatched 3.0 young per nest with young (N=19)
(Temple, 1972c). Although these Newfoundland birds reached
only 707o hatching success, they fledged young in 95% of 20
nests. Central and northern forest-breeding birds have main-
tained the highest productivity of any North American merlins
in recent years. From 1960-1969, northern taiga merlins laid
4.5 eggs/nest (N=2) , hatching 4.0 birds per nest with young
(N=5)--a hatching success of 897o (Fox, 1971). Merlins breeding
in forested areas of the Great Plains maintained good production
24
of young from 1950-1969, laying 4.1 eggs/nest (N=9), and hatch-
ing 98% of these--4.0 young per nest with young (N=16) (Fox,
1971). Only one of two taiga merlin nests recently found in
Alaska fledged young, but this single nest fledged five birds
(Adolphson, 1969).
Richardson's merlins nesting in the Great Plains have decreased
more alarmingly in productivity. Fox (1964) found that 10 pairs
of Saskatchewan-nesting merlins laid an average of 4.5 eggs/nest,
Hatching success, 91% prior to 1950, decreased to 60% for these
birds, which hatched 2.7 young per nest with young. Fledging
remained good, however, with 2.6 young fledged per nest. Most
disturbing, though, is the fact that this population of merlins
ceased to breed soon thereafter (Fox, 1971). Hatching success
throughout the Great Plains from 1950-1969 averaged 49% (Fox,
1971). Richardson's merlins breeding in Montana laid clutches
of five eggs, but hatched only 3.2 young per nest with young
(Ellis, 1974). They fledged 3.2 birds per nest with fledglings;
57.1% of seven nests fledged young. Hunter (1967) reports that
three merlin nests in South Dakota all failed to fledge young.
Five of eight (62.5%) Saskatoon nests fledged young; all five
fledged 4-5 young (Oliphant, 1974).
These data indicate decreasing productivity in North American
merlin populations. Fyfe (personal communication) attributes
lowered Richardson's merlin productivity primarily to desertion
during incubation and failure of incubated eggs to hatch.
Temple (1972c) believed that problems in Newfoundland were
probably due to increased egg loss and decreased hatchability .
Broken eggs and adult mortality resulted in 2 out of 3 nest
failures in Montana (Ellis, 1974).
Pesticide relationships
Research has linked decreased reproductive success and popula-
tion declines in bird-eating raptor populations to accumulation
and concentration of chlorinated hydrocarbon pesticide residues
through the food chain. Zarn (1974) has reviewed briefly the
effects of DDT and other organochlorine pesticides on raptors.
Aberrant behavior and eggshell-thinning seem the most conspic-
uous symptoms of pesticide poisoning. Specifically, chlorinated
hydrocarbons seem to inhibit carbonic anhydrase activity in the
shell gland, resulting in decreased calcium carbonate deposition
in the shells of developing eggs, and consequently, thinner-
shelled eggs.
Merlins, like other bird-eating falcons, are suffering reduced
productivity probably resulting from accumulation of pesticide
residues. The lipid of four adult merlins migrating along the
25
western shore of Lake Michigan contained an average of 302 ppm
DDE, and 196 ppm PCB compounds (polychlorinated biphenyls) . DDE
accounted for 96% of all DDT residues in the birds. The lipid
of three immature merlins contained 50.3 ppm total DDT, of
which 49.3 ppm was DDE. PCB ' s averaged 28.6 ppm. All but one
of the merlins biopsied (both adult and immature) contained
more DDE than PCB ' s . Peregrines trapped here showed the
reverse ratio. Pesticide fallout must evidently vary in the
areas of boreal forest and tundra where these two migrants
respectively breed (Risebrough, e_t a_l. , 1970). These figures
may be compared to DDE figures for lipids from adult peregrines
from the MacKenzie River region (Enderson and Berger, 1968) and
from Alaska (Cade, e_t al_. , 1968) of 392 ppm and 725 ppm,
respectively. The merlins have almost the same level of con-
tamination as the peregrines, and might therefore be expected
to show eggshell thinning.
Fox (1971) calculated eggshell weights for 51 merlin eggs
collected from 1890 to 1969. From 1890 to 1949, eggshells
from Great Plains and taiga-breeding birds did not differ
significantly and averaged about the same through those years--
1.71 grams. From 1950-1969, merlin eggshells averaged 1.31
grams, a 23% decrease. This decrease resembles those reported
by Hickey and Anderson (1968) for declining bald eagle
(Haliaeetus leucocephalus) and osprey (Pandion haliaetus) popu-
lations and extirpated peregrine falcon populations in the
United States. Fox (personal communication) studied 40-50 pairs
of Richardson's merlins nesting in Alberta in 1974, and judged
reproductive success 40-50% of that prior to pesticide use,
while eggshells averaged 15-20%, thinner and pesticide residues
proved high. Swartz (personal communication) has noted high
pesticide residues and eggshell thinning in eggs from interior
Alaskan merlins, although productivity appears fairly normal.
Merlins breeding in Newfoundland contained 267 ppm (dry weight
basis) DDE, while five unhatched eggs averaged 40.4 ppm (dry
weight basis) DDE (Temple, 1972c). These residue counts are
lower than the breeding peregrines and their eggs and lower
than the Lake Michigan migrant merlins. No marked delay in the
breeding cycle of the Newfoundland merlins has occurred, nor
has clutch size decreased significantly since pre-pesticide
times. Eggshells, however, have decreased 97<> in thickness, a
decrease accompanied by reduced productivity.
In Montana, residues (wet weight basis) in 3 non-viable eggs
from one Richardson's merlin nest averaged 9.4 ppm DDE, 8.0 ppm
dieldrin, and 5.6 ppm heptachlor epoxide. PCB ' s , toxaphene,
and other chlorinated hydrocarbons were undetected (Ellis, 1974).
Only one egg contained a dead embryo. Pesticide residues
calculated on a dry weight basis are roughly comparable to 5x the
26
level calculated on wet weight basis; lipid fractions are about
17-20x wet weight (Fox, personal communication). DDE residue
levels in these Montana eggs thus are similar to DDE levels in
Temple's (1972c) Newfoundland eggs. But the Montana merlins
suffered considerably more nest failures--3 out of 7 as compared
to 1 out of 20 in Newfoundland. Embryonic mortality, in con-
trast, was high in the Newfoundland population: all five
unhatched eggs collected for residue samples contained dead
embryos .
HABITAT REQUIREMENTS
Breeding requirements
Taiga merlins breed in the boreal forest, and within this
biome choose somewhat open territories, such as forest edges,
lake shores, bogs, etc. These merlins generally nest adjacent
to water. All nests described from northern Minnesota were in
conifers near the edge of large lakes, many on islands in the
lakes (Beer, 1966). In denser areas of the boreal forest,
taiga merlins usually nest in abandoned crow or common raven
(Corvus corax) nests in coniferous trees. Trees utilized
include many dominant boreal species: black spruce (Picea
mariana ) , red spruce (Picea rubens) , white spruce (Picea glauca) ,
eastern white pine (Pinus strobus) , jack pine (Pinus banks iana),
etc. Along the northern edge of their range, on the fringes
of the tundra, they often nest under tree branches in a scrape
on the ground (Temple, personal communication; Fyfe, personal
communication; Johnson and Coble, 1967; Bent, 1938; Craighead
and Craighead, 1940; Lawrence, 1949).
Taiga merlins may reline the nest with some combination of
small twigs, pieces of bark, feathers, grass, soft rootlets,
and conifer needles. They nest almost anywhere in trees, from
1.5 meters above ground to the top of 18-meter spruce, usually
very close to the trunk. Taiga merlins also have nested in
tree cavities, under the roofs of deserted buildings, and on
cliff ledges in nests made of bits of rock (Bent, 1938; Craig-
head and Craighead, 1940; Fox, 1964; Williams and Matteson,
1947; Lawrence, 1949).
Few nests of the black merlin have been described in detail.
These merlins breed in moist Pacific coastal forests, probably
in areas similar to the open habitat favored by taiga merlins
in boreal forest. Most observed nests of F. c_. suckleyi have
been very high in conifers in dense coastal stands of Douglas
fir (Pseudotsuga menziesii) , Sitka spruce (Picea sitchensis) ,
western hemlock (Tsuga heterophylla) , Pacific silver fir
(Abies amabalis), and western redcedar (Thuja plicata) . Both
stick-nests and cavity-nests are known. Black merlins frequently
nest near large lakes (Temple, personal communication; Laing,
1938; Bent, 1938).
2/
Richardson's merlin breeds only in the prairie-parkland of the
northern Great Plains. Within this biome, the subspecies seems
to prefer isolated groves of trees with open prairie surround-
ings, mixed woods, and wooded areas along prairie river banks
and islands (Temple, personal communication; Fyfe, personal
communication; Fox, 1964).
F. c_. richardsonii tend to breed where native grassland provides
adequate prey species, and quaking aspen (Populus tremuloides)
or other trees provide suitable nesting habitat. On the Sas-
katchewan-Alberta prairie they prefer to nest in deserted
farmstead shelterbelts and groves of deciduous trees where
farmland has gone back to grassland. The merlins choose trees
spaced well apart whose lower 2.4 or 3 meters have been rubbed
bare of branches, and where undergrowth has been trampled and
destroyed by cattle. This appears to be a temporary stage in
the total destruction of trees by cattle, and the number of
sites that fit this description is steadily decreasing. Fox
(1964) found about half of 25 nests in Saskatchewan within .8
km. of water. He also noted that water may be utilized by
merlins for bathing. Captive merlins have been known to bathe
in water, although Wing (1950) observed a wild merlin dust
bathing (Fyfe, personal communication; Fox, 1964, 1971).
In southern areas of the open grassland, Richardson's merlins
nest most frequently in deserted crow and magpie nests, and
seem to have a preference for the latter (Fyfe, personal communi-
cation). Ninety-two percent of Fox's (1964) nests were old
nests of these two species. On the same Saskatchewan study
area, 21 of 25 merlin pairs nested in deciduous trees: aspen,
poplar (Populus sp.), box elder (Acer negundo) , willow (Salix sp.),
and birch (Betula sp.). The key determinant is the nest rather
than the species of tree: in areas where crows and magpies
nest in conifers, merlins will then nest in these nests. All
of seven Richardson's merlin nests on the Montana prairie were
in groves of conifers—Douglas fir, ponderosa pine (Pinus
ponderosa) , and limber pine (Pinus flexilis) --near flat or
rolling grassland. All these merlins nested in old corvid
nests--six of them magpie nests (Ellis, 1974).
The prairie subspecies occasionally nests in tree cavities and
in old magpie nests placed in holes in cliffs (Fyfe, personal
communication). Nests are usually lined with dry inner bark
of poplar (Bent, 1938). Seven of eight Richardson's merlin
pairs in Saskatoon nested high in large spruce, with tall
deciduous trees nearby, always in what appeared to be an old
crow's nest. Two pairs nested in highly populated areas within
about 30 meters of major roads with heavy traffic (Oliphant,
1974).
28
No observers have recorded Richardson's merlins nesting on the
ground, a most important fact considering the serious shortage
of suitable nest trees in the subspecies' range. A shift in
nesting patterns to include ground-nesting would greatly in-
crease habitat available for this bird (Fyfe, personal communi-
cation), though predator pressure would also increase (Ellis,
personal communication) .
Wintering requirements
Wintering taiga merlins tolerate an amazing range of habitats--
from beaches, dunes, and swamps to tropical forests and scrub,
to farmland and urban areas. Black merlins also range through
a variety of habitats on their wintering grounds--desert scrub,
coastal forest, coniferous forest, farmland, and urban areas
(Temple, personal communication).
Richardson's merlins, on the other hand, seem more selective in
choosing wintering grounds, preferring prairie habitat with
scattered trees, utility poles, or other hunting perches-
habitat very similar to their breeding habitat (Temple, personal
communication). Fence posts and lower perches attract the
falcons even where telephone poles are available (Fox, 1964).
Wintering Richardson's merlins favor winter wheat stubble in
Wyoming, and in Colorado prefer very open country on the plains
or land along or near creek beds overgrown with cottonwoods
and willows (Fox, 1964) . In Utah, where all three subspecies
winter, Richardson's merlins occur with greatest frequency in
open farmland; the other two subspecies show no strong habitat
preference (Eyre and Paul, 1973). In northern parts of its
wintering range, Richardson's merlin also frequents urban
areas; many immature birds, for example, winter in Saskatoon
(Temple, personal communication; Fyfe, personal communication;
Oliphant, 1974 and personal communication).
LIMITING FACTORS
Three factors presently limit merlin populations. Two are com-
ponents of the environment required for successful reproduction:
first, an adequate food base and second, available, and suitable,
nest sites. The third factor exists in varying strength, and
results in varying decreases in productivity: pesticide resi-
dues (Fyfe, personal communication).
Habitat destruction
Habitat destruction to date is of minor importance for taiga and
black merlins. Hence, their supplies of food and nest sites
remain adequate (Fyfe, personal communication; Temple, personal
communication). This is not true for Richardson's merlins,
however.
29
An adequate food base for prairie merlins means an adequate
supply of grassland birds—which can only mean uncultivated
grassland habitat. Without native grassland, there will be
few Richardson's merlins (Fyfe, personal communication). Like-
wise, destruction of habitat reduces the number of adequate
nest sites—which, for this subspecies, are narrowly defined.
Fox (1971) documents a steadily decreasing number of suitable
nest trees on the Saskatchewan prairies. Merlins have ceased
breeding entirely in some areas of intensive cultivation (Fox,
personal communication). Habitat destruction continues to
accelerate as more and more land goes under intensive agriculture,
putting ever greater pressure on merlins of the prairies.
Chemical contamination
Merlins are carrying high pesticide residue levels— organo-
chlorines, mercury, and PCB ' s (Fyfe, personal communication).
Thin-shelled eggs and poor overall productivity have been
noted in varying degrees throughout the range of the species.
Richardson's merlins, however, breed in the most contaminated
habitats and predictably their reproductive success has de-
creased drastically over most of their range. Pesticide
contamination may be the single biggest problem facing North
American merlins (Temple, personal communication; Fox, personal
communication) .
Little seasonal variation exists in the pesticide content of
merlin diets, since most of these falcons prey on the same
species on both breeding and wintering grounds (Temple, personal
communication). Migration patterns in prey species can, how-
ever, dramatically affect pesticide levels in prey and predator.
In Newfoundland, brain tissue of resident gray jays contained
only .24 ppm (dry wt. basis) DDE, while brains of migrant robins
and Savannah sparrows contained 3.17 ppm and 2.10 ppm, respect-
ively. These three species accounted for the bulk of the breed-
ing merlins' diet (Temple, 1972c).
Other mortality factors
Several specific merlin mortality factors have been noted.
Nestlings or eggs may not survive bad storms (Oliphant, 1974),
and may also succumb to disease or parasites, such as Simulium
flies (Ellis, 1974). Adults and young may catch fatal
Trichomonas gallinae (frounce) from consuming infected doves
(Stabler, 1969). Before protective legislation, merlins were
often shot; they allow a close approach when perched and will
fly quite near a man in the open (Bent, 1938; Lawrence, 1949;
Jewett, 1948). Temple (personal communication), however, feels
that shooting no longer poses a serious threat to the species,
30
at least as indicated by banding recoveries. Merlins have
also been known to die on impact with windows in cities
(Porter and Knight, 1952).
Museum specimens of Richardson's merlins examined by Fox (1964)
showed a decrease in the ratio of subadults to adults from 607o
in August to 30-407» in January, when molt begins. Over 707»
of total returns for merlins banded as nestlings occurred during
their first year of life.
Human interaction with nesting merlins may have serious and
harmful effects on reproductive success. Oliphant (1974)
believed that two Saskatoon Richardson's merlin nests may have
failed because the nest tree was climbed during the early
stages of incubation. As far as he knew, no successful nest
was climbed prior to hatching. Temple, Fox, and Ellis all
climbed nests which were ultimately successful, however (Ellis,
personal communication). Bent (1938) notes that if the nest of
a Richardson's merlin is climbed before an egg is laid, the
adults will always desert the nest, though merlins do not desert
easily after the clutch is complete (Fox, personal communica-
tion). In contrast, a pair of taiga merlins moved their nest
and two eggs some 36 meters after two near misses by observers
attempting to collect the adults as specimens (Bent, 1938).
Falconers trap numbers of merlins annually, and have definite
impact on the population of the species. Such impact may prove
positive, since some trapped birds are used in captive breed-
ing projects. But as the species decreases in abundance, the
negative impacts of trapping birds from the wild increase.
SPECIES AND HABITAT MANAGEMENT RECOMMENDATIONS
1. We know little of merlin distribution, breeding behavior,
habitat requirements, or reproductive success in Alaska.
We know virtually nothing about black merlin ecology.
Breeding population surveys, as well as any black merlin
research, are essential before we have the basic data
necessary to formulate management plans. Merlins in
Alaska, although reproducing adequately now, as far as we
know, should be watched very closely. Development of the
state's resources has increased sharply in pace, and wilder-
ness country has begun to shrink. Merlins no longer remain
isolated from the drastic changes in habitat wrought by man.
Managers of public lands have an enormous responsibility to
carefully consider the effects of each of their decisions on
merlins, as well as on all wildlife species.
2. Ban, or at least severely limit, use of persistent pesticides
in North America and in Central and South American wintering
areas .
31
3. In the northern Great Plains, habitat management for
Richardson's merlins seems possible. First, land managers
should give careful consideration to preservation of native
prairie whenever development alternatives exist. BLM-
administered public lands as well as National Grasslands,
National Wildlife Refuges, and National Parks and Monuments,
are prime candidates for preservation of grassland habitat.
Without the native grassland, the merlins' food base will
disappear, along with the merlins themselves soon after
(White, 1974).
Merlin nesting sites should also be provided both on grass-
land preserves and on other managed grasslands. Scattered
small groves of trees managed for merlin nest sites would
benefit the falcon, even in agricultural areas and on
wintering grounds (Temple, personal communication). Con-
struction of artificial nest sites, as described for
ferruginous hawks (Buteo regalis) in Olendorff, 1973, should
also be investigated. Anywhere in the northern plains where
such management occurs, even though perhaps not presently
within the Richardson's merlin breeding range, could prove
valuable for the falcon if it shifts its range slightly to
adapt to the change in habitat.
White (personal communication) has pointed out that such
shifts in range can indeed take place. Observers in Utah
collected one probable Richardson's merlin about 1870, but
from 1920-1960 only non-richardsonii were seen and collected
in the state. In about the mid-1960' s, Richardson's merlins
suddenly increased in frequency in the state, until today
(1975) about half of the eight to ten merlins trapped
annually along the Wasatch front are F. c_. richardsonii .
Fox (personal communication) believes that Richardson's
merlins have already begun to shift their range in response
to increased cultivation and habitat loss in rural Saskat-
chewan and Alberta, and cites recent nestings in Saskatoon,
Edmonton, Montana, and Wyoming are evidence. He predicts
an increase in Montana, Wyoming, and North and South Dakota
nestings .
4. Carefully regulate capture of merlins for falconry purposes.
PROTECTIVE MEASURES INSTITUTED
A. Legal or regulatory
1. Regulations administered by the U. S. Environmental
Protection Agency limiting the use of DDT and other
persistent pesticides within the United States should
32
'•
benefit merlins by reducing pesticide burdens in merlin
prey. Similar regulations are also in effect in
Canada.
2. On March 10, 1972, the United States and Mexico added
merlins to the list of bird species protected by the
convention for the Protection of Migratory Birds and
Game Mammals, originally ratified on February 7, 1918.
B . Captive rearing
One method of protecting species endangered by pesticide-
contaminated environments is to maintain the species in
captivity until the environment becomes safe, and reintro-
duction is possible. Merlins bred successfully in captivity
for the first time in 1974. The following list details
recent captive merlin breeding projects:
1. John Campbell, of Black Diamond, Alberta, raised four
young merlins in 1974--the first successful captive
rearing of the species (Fyfe, personal communication;
Swartz, personal communication).
2. In 1974, Richard Fyfe's (Edmonton, Alberta) captive
merlins laid fertile eggs, but they didn't hatch; those
eggs left with the females (rather than placed in an
incubator) disappeared, perhaps eaten by the adult
falcon (Fyfe, personal communication).
3. L. G. Swartz, at the University of Alaska, had a near-
miss in 1969, when his female merlin died with a full
clutch of five eggs "on the way" (Swartz, 1972).
4. Two other captive breeding projects in the United States
have not been successful to date. The first is run by
a private individual in California. The second, the
U. S. Fish and Wildlife Service project at Patuxent,
Maryland, has succeeded in getting fertile eggs, though
none have hatched (Fyfe, personal communication).
CURRENT RESEARCH
Richard Fyfe and his coworkers of the Canadian Wildlife Service
conduct merlin research in western Canada. Their work has
emphasized Richardson's merlins, but concerns taiga merlins as
well. They have considerable data on merlin courtship and
breeding biology, population changes and habitat degradation,
and residue levels (Fyfe, personal communication). L. W.
Oliphant (Dept. Vet. Anat., Coll. West. Vet. Med., Univ. of
Sask. , Saskatoon) has recently received funds to conduct merlin
research in the Saskatoon area (Oliphant, personal communication)
33
AUTHORITIES
1. Dr. Stanley A. Temple
Laboratory of Ornithology
Cornell University
Ithaca, New York 14850
2. Richard Fyfe (F. c_. richardsonii and columbarius)
Canadian Wildlife Service
Room 1110
10025 Jasper Avenue
Edmonton, Alberta T50 156
Canada
3. Glen A. Fox (F. c_. richardsonii)
Toxic Chemical Section
Canadian Wildlife Service
Environment Canada
Ottawa, Ontario K1A 0H3
Canada
4. Dr. L. G. Swartz (Alaska)
Biology Department
University of Alaska
College, Alaska 99701
5. Dr. David H. Ellis (Montana)
U. S. Fish and Wildlife Service
Room B-3
301 W. Congress St.
Tucson, Arizona 85701
SUMMARY
Merlins, small swift-flying falcons, range through much of the
northern hemisphere. Three subspecies breed in North America,
each in an ecologically distinct region. Male merlins have
blue-gray dorsal plumage, females brownish plumage. Immatures
resemble adult females. Merlins average 25 to 34 centimeters
long, with a wingspread of about 60 cm. Small size, a distinctly
banded tail, dark dorsal plumage, and indistinct facial markings
distinguish merlins from other falcons.
Falco columbarius columbarius , the taiga merlin, breeds in the
boreal forest from Alaska to Newfoundland, and winters south to
South America. F. c_. richardsonii, the Richardson's merlin,
breeds in the prairie-parkland of the northern Great Plains,
wintering mostly in Colorado and Wyoming. The black merlin,
F_. c_. suckleyi, resides semipermanently in the moist coastal
forests of western British Columbia, and perhaps, Washington
and southeast Alaska. Some black merlins winter south as far
as California and New Mexico.
34
All North American merlins have recently decreased in reproduc-
tive success. Organochlorine pesticide residues have contributed
to these decreases. Intensive agriculture has decreased both
the native grassland necessary to support the food base of
Richardson's merlins and the number of suitable nest-trees.
Taiga and black merlins, as well, face habitat destruction as
man develops the natural resources of the northern forests.
Merlins hunt diurnally, occasionally into twilight. They consume
birds, almost exclusively, with insects a minor, but significant,
part of their diet. Mammals are taken occasionally. Merlins
hunt avian prey ranging in size from sparrows to ptarmigan.
They attack with a direct, very fast dash, but also may stoop
from high above prey.
Most males arrive on the breeding territory in April, about a
month before females. Males display to females during courtship.
Merlins generally do not nest in the same site in successive
years, but may nest in the same area. Nesting density is low.
Merlins lay clutches of four or five eggs from April to July,
mostly in May. Females do most, and usually all, of the incu-
bating. Incubation lasts 28-32 days. Males provide food for
the incubating female, and later the young as well. Males,
however, rarely feed the young. Young merlins fledge at about
one month of age. About five weeks later they reach full
independence.
Nest defense by adults discourages most predators. Merlins
react very aggressively to large birds and raptors near their
nests, as they do to human intruders. Some merlins nesting
in urban environments refrain from such aggressive nest defense.
Merlins vocalize often when breeding, using their primary nest
defense call most frequently.
Fall migration is much more conspicuous than spring migration.
Migration southward peaks between mid-September and mid-October.
In spring, most merlins pass through central North America in
March.
Productivity has declined significantly in North American merlin
populations during the past 20 years. Desertion during incuba-
tion, and increased egg loss and decreased hatchability seem
the major problems. These all derive primarily from organo-
chlorine pesticide residues in breeding merlins, resulting in
eggshell thinning, dead embryos, and aberrant parental behavior.
35
*
Taiga merlins breed in open areas of the boreal forest, usually
in abandoned corvid nests in conifers, but occasionally on the
ground. Taiga merlins usually nest near water. Black merlins
favor open habitat in the Pacific coastal forests, nesting very
high in conifers. Richardson's merlin breeds in old corvid
nests on the prairie-parkland, in deciduous trees or conifers--
wherever the crows and magpies choose to build nests. Suitable
nest sites for Richardson's merlins on the Canadian prairies t
are diminishing fast in number. Richardson's merlins do not .
nest on the ground, an adaptation which would greatly expand
suitable habitat.
Wintering taiga and black merlins tolerate a wide variety of
habitats. Richardson's merlins have narrower tolerance for
wintering grounds, seeking open farmland and prairie similar
to their breeding habitat.
Three main factors limit merlin populations: availability of
nest sites, adequacy of the food base, and pesticide residue
levels. All three subspecies depend on these same factors, but
Richardson's merlin populations have been most pressured, and
therefore are in the greatest trouble. Other mortality factors
(disease, weather, etc.) are not as immediately important.
Nests may fail, however, if the nest tree is climbed during
incubation.
36
•
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37
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•
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c
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38
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Miller, L. 1941. The black merlin in southern California.
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southern California. Condor 36:176.
White, C. M. 1969. Breeding Alaskan and Arctic migrant popu-
lations of the peregrine. pp. 45-51 In. J. J. Hickey, ed.
Peregrine falcon populations: their biology and decline.
Univ. Wisconsin Press.
. 1974. Current problems and techniques in raptor
management and conservation. Trans. 39th N. A. Wildlife
and Nat. Resources Conf . :301-312 .
Williams, R. B. and C. P. Matteson, Jr. 1947. Wyoming hawks:
American merlins or pigeon hawks. Wyoming Wild Life 11(12):
21-24.
Wing, L. 1950. Dusting by the merlin (Falco columbarius)
Condor 52:275.
*.-
Wolfe, Col. L. R. 1946. Pigeon hawk breeding in Utah.
Condor 48:97.
40
Zarn, M. 1974. Osprey (Pandion haliaetus carolinensis) .
Habitat Management Series for Unique or Endangered Species,
Report No. 12. Bureau of Land Management, Denver, Colorado.
41 pp.
Additional references (not consulted for this report)
Beebe, F. L. 1955. The black merlin. Falconry News and Notes
l(6):25-29.
Newton, I. 1973. Egg breakage and breeding failure in British
merlins. Bird Study 20(4) :241 -244.
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