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Title: Sooty tern behavior
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Title: Sooty tern behavior
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Creator: Dinsmore, James J.
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Copyright Date: 1972
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Table of Contents
    Front Cover
        Front cover 1
        Front cover 2
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    Back Cover
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Full Text


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of the
Biological Sciences


Number 3



James J. Dinsmore


Volume 16

C-- ).


SCIENCES, are published at irregular intervals. Volumes contain about 300
pages and are not necessarily completed in any one calendar year.

FRED G. THOMPSON, Managing Editor

Consultants for this issue:

Communications concerning purchase or exchange of the publication and all
manuscripts should be addressed to the Managing Editor of the Bulletin,
Florida State Museum, Museum Road, University of Florida, Gainesville,
Florida 32601.

Publication date: 21 January, 1972

Price: $1.00



SYNOPSIS: A 4-year study of the breeding behavior of Sooty Terns (Sterna fus-
cata) 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 differ-
ences 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 Sooty Terns consists mainly of the high flight in which two
birds ascend, usually by jerk-flying, circle, and then descend together in a co-
ordinated glide. Sooty 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 caring for it
somewhat more than the female. The birds are attentive over 95 per cent of
the time, incubating when it is cool, shading when it is hot, and engaging in
other activities such as preening, fighting, and loafing most commonly when
they change between incubating and shading. Nest relief usually occurs in
the evening after 24 or 48 hours of care. Dipping, in which adults dip their
feet, bill and/or breast feathers in the ocean, is common during incubation
and may cool the egg or provide it with needed moisture.
Chicks are closely brooded for 4 or 5 days, after which adults seem to
recognize them individually and spend progressively less time with them. By
the end of the third week, chicks are alone much of the time except when fed.
Adults feed the chick by regurgitation, spend about 3.5 hours per foraging
trip, and often feed a chick several times after one trip. Males feed the chick
somewhat more than females do. Vocal exchanges between parent and chick

The author is currently Assistant Professor of Biology at the University of
Tampa, Tampa, Florida, 33606. He submitted an earlier version of this paper to
the University of Florida in partial fulfillment of the Ph.D. degree (1970). Manu-
script accepted 1 May 1971.

Dinsmore, James J. 1971. Sooty Tern Behavior. Bull. Florida State Mus., Biol.
Sci., Vol. 16, No. 3, pp. 129-179.

5-70. ',2

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Vol. XVI No. 3

seem most important in individual recognition. Occasionally adults feed a
chick other than their own. Chicks first fly when about 8 weeks old and leave
the colony soon 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 Sooties carry food and the distance they travel
to forage.
Sooty Terns have a lower clutch size, longer period of development of the
chick, and first breed when older than most other terns, many of which feed in
marshes and coastal waters. These characteristics of Sooty Tern breeding
biology are similar to those of many other pelagic birds. A distant food supply
and high adult survivorship apparently have contributed to these differences
from other terns.


INTRODUCTION ............... 130
ACKNOWLEDGEMENTS ........... 131
THE DRY TORTUGAS ........... 131
METHODS .................... 133
Formation of the Colony...... 134
Aerial Flocking ............ 134
First Landing ........... . 136
Pre-laying Activities ......... 137
Aerial Display ............. 137
Ground Activities .......... 139
Establishing Territories ..... 142
Incubation Period ........... 144
Egg-laying ................ 144
Behavior During Incubation. 145
Attentiveness .............. 147
Dipping .................. 147
Temperature Regulation ..... 149
Nest Relief ............... 150
Length of Incubation Shifts 151

Influence of Weather.......... 152
Fledging Period ................ 153
Hatching ................... .153
Attentiveness ................. 153
Feeding ..................... 155
Frequency of Feeding......... 157
Individual Recognition ........ 159
Chick Behavior ............... 161
Other Aerial Activities .......... 162
Thermal Soaring..............162
Fly-ups .....................162
Panics ...................... 164
Predators .................... 165
Other Terns .................166
Site Tenacity ................. 168
DISCussION ......................169
Behavioral Comparisons ......... 169
Sooty Terns as a Pelagic Species .171
LITERATURE CITED ............... 176


In the recent resurgence of work on seabirds, one species that has re-
ceived considerable attention is the Sooty Tern (Sterna fuscata), an ex-
tremely abundant bird of tropical and subtropical marine habitats. Recent
studies show that Sooties follow a nonannual breeding cycle at some local-
ities (Chapin, 1954; Ashmole, 1963), that perhaps they fly continuously
for months on end (Ashmole, 1963), and that juveniles undertake a long
migration (Robertson, 1969). Such tantalizing bits of information suggest
that further studies of this species would be rewarding.



Although the ethology of several species of terns is well known, the
basic studies of Sooty Tern behavior were made over 50 years ago (Wat-
son, 1908; Watson and Lashley, 1915; Lashley, 1915). Most terns that
have been studied feed in coastal waters or fresh water, but the Sooties
are pelagic. From 1968 to 1971 I studied the behavior of Sooty Terns at
the Dry Tortugas. Besides describing their behavior, I have attempted to
compare their behavior with that of coastal feeding terns and then to re-
late the Sooties' behavior to their pelagic habits.


I have benefited greatly from the counsel and interest of many individ-
uals. In particular, the advice and encouragement of Oliver L. Austin, Jr.
and William B. Robertson, Jr. have been of great help to me throughout this
study. I have also profited from the comments and criticisms of George W.
Cornwell, Brian Harrington, Robert W. McFarlane, Frank G. Nordlie, John
C. Ogden, Thomas J. Walker, S. David Webb, Glen E. Woolfenden and others,
especially many members of banding parties to the Tortugas. Personnel at
Fort Jefferson National Monument and Everglades National Park were es-
pecially helpful in providing a place for me to live at Fort Jefferson and then
making that living enjoyable. 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. Chap-
man 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 pos-
sible for me to spend an extended period at the Dry Tortugas. My wife, Pat,
encouraged me throughout the study and typed many drafts of this paper.
The text figures were done by Paul Laessle. To all these individuals and or-
ganizations, I am grateful.


I studied Sooty Terns at Bush Key, one of seven islands in the Dry
Tortugas group at approximately 240 38' N, 820 52' W, about 70 miles
west of Key West, Florida. The islands are located on a large shallow
bank, much of it less than five fathoms deep, in the southeastern Gulf of
Mexico (Figure 1). Sooty Terns have nested at the Tortugas since at least
1832 (Audubon, 1844). In recent years some 80,000 Sooties have bred
mainly on Bush Key, a low sand island some 20 acres in area. Nesting
terns of the Tortugas have had some protection since 1903, and since
1935 Bush Key has been part of Fort Jefferson National Monument. The
history of the tern colonies at the Tortugas has been reviewed in detail
(Robertson, 1964) and will not be covered further here.
Three small brackish ponds in the center of Bush Key are rimmed by
mangroves (Rhizophora mangle and Laguncularia racemosa) and button-
wood (Conocarpus erectus). Around them a thicket of bay cedar (Suriana
maritima) 6 to 8 feet tall covers much of the island. Outside the bay cedar


FIGURE 1. Map of Dry Tortugas Islands, based on Coast and Geodetic
Survey 585 "Dry Tortugas."

thicket and extending from it to the tide line, the vegetation is relatively
low and open. Most Sooty Terns nest on these flats, although some 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 lanceolata),
sea oats (Uniola paniculata), prickly pear (Opuntia sp.), a grass (Sporo-
bolus sp.) and sea purslane (Sesuvium portulacastrum) with some sea lav-
ender (Tournefortia gnaphalodes) around the edges. The density of these
plants varies from year to year, primarily depending on the amount of
Royal Terns (Thalasseus maximus), Sandwich Terns (T. sandvicensis),
and Least Terns (Sterna albifrons) formerly nested at the Dry Tortugas
(Robertson, 1964). Currently several hundred Roseate Terns (S. dougal-
lii) nest on the Dry Tortugas and several thousand Brown Noddies (Anous
stolidus) nest on Bush Key with the Sooties. The only mammals present
are the introduced rat (Rattus rattus) and a few humans who man the
Fort Jefferson. National Monument and a nearby Coast Guard lighthouse.
The climate at the Dry Tortugas is best described as hot and dry.
Daily temperatures often reach the low 90's (0F) and seldom drop below
the low 60's 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 microhabitat in which Sooty Terns
nest. Rainfall averages less than 40 inches per year (Florida State Board of
Conservation, 1954), much of it coming in sudden squalls. Several hurri-

Vol. XVI No. 3


canes have struck the Tortugas in recent years. On 8 June 1966 the winds
of Hurricane Alma drove water over parts of 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 1968 the center of Hurricane
Abby passed about 60 miles 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.
In recent years much of the work at the Tortugas has centered around
long term banding studies of the terns. From 1936 to 1941 some 13,300
Sooty Terns, mainly juveniles, were banded at the Dry Tortugas. Some
of these are still alive and provide information on the longevity of the
species. In 1959 the National Park Service, the Florida State Museum,
and the Florida Audubon Society began a cooperative banding program
that to date has banded almost a quarter of a million Sooty Terns on the


I lived on Garden Key from 29 March to 10 July 1968 and worked almost
daily on nearby Bush Key. I erected a blind near a small plot (25 x 30 feet)
from which I cleared nearly all vegetation. I captured terns in mist nets and
color-marked 182, each with a unique pattern of three-colored leg bands and
a Fish and Wildlife Service (FWS) band for individual recognition. The terns
gradually became accustomed to my presence in the blind, and their behavior
appeared normal once I was out of sight.
I determined the sex of the terns by noting their position in copulation,
keeping in mind that male-male mountings may occur. Once the birds were
color-marked, I recorded their activities and the roles of males and females in
the care of their eggs and chicks. As Sooty Terns spend much of their time
flying, I had to limit my studies to their activities at and near Bush Key. After
the eggs hatched, I tethered some chicks to prevent them from hiding through-
out 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,
1970, and 1971 I spent shorter periods on the island after the young hatched.


Starting in mid-January, Sooty Terns appear near the Dry Tor-
tugas in numbers at night only, circling, calling, and sometimes landing,
but generally are absent during the day (Robertson, 1964; pers. comm.).
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 day-
light and land on Bush Key. There they display, copulate, dig a scrape, and


lay the single egg that they incubate 29 to 30 days. The chicks are closely
attended for the first week or two and then are alone much of the time ex-
cept when fed. Chicks first fly when about 8 weeks old and apparently
leave Bush Key soon afterward. Some juveniles are flying by late June,
and most terns have left the Tortugas by late 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 80 have been re-
covered away from the colony. After leaving Bush Key juveniles appar-
ently 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. They spend 2 to 4 years in the Gulf
of Guinea off West Africa, and gradually drift back across the Atlantic
(Robertson, 1969). A few 3-year-old birds appear at the Tortugas late in
the breeding season, but almost certainly do not breed. At least a few
6-year-old birds breed, but some Sooties may not breed until older (B. Har-
rington, pers. comm.).
Among those banded as adults the recovery rate is lower, with only
about 25 recoveries away from the colony out of 81,100 banded through
1970. These recoveries suggest that most adults range over but remain es-
sentially within the Gulf of Mexico throughout the year (Robertson,


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.

Robertson (1964) describes a period of night flocking, starting some
2-3 months before eggs are laid. The terns approach Bush Key from the
northwest and 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 night and leave
before daylight. Tracks in the sand on Bush Key show that night groups
form there too, although I never saw them.
When I arrived at the Dry Tortugas on 29 March 1968, Sooty Terns
had not yet laid any eggs, but they were flocking in well-developed pat-
terns every evening. Every morning by about 08:00 (all times are EST)
most Sooties had left the island. Occasionally during the day flocks of up
to 10 Sooty Terns circled rapidly over Bush Key, giving loud "wide-a-
wake" calls or sharp "yip yip" notes as one bird chased another. The birds
passed over the island once or 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, 1941; Cul-
len, 1960a), except that they did not carry a fish in their bill.

Vol. XVI No. 3


In early evening, between 16:30 and 17:00, the terns gathered to
circle and call northwest of Bush Key. 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 to the flock over the
water. Gradually more circled over the island and a few started landing on
the open sand beach on the north side of Bush Key. 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
terns continued to join the circling flocks and their calls grew louder.

Out over the water the flocking pattern changed slowly. At first all the
terns were in one circling flock, from which they flew to the island. As
more birds arrived near Bush Key, more 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 they approached the island.

On 5 April at 17:30 I could see three such flocks, one about 1,000
feet north of Bush Key and the other two beyond. Between the flocks
lines of birds flew with slow, exaggerated wing beats about 50 feet above
the water. Beyond the last flock a line of terns, again about 50 feet above
the water, was stretched out for several miles. Perhaps some birds ap-
proach Bush Key low over the water as they normally do later in the sea-
son, but most drifted in with the flocks described above. Most terns cir-
cled northwest of Bush Key but occasionally small flocks formed south of
Garden Key. After sundown more terns landed and gathered on the
At sunrise no distinct flocks were present, although many terns cir-
cled 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 Sooties is apparently a significant social activity. Al-
though the adults apparently are spread throughout the Gulf of Mexico
during the nonbreeding season, most individuals still commence breed-
ing on Bush Key within a relatively short period of time. Even on Ascen-
sion Island where the breeding cycle is'nonannual and there apparently
is no selection for breeding in one given calendar month each year, the
breeding cycles of most birds are still synchronized (Ashmole, 1963). Pre-
sumably highly synchronized egg-laying would reduce the time when
individuals were vulnerable to predation at the nesting colony. Thus it
seems possible that the flocking by terns prior to egg-laying synchron-
izes the reproductive cycles of individual birds.


In 1968 the first daylight landings of Sooty Terns appeared to be
gradual extensions of time spent on the ground by night groups. Similarly
on Ascension Island, after a period during which Sooties were present
only at night, they suddenly started to return earlier in the evening and
stayed later in the morning, and egg-laying soon followed (Ashmole,
When I arrived on the Dry Tortugas in late March 1968, the terns
had already started staying later in the day so that some were present until
around 08:00. Egg-laying started on 1 April 1968 and by 21 April eggs
were present virtually throughout Bush Key.
When first landing the terns seemed nervous and were easily fright-
ened. They hovered over open ground, landed briefly, and then flew in a
panic 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 repeat the whole sequence.
As soon as the birds landed and settled down, displaying became
common. One display is the parade that will be described later. In an-
other display the bird points its bill down, sometimes almost burying it in
its breast feathers (Figure 2). A common posture of Larids, Harrison (1965)
calls it the "stare-down" and says it seems to "cut off' or end a low inten-

a- -

FIGURE 2. Sooty Tern in stare down, a display that commonly is given
almost immediately after the terns land at the colony. (Photograph by O. L.
Austin, Jr.)

Vol. XVI No. 3


sity agonistic or conflict situation. Sooties often give a stare-down almost
immediately after landing and then preen their back and wings.
Apparently little displaying occurs in the night clubs (Ashmole,
1963) but generally little is known about the birds' nocturnal activities. I
made only one night visit to Bush Key during this period, and I found
that most terns were quietly resting on the beaches or in open parts of the
island with few in the air. Generally the terns were highly excitable at
night so I avoided night work and have little information on their noc-
turnal activities.


All of the displays and activities preceding egg-laying may occur
within a few days after the birds first land in daylight. I was marking birds
at this time and had few that I could follow through the entire behavioral
sequence. I also found that birds were-still moving around within the
colony, as many color-marked birds Aimediately left my study 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

Early in the nesting season and to a lesser extent later, aerial displays
are a characteristic activity of Sooty Terns. Cullen (1960a) 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
Except for the chases over the colony prior to egg-laying, which may
be homologous to the low (fish) flight that is common among Sterna
terns, aerial display by Sooties seems to consist solely of a high flight sim-
ilar to that of the Arctic Tern (Cullen, 1960a).
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. I noted two means of ascent. In one the two birds ascended
almost vertically in small 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 (1960a) describes. I saw the upward flutter at the start of about
36 percent 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 one was
chasing the other. In these chases the 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 adja-
cent water, often reaching a height of several hundred feet. They some-
times 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 man-
euvered close to each other by flapping their wings in slow, almost half
beats with the wings barely moving to below 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, reaching a maxi-
mum of about 450 to 600 above horizontal. 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 (1960a) 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 scissoredd" to-
gether 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 of the first. Sometimes they started the glide
several times, leveled off after descending a short way, and then dove
again. At other times they separated and each flew away alone. In a com-
plete display they glided together down to just above water level,
swooped up a short ways and then separated, each flying away in a
normal flight. Several times I followed both birds after the glide, but I
never saw them 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; 13 times the birds changed from the upward flutter to
jerk-flying. Thus jerk-flying was part of the ascent in 45 of the 50 high
flights. The other 5 times the entire ascent was with upward flutter, In 40
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 con-
tinued to glide down to about sea level in only 28. Thus ascending by
jerk-flying and descending in a glide are the two most common parts
of the high flight, although they do not occur in all of them.
The birds often changed 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 Torns sometimes carry a fish in the high flight

Vol. XVI No. 3


(Cullen, 1960a), the high flight of Sooties is very similar to that described
by Cullen for the Arctic Tern.
Aerial displays are most common in early morning and from 15:00
to 19:00. Most occurred early in the breeding season in April but I also
saw some in May, June, and July when they may have been 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 Cullen (1960a)
suggests that it may be part of pair formation. If so, perhaps the high
flights late in the nesting season involve subadults. 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 before
they first breed. It is not known how long Sooty Tern pair bonds last
but it would not surprise me if the same birds pair in successive years.
Such an extended "engagement" period is known for another long-
lived pelagic species, the Laysan Albatross (Diomedea immutabilis)
(Fisher and Fisher, 1969). Moynihan (1962) saw many aerial displays
by Brown Noddies and Inca Terns (Larosterna inca) late in the nest-
ing 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.

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 the tern assumes 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
(1941) describes a similar display in the Common Tern (Sterna 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 for-
ward, often sleeking the feathers. The wings are usually held well away
from the body, particularly at the carpals, and are sometimes lowered
until they drag on the ground (Figure 3). Occasionally the 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 Palmer
(1941) says the display shows submission. Sooties may point the bill
horizontally or slightly above horizontal, but they seldom point it
straight up as some other terns do. Sooties also tilt their head to the
side, especially when displaying with or to another bird. Then they tip
their head away from the other individual but seem to lean the body
even more toward it so that the near wing often drags on the ground.



FIGURE 3. Parade display by Sooty Terns. In the top photograph one adult
is parading around a second tern, the parading bird holding its wings well
away from the body and the head tilted away from the other bird. At the
bottom the two birds in the right foreground are parading around each other,
each holding its wings away from the body and the head and neck extended
forward with the head tilted away from the other bird. (Photographs by
O. L. Austin, Jr.)

I saw Sooties give this display almost immediately after landing.
Often it preceded copulation, but at other times two birds separated
after parading. Both sexes 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. The erect form
of the parade was common almost immediately after birds landed and
before they established territory, and seems to be part of pair formation.
In the other form of the parade the tern points the bill down, but
Sooties seldom point it directly at the ground. Palmer (1941) 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 wings so far away from the body or stretching the neck so
far forward. Several times I saw a parade start with the bill pointing

Vol. XVI No. 3


upward and end with it pointing toward the ground. Thus the position
of the bill varies considerably, although the basic wing and body postures
are similar in the bent and erect forms of the parade.
Both Palmer (1941) and Cullen (1960a) note these two bill posi-
tions occurring in aerial displays of terns. In aerial displays Sooty
Terns always seemed to point the bill straight forward, the aerial posi-
tion 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 al-
though I was only 10 feet away I heard no sound. Notes too low for
me to hear may accompany the display.
Several times I saw a bird on the ground raise its head and give
a loud "ke-wat-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 a male called to a
female in the air, she landed, they paraded and then the two copulated.
In such exchanges the erect form of the parade was most common and
it seemed to be part of pair formation.
COURTSHIP FEEDING. I saw courtship feeding only 17 times, mainly
from 13 to 20 April but once as late as 17 June. Not all of these termin-
ated with the actual transfer of food. In five instances where the sex
was known, the female always begged. Generally she crouched, turned
her head toward the male, and uttered a rapid series of low chuckling
"ka ka" or "yip yip" notes, raising her open bill toward him, and biting at
his bill, much as a young tern begs for food. A few times I actually saw
the male transfer food to the female, sometimes repeatedly in one bout of
courtship feeding.
Occasionally the parade preceded courtship feeding, but the latter
is certainly not a regular component of the parade. Courtship feeding is
a typical precopulatory behavior in most Larids (Cullen and Ashmole,
1963) but in Sooty Terns courtship feeding is relatively rare and only
occasionally precedes copulation.
COPULATION- The parade commonly precedes mounting and copula-
tion. 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 their body feathers and then
Copulation was commonest on my plot from 6 to 23 April. Of
226 times I noted a male trying to mount a female, in only 60 (26.5
percent) did they seem to copulate. In 77 (34.1 percent) attempts the
male fell off the female, the female did not crouch, or he simply dis-


mounted without trying to copulate. More commonly (89 times, 39.4
percent), another bird interfered and either knocked the male off the
female or the male dismounted and fought with the intruder. Inter-
ference from other birds was especially common from 11-20 April.
Then almost half of the attempted copulations ended with interference
by another bird and few pairs could copulate without other birds inter-
fering. Several times I saw two to six males try to mount one female.

Sooty Terns apparently do not establish territories until after
copulation. Even then the territory is not permanent until they dig a
scrape and the female lays an egg. Normally Sooties claim an area span-
ning little more 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. Although Sooty Tern nest throughout Bush Key,
they nest in greatest concentration outside the bay cedar thickets and
mangroves of the central part of the island.
SCRAPE-BUILDING.-By 10 April scrape-building was common on my
plot and from then until 16 April it was the bird's most conspicuous
activity. Although scrape-building obviously provides a site for the
egg, the incipient scrape-building behavior associated with it often oc-
curs before copulation and seems to be part of courtship in Sooties,
much as it is in Common Terns (Palmer, 1941).
Pairs of Sooties usually spend some time selecting a site and may
make several false starts over a period of several days. 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.
Nearly always a pair works together to dig the scrape. Typically
they stand close together, point their bills toward the ground, then
lower their bodies, and kick dirt back with their feet. They often give
low "puck puck" notes as they poke at the ground. 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 the scrape.
Scrape-building generally stops once the female lays her egg, but
the birds occasionally work on the scrape after it contains an egg.
Several pairs that lost their chick shortly after it hatched began work on
new scrapes, but I did not see any bird lay again.
FIGHTING. Conspicuous fighting started around 11 April when
scrapes were being dug. In fighting, two birds face each other, sometimes
approaching each other with their heads and bodies held low and for-
ward 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 at the head and bill of the other bird or grips
the other bird's bill and then both shake their heads vigorously. Occasion-

Vol. XVI No. 3


ally one beats the other with a wing, but normally they use the wings
only for balance. Sometimes they give low 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
and faced each other with wings against the body, bills forward. First
one and then the other lowered its bill into the bent posture, turned its
head slowly away, and then broke off the altercation (Figure 4). Occa-
sionally they gaped at each other before lowering the bill. Gaping appar-
ently is an aggressive signal (Moynihan, 1962) as is the bent position.
Thus the aggressive display apparently ended the fight.

/i /

- --

/ --

FIGURE 4. Face off between two Sooty Terns. After staring at each other,
the birds will lower their bills, turn away from each other, and end the
fight. (From photograph by Brian Harrington)

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 turning its attention
back to its mate.
Thus the sequence of events preceding egg-laying seems to be as
follows: First the terns flock near and land on Bush Key every evening
in a mass social activity, perhaps synchronizing the breeding cycles.
Gradually they spend more time on the ground and, as flocking de-
creases, they begin individual sexual activity. Males seem to land, per-
haps near where they bred in previous years, and call to try to attract
females to them. The parade and high flight that follow probably con-
tribute to pair formation. Pairs 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.



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: 5 be-
tween 12:00 and 14:00, 5 between 14:00 and 16:00, and 4 from 16:00
to 18:00. Ridley and Percy (1958) 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 15 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. After laying
the egg the female almost immediately leaves, walks around it, and the
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. Ex-
changes 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 not-
ing which adult cared for the egg and the adult's general behavior, I
maintained records of activity patterns of 15 pairs in the following man-
ner: 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.). I used this
method to make 120 observations per hour per nest for 1-hour periods
throughout incubation.
One fault of this method is that by cataloging activities, some
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 percent of the time, this variation is only a small fraction of all the
birds' activities.
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. During incubation, I have records for 1,632 bird-hours, each

Vol. XVI No. 3


100- 1-00

I --

\- /6 I.60

S40 ACTIVITIES ....... 0

20- / -20

... .... \ t''
I- 60- 0. -60

06:00 08:00 10:00 12:00 14;00 1600 18:00

FIGURE 5. Activities by adult Sooty Terns during incubation, showing the
percent of time they spend incubating or shading the egg.

including 120 spot observations for a total of 195,840 spot observations.
These include at least 1 hour of watching for each sex for each of the 12
daylight hours for most days during incubation, and for some I have
many more. These were used to prepare Figure 5 and Table 1.

During incubation Sooty Terns primarily tend the egg or perform
a few maintenance activities during brief spells away from it. I categor-
ized their activities into several types.
Sooty Terns incubate, placing one of their two brood patches
against the egg, mainly at night, in the early morning, and in late after-
noon (Figure 5). As the air temperature increases in the morning, adults
gradually 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 incu-
bating again (Figure 5.) To shade the egg the adult 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 maxi-
mum surface area of the body exposed to the sun and, as the upper sur-




Activity Spot observations Percent
Preening 4258 48.4
Poking at egg 1540 17.5
Standing beside egg 1171 13.3
Walking 886 10.1
Fighting 673 7.6
Away from scrape 276 3.1
Total 8804 100.0

face of a Sooty Tern is black, would seem to present problems of heat
load for the bird. However keeping the back toward the sun best in-
sures shading of the egg. The adults probably reduce their heat load by
erecting their backfeathers and holding their wings out from their sides,
increasing insulation and the area for heat loss (Figure 6.) 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
5 indicates 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 5.
Among these other activities the most frequent is standing at 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

.1 7.-

FIGURE 6. Typical sunning posture by adult Sooty Terns. Note the erect
back feathers, lowered wings, and open bill. The chicks in this photograph
are close to fledging, but the adults assume a similar posture when they are
shading eggs. (Photograph by O. L. Austin, Jr.)

Vol. XVI No. 3


adult walks around the scrape or to the edge of its territory and stands.
Occasionally a bird leaves its egg to spar briefly with another tern, either
a neighbor or one that has come too close. Generally Sooty Terns are
much less bellicose during incubation than they are at other times in
the nesting cycle. Perhaps home 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. Some-
times the bird stretches both wings vertically above the body, leans for-
ward with head outstretched, and defecates, thus stretching and defecat-
ing 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 their absence
seems to be associated with the aerial activity called dipping, described

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 percent
between sexes. In all, the adult caring for the egg spends over 95 percent
of the daylight hours either incubating or shading and only about 4.5
percent in a variety of other activities (Table 1). These other activities
are commonest from 07:00 to 10:00 and from 17:00 to 18:00, the times
when birds shift between incubating and shading (Figure 5). Presum-
ably at these times 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 percent of their time directly caring
for the egg, mostly by shading. This is certainly the time when the en-
vironmental temperatures are highest, 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 ex-
Usually each tern flies directly and rapidly from the colony at a
height of about 10 feet. It then drops down, dips the bill in the water,


Flights that Number of times body
included dipping: parts were dipped:
Where flight Total
ended flights Feet Bill Breast Feet Bill Breast Total

On beach 35 13 34 0 22 60 0 82
In colony 78 45 74 19 102 158 49 309
Totals 113 58 108 19 124 218 49 391

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 1 to as many as 10 times
on one flight. In 113 flights between 13 May and 7 June in which I
recorded all dipping movements, and also whether the bird landed back
in the colony or with groups of terns sunning on the beach, over 80
percent of all dipping occurred on the flight out. In about 95 percent
of the flights the birds dipped the bill at least once (Table 2), and in only
about 17 percent they dipped their breast feathers. As one dipping motion
might result in more than one part of the body being dipped, I recorded
the number of times each part of the body was actually 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 terns sunning or resting included dipping the breast feathers,
while about 25 percent 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

Number seen during:

Time First 15 days Second 15 days Total
of incubation of incubation
06:00-07:00 41(4)2 1 (1) 5 (5)
07:00-08:00 6 (3) 0 (0) 6 (3)
08:00-09:00 8 (7) 5 (4) 13(11)
09:00-10:00 31(24) 4 (3) 35(27)
10:00-11:00 31(25) 11 (8) 42(33)
11:00-12:00 24(18) 5 (5) 29(23)
12:00-13:00 21(19) 17(15) 38(34)
13:00-14:00 24(20) 14(11) 38(31)
14:00-15:00 23(20) 4 (4) 27(24)
15:00-16:00 18(12) 3 (3) 21(15)
16:00-17:00 20(14) 1 (1) 21(15)
17:00-18:00 0 (0) 1 (1) 1 (1)
Totals 210(166) 66(56) 276(222)
Number of times in activity records birds were away from egg.
2 Number of actual flights represented, i.e., one flight might involve two or more spot
observations in activity records.

Vol. XVI No. 3


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, I often could see that its bill, feet, and some-
times the breast feathers were wet. Obviously these direct flights from
the egg were the start of the dipping flights that I watched over the
water. As dipping is most common when the day is hottest, it seems
possible that this behavior is thermoregulatory, both for the adult and
the egg. As both sexes dipped with equal frequency (139 to 137), I com-
bined their records. Dipping by incubating adults is most frequent dur-
ing the first 15 days of incubation (Table 3).
Watson (1908) mentions an activity similar to dipping but says
the terns were bathing. The Ashmoles (1967: 62) note that incubating
Sooties sometimes fly off to drink. Brian Harrington (pers. comm.) has
seen Sooties dip at Johnston Island in the Pacific but less frequently
than at the Dry Tortugas. Simmons (1970) also noted aerial drinking
by Sooties at Ascension Island. Both Tompkins (1942) and Hardy
(1957) report Least Terns dipping their breast feathers during incubation
and suggest that it provides water necessary for the eggs.

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 tem-
peratures they would reach in open sunlight. On Midway the adult
terns maintain the egg temperature above that of the surrounding air
and below that of the surrounding sand, but it is not stated whether
the adult is incubating or shading the egg.
I attached thermisters to several eggs and monitored surface tem-
perature 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 my blind, and the adults caring for them were easily dis-
turbed by my movements. 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 egg on 8 June
and conflicting information from the same egg the next day. On 8 June,
in 23 temperature readings taken between 10:20 and 13:05, the egg
averaged 101.5F (range 99 to 1050F). and the air at 4 inches averaged
105.50F (range 103 to 108.5F). 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 egg. The next
day in 35 readings the egg temperature averaged 105.2F, somewhat
above the air temperature that day (1050F) and 3.7oF 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 lower the egg temperature.
On the 8th the adult had been able to maintain the egg's tempera-
ture 4F below the air temperature. When the adult flew off to dip, the
exposed egg's surface temperature rose 1 to 2F while the bird was ab-
sent but then dropped 2 to 5F 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 I have limited information, I suggest that dipping the
breast feathers by incubating terns helps regulate either the egg's temper-
ature or humidity. Some adults dipped often during incubation while
others did so only rarely. If dipping the breast feathers is thermoregula-
tory 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 any moisture the egg may need.
Late in the breeding season adults continue to dip but seldom dip
their breast feathers. On 27 June 1970 when most chicks were 6 to 8
weeks old and few eggs were present, nearly all of the hundreds of adults
flying out from Bush Key dipped only their bill or occasionally their

Most of the 61 nest reliefs observed during incubation occurred
during early morning or late afternoon (Table 4). Generally the adult
lands near its scrape and walks up to its mate on the egg. The bird on


Time Number of Hours of Exchanges/hour
exchanges Observation Observation

06:00-07:00 13 21.52 0.60
07:00-08:00 4 27.92 0.14
08:00-09:00 4 29.03 0.13
09:00-10:00 3 23.50 0.12
10:00-11:00 5 19.33 0.25
11:00-12:00 1 13.38 0.07
12:00-13:00 1 10.33 0.09
13:00-14:00 3 10.25 0.29
14:00-15:00 1 10.97 0.09
15:00-16:00 3 14.78 0.20
16:00-17:00 12 16.98 0.70
17:00-18:00 6 17.00 0.35
18:00-19:00 3 4.50 0.67
19:00-20:00 2 1.17 1.70
Total 61 220.66 0.28

Vol. XVI No. 3


the egg 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 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 and the other
leaves the island. I never saw an adult feed another one at a nest ex-
change 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, return-
ing adults fly directly to the island and presumably to their scrape. The
number of hours I watched in early evening is much smaller than for
other times. Otherwise I certainly would have seen more exchanges then.
At Bush Key few adults regurgitate when mist-netted in the morn-
ing or early afternoon, but many do so in the late afternoon, indicating
they just returned from foraging. 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
generally incubate.quietly from about 08:00 to 16:00.
Ashmole (1963) found nest relief most frequent between 21:00 and
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 Ascen-
sion Island. If Ashmole's reasoning holds, then Bush Key Sooties feed
within a few hours flight of Bush Key at most and can return quickly
when finished feeding.

To determine the length of incubation shifts (the length of time
one adult cares for the egg without relief), I recorded which adult incu-
bated each day for 13 of the nests on which I kept activity records. I
found that the bird present in the morning usually remained there until
at least late afternoon. Because most adults seemed to return and exchange
in early evening, I assumed this was true unless I had evidence to the
contrary. 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 incuba-
tion shifts generally are 24 or 48 hours long.
Of 231 incubation shifts computed as multiples of 1-day periods,
148 (64.1 percent) were 1 day in length, 70 (30.3 percent) were 2 days


long, and only 13 (5.6 percent) were 3 days long. Figured differently,
the Sooties did 45.3 percent of their incubating in 1-day shifts, 42.8
percent in 2-day shifts, and 11.9 percent in 3-day shifts. Even though
1-day shifts are more than twice as common as 2-day shifts, the two con-
tributed 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 somewhat more time (175 to 152 days) caring for
the egg than did females, but the difference is not statistically significant
(X2=1.62, P>0.20). In the hourly activity records, males again were
present more often (880 to 752 hours), the difference being statistically
significant (X2=10.04, P<0.005). As the female left the egg shortly after
laying and the male cared for it on the first shift, some of this difference
occurred then, but even excluding the records for the first day of incu-
bation, the male still cared for the egg more than the female (X2=3.95,
Watson (1908) found Sooty Tern incubation shifts on the Tortugas
averaged somewhat over a day long with the longest a little over 3 days,
well within the range I found in 1968. On Ascension Island incubation
shifts average 132 hours (Ashmole, 1963), 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, 1967); on the Seychelles
they vary from 2 hours to 3 days (Ridley and Percy, 1958).
The striking differences in incubation shifts between Bush Key
Sooties and those at two other colonies are probably related to food
availability and its distance from the island. At Ascension many young
died of starvation one year, apparently because of a failure in their food
supply (Ashmole, 1963). On the Dry Tortugas Sooty Tern chicks have
never been known to experience heavy mortality from food shortage
(Robertson, 1964). The shorter incubation shifts seem to indicate that
the terns have an adequate food supply near the island.

The most obvious effects of weather on incubation patterns of Sooty
Terns are the shifts from incubating to shading and back to incubating,
depending on the ambient temperature (Figure 5). 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 vir-
tually never leaves its egg between 11:00 and 15:00 except to dip. The
few records I have of terns performing other maintenance activities
during those hours nearly always occurred on cloudy days.
Rain also changes activity patterns. At the start of a rainstorm,
swarms of Sooty Terns rise and circle over the colony, calling noisily

Vol. XVI No. 3


As nearly all birds that are caring for an egg incubate throughout the
storm, it is primarily free birds that circle over the colony.
Apparently Sooty Tern feathers are not water repellent and are
easily soaked by rain. After rain Sooties preen their body feathers and
flap their wings vigorously. This apparently helps dry the feathers to
permit flight. Sooties have difficulty taking off in the early morning
after a heavy dew has soaked their feathers. The flocks of terns rising at
the start of rainstorms are probably birds getting airborne before they
become waterlogged. Once in the air, many remain there until the storm
is over, the motion of their wings preventing them from getting soaked.


Sooty Terns incubate their eggs 29 or 30 days (mean 29 days 12.3
2.4 hours, range 28 days 22 hours to 30 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 seems somewhat short,
though Ridley and Percy (1958) claim Sooties incubate for 26 to 29 days
on the Seychelles.
The chick may pip the egg as much as 36 hours prior to hatching,
but usually does so only the day before hatching. For eggs that I could
determine the time of hatching exactly, most hatched around sunrise or
around noon.
Once the egg pips, adults continue to care for it very closely. The
only time that Sooty Terns ever actually mobbed or struck me in the
colony was when eggs were hatching. Howell and Bartholomew (1962)
show that a pipped egg is particularly sensitive to heat stress and the
chick's survival is thus dependent on care by the adult.
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. While I saw adults pick up and carry off a piece of egg shell
perhaps 20 times, often one from a neighboring scrape, usually they let
the empty shell roll around until it is crushed.
Sooty Tern eggs (like those of most terns) are speckled and camou-
flaged on the outside, whereas the white inside lining contrasts sharply
with the sand background of the nesting colony. Tinbergen et al. (1962)
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 difference to
chick survival. Any predator reaching the colony would have little
trouble finding a chick, whether the white inside lining of an empty egg
marked the scrape or not.

My data on parental care of chicks are less complete than those for


Vol. XVI No. 3

care of the egg. Every time I entered my blind the chicks more than a
few days old scattered and hid under the nearest cover. It sometimes took
them several hours to return to their scrapes where I could watch them
again, and some never returned.

100-. *100

80s- -80
I\ I
I \
I \\ I

60- \ -60

S /\'" "./ SHADING .--- \'-
0.. 40- .. OTHER -40


FIGURE 7. Activities by adult Sooty Terns during the first two weeks of
caring for -the chick, showing the percent of time they spend brooding or
shading the chick.

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 7). As in incubation,
adults rise up off the chick to shade it in hotter hours of the day and brood
it when it is cool. Females spent somewhat more time with the chick
than males did (157 to 132 bird-hours), but statistically these are not
significantly different (X2=2.16, P>0.10). I found little difference be-
tween males and females in care of chicks, and I have combined the
records in Figure 7.
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 chicks even seem to spend the night
unbrooded by an adult.

Chick care centers around the provision of food. Sooty Terns feed
almost exclusively on fish and squid (Ashmole, 1963; Ashmole and Ash-
mole, 1967). They catch much of this food when schools of tuna, mackerel,
or other large predacious fish drive smaller fish to the ocean surface. The
Sooties dip down to seize food from the surface or the air above it, and
seldom if ever dive headlong into the water for their prey as most other
terns do (see Ashmole and Ashmole, 1967).
The Dry Tortugas Sooties have been reported eating fish of the
families Carangidae and Clupeidae (Watson, 1908). In recent years
Robertson has collected many fish and squid that Sooties and Brown
Noddies regurgitated when mist-netted. At present only the Scombridae
have been analyzed (Potthoffand Richards, 1970).
I saw one chick fed about 4 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 also may beg with some rapid "cheep" calls. 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. The adult
may hold the fish in its bill before feeding the chick, or it may slide the
fish directly down the bill and into the chick's mouth. Sooty Terns
returning to the colony always carry the food internally and then re-
gurgitate it for the chick.
Older chicks seem to beg more by opening their bills and giving
rapid "cheep" calls than by pecking at the adult's bill. They also grab at
the fish while the adult is 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 seemed to be scaleless, and in some cases they
were partly digested or fragmented, especially the last ones fed to the
chick (presumably the first ones the adult caught). The regurgitated food
is often coated with mucus, which may retard digestion of the food while
the adult carries it back to the colony (Ashmole and Ashmole, 1967).
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
times a whole meal might be one large bolus containing several fish.
Of 349 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 those periods (Table 5). Passing several fish to a
chick in the space of a few minutes is counted as one feeding. As the num-
ber of observation hours varied during the day, the number of feedings


Adult feeding young

Number Extra
of times Undet. adult Hours of Feedings
me fed Male Female Both sex present obs. per hour ^

06:00-08:00 74 22 21 9 22 16 54.78 1.35

08:00-10:00 58 27 23 1 7 27 34.33 1.68

10:00-12:00 26 13 8 0 5 5 16.82 1.54

12:00-14:00 12 5 6 1 0 5 14.42 0.83

14:00-16:00 44 18 6 1 19 8 22.88 1.92

16:00-18:00 85 34 34 4 13 29 33.48 2.53

18:00-20:00 50 19 4 1 26 1 20.83 2.40

Totals 349 138 102 17 92 91 197.54 1.77


per hour of observation (Table 5, last column) is a more valid comparison.
This shows that peak chick feeding occurred in late afternoon and early
evening with a smaller peak early in the morning. These counts were
made from 16 May to 10 July, the time when chicks were present on my
In about 5 percent 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 male did
so more often than the female (138 to 102) and the difference is statis-
tically significant (X2 = 5.40, P <0.025).
In addition I recorded some 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 reswallowed it, even
when a chick begged loudly. The adult might do this several times, but
usually eventually fed the chick. At other times an adult was unable to
regurgitate any food.
With small chicks the adult places all food directly in the mouth.
Any that drops on the ground remains there unless an adult picks it up
to eat or feed to the chick. The youngest chick I saw pick up food from
the ground was about a month old, but they may do so when younger.
These older chicks pick up food from 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 the fish and swallow it.

Sooty Terns on Bush Key feed their young infrequently enough to
make if 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 a chick, leaving it to 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 return
to the island and feed the chick several times over a period of several
hours. I saw one 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 4 or 5 hours and fed it again, I could not be sure whether
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 I
have determined the rate of feeding two ways.
Continuous observations on 26 May produced five records of an
adult leaving a 2 to 8-day-old chick and returning later to feed it. The
length of time the adult was absent (foraging time) averaged 3.87 hours
and ranged from 2.63 to 5.30 hours.
I also tabulated the number of times chicks were fed during long
periods of continuous observation, separating these into chicks less
than 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 feed-



Number Probable
of number of Bird-hours Hours per
Age of chick feedings foraging trips of observation foraging trip

1-15 days 144 113 369.42 3.27
16-27 days 20 19 84.58 4.45
Totals 164 132 454.00 3.44

ings that occurred when an adult fed a chick more than once after return-
ing to Bush Key. This gives the probable number of foraging trips. Divid-
ing this into the bird-hours of observation gives a rough estimate of the
time an adult spent foraging, about 3.44 hours. Apparently foraging
trips are about an hour longer when the chicks are older, but the sample
size is small and the adults may just loaf part of the time. Note that here
time 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 at a time when they are feed-
ing their chicks. Adults seem to pass food to older chicks more rapidly
on their return. Rather than standing with the chick and feeding it sev-
eral times in a few hours, they seem to give it all the food they have and
then leave.
Often I saw exchanges that appeared to be after foraging trips of
4 to 8 hours, but the birds may have exchanged in shorter intervals when
I was not watching. Thus my records of foraging time are biased for
shorter periods. Watson (1908) gives 4 to 7 hours as the interval between
feedings by Dry Tortugas Sooties.
Sooty Tern chicks on Bush Key are fed more frequently than those
in other colonies that have been studied. On Ascension Island and
Christmas Island it appears that chicks are fed only about once a day, or
even less frequently, but good information is lacking (Ashmole, 1963;
Ashmole and Ashmole, 1967).
As the chicks grow, the adults become less attentive and often
gather by the hundreds on the Bush Key beaches and sun much of the day.
Other adults soar over the island during the hotter times of day. Perhaps
adults away from their chicks spend part of their time in these groups.
They must spend some of their time foraging but exactly how much is
difficult to determine.
At Bush Key more adults are present and the colony is noisier in
the evening than at any other time of day. Hence I think that both
adults are probably in the colony at night, though both may not be with
the chick at all times. Elsewhere Sooty Terns feed at night at least oc-
casionally (Gould, 1967), but at Bush Key, with the noticeable influx of
birds in the evening and exodus in the morning, I doubt that many

Vol. XVI No. 3


adults feed at night. As during incubation, mist-netted birds regurgitate
far more frequently in the evening than at other times.
The average flight speed of Sooty Terns is about 27 mph (Schnell,
unpublished). Thus on longer trips they may forage as much as 100 miles
from Bush Key, but the average foraging trip of about 3.5 hours gives
them a maximum range of about 47 miles. Sooties 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 forage 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 seem to check their chick periodic-
ally 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 4 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 forage during
the storm and had left as soon after the storm as possible to obtain food
for themselves and their chick. Mason and Steffe (1966) noted a similar
disruption after Hurricane Alma. Some of the returnees undoubtedly
were birds displaced by the storm.

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 recogni-
tion seems to develop when the chicks are about 4 to 5 days old. As
Davies and Carrick (1962), Hutchison et al. (1968), and Stevenson et al.
(1970) suspect for other terns, this recognition probably is based largely
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 uses such behavior to lure the
chick back to the scrape, backing away from the chick and toward the


scrape until the two are 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 to lure it back to the scrape.
By 3 weeks of age most of the chicks on my plot spent much of
their time hiding under the nearby bay cedar. By removing most of the
vegetation from the nesting plot I may have forced them to move to this
cover. Typically these older chicks emerged from the bay cedar late in
the afternoon and stood at its edge or ran out 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 come and feed them. When-
ever I 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 home scrapes and returned
there to be fed.
Sometimes the adult landed at the scrape before the chick arrived.
At first the adult circled 10 to 15 feet over the scrape and delivered loud
"wid-ik" or "ka-wid-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-wake" 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. Occasion-
ally the adult pecked the begging chick sharply. I was not able to deter-
mine the relationship of the two in these cases, but I suspect that the two
had made a mistake in recognition and the adult did not realize it until
the chick came close.
Although chicks are usually fed by their parents (or by adults that
adopt them in shuffles before chicks are individually recognized), at least
eight times 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 times an unmarked adult
fed a chick whose parents were both marked and thus recognizable.
Some chicks seemed to approach and beg of any adult that came near.
Although these chicks were not marked, I am fairly sure that some were
fed by adults other than their parents. Often the adult pecked the chick
and drove it away, but at times the adult tried to regurgitate, and oc-
casionally it brought up food and 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 them or tried to
feed them may have been their lost parents, or other adults that had lost
their chick or egg.
Thus older chicks seem to find their parent both by knowing where

Vol. XVI No. 3


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 be-
leive 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.

Sooty Tern chicks can walk almost immediately after hatching and
stand and beg for food within 4 hours. For the first few days they are closely
brooded by one of the parents, the chick resting either between the par-
ent's feet or crossways in front of them. At first 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 them. The extra chick eventually returned to its own
scrape, usually when called by an adult. A common alarm response of
these small chicks is to lie flat on the ground with the head and bill ex-
tended forward. Chicks apparently do this to avoid being pecked by
adults, as adults peck and sometimes kill strange chicks that come 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. During the 3rd week this attention gradually diminishes, and
by the end of this 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 on
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. Occasionally adults feed chicks in such flocks but
usually chicks seem to return to their home scrape or some other nearby
place to be fed.
I saw 6-week-old chicks jumping into the air and flapping their wings
vigorously. I lack exact records of when they begin to 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. In
1971 some chicks at Bush Key were flying at 8 weeks of age. One year
chicks at Ascension were flying when about 8 weeks old and the next year,
when food apparently was scarce, birds around 9 weeks old still could not
fly (Ashmole, 1963). Burckhalter (1969) saw 8-week-old chicks flying on
the Hawaiian Islands, but they stayed at the island another 2 to 3 weeks.
Most of the chick's activities before it starts flying seem to be related
to feeding and temperature maintenance. Obviously the adults help with
the latter when the chicks are small, but 3- to 4-week-old chicks start as-
suming 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 my tethered chicks died in open sun
when about this age, apparently from heat stress. Thus body temperature
must closely approach lethality when chicks 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 Key, little is known of their activities. In all
probability, the adults stay with the young and continue to feed it until
it can capture food by itself (Robertson, 1964: Burckhalter, 1969) as do
adult Royal Terns and other terns (Ashmole and Tovar, 1968). Bush Key
adults cannot feed flying Sooty Tern young more than about 2 months,
as all juveniles leave the range of adults by mid-October and some have
done so by early August (Robertson, 1969). This certainly must be a diffi-
cult time for young Sooties, as they must learn to capture food while on
the wing, a skill requiring good eyesight, coordination, and timing. Per-
haps the long fledgling period of Sooty Terns in comparison to those of
coastal terns (see Table 7) is an adaptation to compensate for the reduced
period of time when adults can care for the young.


Sooty Terns at Bush Key commonly soar in dense circling columns
during midday hours on hot days. Possibly at these times updrafts or ther-
mals develop near Bush Key. The columns approach 200 feet in diameter
and may go as high as 1,000 feet in altitude, although most birds are usually
below 200 feet. Within the columns the birds circle as they climb and may
then soar off to join another column. Most common late in the breeding
season, these columns of soaring birds are taller and the birds appear to
fly faster than in the flocks seen as the colony forms. 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 continually and birds join or leave it. The columns
often move laterally and two may merge into a single column. Magnifi-
cent Frigatebirds (Fregata magnificens) and occasionally Brown Pelicans
(Pelecanus occidentalis) and Laughing Gulls (Larus atricilla) (Har-
rington, pers. comm.) join these flocks, but I did not see Brown Noddies
do so.

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 "wide-a-wake" call at the peak, and drop back to the

Vol. XVI No. 3


2 I

FIGURE 8. A fly-up by Sooty Terns. Just after the peak the birds have the
somewhat humpbacked appearance shown here. (From drawing by Brian

ground (Figure 8). I call these flights "fly-ups", and I believe they are a
type of social behavior, often used in greeting. Besides the birds that ac-
tually fly, other birds around them often raise their wings vertically over
the body in a flight intention movement, but do not fly. These fly-ups
usually involve a small group of terns. During a fly-up, other nearby
birds become very active and noisy for 15 to 60 seconds and then gradually
quiet down.
The few fly-ups I saw during incubation usually occurred when a
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 reports
this group activity when an adult arrived at the colony.
Once the chicks were present, I often saw fly-ups when an adult landed
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
them. As the chick approached, the adult usually flew up, called, and then
landed to feed the chick. In these instances the chick seemed to stimulate
the fly-up. Several times I saw a chick run past several 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 were
their own while the chick went by and approached another adult, pre-
sumably its parent, to be fed.

Sooty Terns exhibit two distinct types of disturbance flights, com-
monly called panics. Of these, dreads apparently include the flights Pal-
mer (1941) calls dreads and panics for Common Terns while those termed
alarms are similar in both species.
ALARMS.-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 so, 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 Sooties attack Cattle Egrets
(Bubulcus ibis) and once a Purple Gallinule (Porphyrula martinica) that
had broken a tern egg.
A loud, 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 all 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 boom, boat whistle) or a sudden movement. These
elicited a different response the dread.
DREADS. 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 call-
ing 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, poorly 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, they scare up birds near them
and so on until the whole colony is in the air.
Lind (1963) suggests that in Sandwich Terns these disturbance flights

Vol. XVI No. 3


may have several functions, including synchronizing reproductive be-
havior, but I have no evidence of this for Sooty Terns.


The isolation of most islands where Sooty Terns nest means that they
come in contact with few vertebrate predators. Probably the most serious
predator of adult Sooties at Bush Key is the Peregrine Falcon (Falco pere-
grinus). In 1Q68 I saw a peregrine swoop at Sooty Terns 23 times without
capturing a bird, but I did find the decapitated remains of four Sooties
that the falcon almost certainly had killed.
At the Tortugas several other bird and animal species prey on the
eggs and young of Sooties. I have seen both Ruddy Turnstones (Arenaria
interpres) and a Purple Gallinule break and eat eggs, and the former has
done so elsewhere (Ridley and Percy, 1958; Crossin and Huber, 1970).
Rats are also known to take Sooty Tern eggs and chicks (Russell, 1938;
Kepler, 1967).
Frigatebirds are perhaps the greatest threat to Sooty Tern chicks. At
the Dry Tortugas Magnificent Frigatebirds have taken many chicks some
years (Beard, 1939; Sprunt, 1948) while in 1968 I saw only one chick
taken. When frigatebirds do prey on tern chicks, it seems to be a prey
preference of only a few individuals rather than of all the frigatebirds pres-
ent (Robertson, pers. comm.). Ashmole (1963) reports Fregata aquila tak-
ing many young Sooty Terns on Ascension Island, and F. minor does the
same on Christmas Island (Ashmole and Ashmole, 1967).
On the Dry Tortugas migrating Cattle Egrets experience an extreme
shortage of food and feed on almost anything they can find including an
occasional Sooty Tern egg or chick (Robertson, pers. comm.) as well as
fish dropped by the terns. Cattle Egret predation on Sooty Tern eggs and
chicks is also known at the Seychelles (Ridley and Percy, 1958). The
Great White Heron (Ardea occidentalis) also has been reported preying
on Sooty Terns at the Tortugas (Robertson, 1962).
More commonly the Cattle Egrets forage on insects on Bush Key
and in doing so disturb the adult terns from their care of their egg or
chick, an activity that can be fatal to the offspring. 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 comes closer the
tern rises up off its egg or chick, erects the crown feathers, and continues
to direct "wuk" calls at the egret, the calls becoming louder and more
rapid and often changing to rapid "ka ka ak" calls. If the egret comes
within a few feet of the tern, the tern flies from the scrape and circles
over the egret until it departs. A few times a tern stretched its head and
bill forward, raised the wings almost vertically, and ran at the egret.
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 them, mobbing them
more commonly in 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.
A few Herring (Larus argentatus), Ring-billed (L. delawarensis),
and Laughing Gulls frequently linger around Bush Key. I never saw one
enter the Bush Key colony to prey on tern chicks or eggs, although they
could do so easily. Watson and Lashley (1915) saw Laughing Gulls prey
on Sooties at the Tortugas.
Adult Sooties often peck and kill chicks that wander onto their
scrape and can be a serious cause of mortality.
Despite the isolation of most Sooty Tern colonies, humans still
collect eggs for food at some of them (Cott, 1954; Ridley and Percy,
1958), but National Park Service protection eliminated egging at Bush
Key after 1935.
I could not help disturbing the birds every time I entered or left
my blind. Although they soon habituated to this and rapidly returned
to the scrape once I was out of sight, I occasionally stepped on eggs or
separated chicks from their parents. Such mortality is inevitable when-
ever a human works extensively in a Sooty Tern colony.
Disturbances such as sonic booms, boat whistles, and low flying
airplanes may cause the birds suddenly to leave the island in a dread.
However I did not see any of these disturbances keep the birds away from
their scrape for more than a few minutes unless they were repeated.

Sooty Terns and Brown Noddies 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 flat parts of the island outside the bay cedar, again separating
them from the Noddies. Some Noddies nest in low vegetation within
inches of Sooty scrapes. I often saw young of both species in such situa-
tions, so apparently they can breed successfully in close proximity.
Early in the nesting season Noddies often searched for nest mater-
ial on my study plot. In all 35 instances of direct supplanting between
these species that I saw on my study plot, the Sooty drove off the Noddy.
In two of these a Sooty chick drove an adult Noddy off the plot. Gen-
erally the Sooty stretched its head and neck forward, held the wings either
tight against its sides or slightly away from the body, and advanced to-
ward 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

Vol. XVI No. 3


growling "urr" note as it advanced at a Noddy but usually both were
silent. The few times I watched for the reverse encounters, I saw Nod-
dies drive off Sooties that had come close to Noddy nests.

Noddies arrive at Bush Key at about the same time as Sooties, but
start work on their nest almost at once instead of having a pronounced
flocking period 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 Sooty eggs. Young Noddies mature faster than Sooties and many
were flying by late June, well before most Sooty chicks.

Both species forage 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 (1967) show
that Sooties and Brown Noddies have very similar diets on Christmas
Island, but the Brown Noddies seem to forage much closer to the island.
They suggest that the wing of the Brown 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 forage within sight of Garden Key,
dipping down and capturing food at the sea surface. The incubation
shifts (30 minutes to 5 hours) and to some extent the intervals between
feeding their chick (2 to 4 hours) (Watson, 1908), are shorter for the
Noddy than for the Sooties. The fact that Sooties seldom forage within
sight of Bush Key and the somewhat shorter foraging times of Sooties,
especially during incubation, suggest that the two terns forage in some-
what different waters. The two species apparently forage in different
zones at colonies in the Pacific and Indian Oceans (Ashmole and Ash-
mole, 1967; 65-66; King, 1970).

In recent years the few hundred Roseate Terns at the Dry Tortugas
have nested in the coral rubble on Long Key and on the open sand on
some of the other islands, habitat quite different from that the Sooties
use. When they have nested on Bush Key, it usually has been at the far
east end away from most Sooties. Roseates usually arrive later than
Sooties and in 1968 did not start nesting until mid-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 any Sooty Tern chicks. Roseate Terns often forage within
sight of Bush Key and probably avoid competition with Sooties by for-
aging in different areas. They dive from the air and plunge into the water,
thus capturing food that is unavailable to the Sooties.



Common Terns have been shown to have a tendency to return to
the same nest site year after year (Austin, 1949). Later sightings of the
182 Sooties I color-banded on my small study plot on Bush Key in 1968,
provide some information on the species' site tenacity. I later saw 106 of
the 182 (58.2 percent) in the general area of banding, and most of them
probably nested in that general part of the colony.
From 28 April to 2 May 1970, O. L. Austin, Jr. sat near the plot for
several hours each day to watch for and record color-banded birds. He
saw at least 100 birds with color bands on the plot or immediately ad-
jacent to it. I saw three more in June and July, and four others were
caught in other parts of Bush Key. Of the 103 birds on the plot, 73 could
be identified individually; the others had lost some of the bands and
were not caught to read the FWS band. Of the 73 invidivually recogniz-
able Sooties, 53 (72.6 percent) were among the 106 that had been seen on
the plot in 1968 after banding. Thus, of 106 color-marked birds that prob-
ably nested on or near the study plot in 1968, at least half of them (53 of
106) were on the same plot 2 years later. The 30 birds that could not be
identified individually in 1970 may well have included birds that nested
on the plot in 1968. Thus 50 percent is a minimum figure and perhaps
70 percent 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 1968 and 1970. As my plot and the ground immedi-
ately around it covered much less than 1 percent of the habitat suitable
for Sooties on Bush Key, it seems unlikely that so many color-banded
birds would return to this same area in 1970 by chance alone.
In early July 1971 I spent a few hours watching for color-banded
birds on my plot and saw at least 13 different individuals. Many of these
had lost one or more color bands. Of eight that were individually recog-
nizable, seven probably had nested on the plot in 1968, again suggesting
that the terns were returning to the same plot to nest.
Site tenacity implies individual attachment to a specific locale in the
colony. Group 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 Com-
mon 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
recognize other chicks around them and 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 stay together within the colony could also explain
the clustered nesting of color-banded birds seen at the plot in 1970, but

Vol. XVI No. 3

this implies a fairly sizable group of perhaps several hundred birds func-
tioning 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 successive years.
If individuals repeatedly nest in the same part of the colony, they would
of necessity contact the same birds year after year and then possibly form


Two broader aspects of Sooty Tern behavior are particularly impor-
tant and warrant further discussion. First is the comparison of the behav-
ioral repertoire offuscata to that of other 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 fuscata that have allowed
it to survive and thrive in a pelagic environment. By 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 Procellariiformes.

Behavioral comparisons can be made with several other temperate
Sterna terns, the White Tern (Gygis alba), the two Anous, several species
of Thalasseus, the Black Tern (Chlidonias niger) and the Whiskered
Tern (C. hybrida).
The high flights of the Common, Arctic, Roseate, and Sandwich
Terns are all quite similar (Cullen, 1960a). The high flights of the Cas-
pian Tern, Black Tern, and Whiskered Tern resemble those of the others,
although in the latter two many birds may ascend together (Swift, 1960).
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 (1960a). In the high flight of the Brown Noddy the ascent is some-
what like the upward flutter of Sooties, but the birds descend in a circling
glide rather than a long, essentially straight glide as the Sterna terns do
(Moynihan, 1962; pers. obs.). The high flight of Gygis is similar to that
of the dark Noddies (Moynihan, 1962).
The low flight is poorly developed in fuscata and not nearly so com-
mon as in other terns except Gygis (Dorward, 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 Sooties may
be due to their different manner of carrying food. Cullen (1960a) notes


that food has little importance in advertising by unmated birds of both
species of Anous, both of which carry food in the esophagus as Sooties do.
Perhaps carrying food in the esophagus allows Sooties to transport
more food (Ashmole and Ashmole, 1967) than if they carried it 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, S. anaethetus and both Anous carry fish in their esophagus.
The ground displays of Sooties are similar to those reported for
other Sterna terns, all having some form of a parade display. In particular
the parade of Sooties seems to resemble that of the Common Tern de-
scribed by Palmer (1941) Incipient scrape-building activities are part
of courtship in Sooties, just as they are in Common Terns.

The most conspicuous ground display of the dark Noddies is nod-
ding, and generally their behavior is gull-like (Moynihan, 1962). Nod-
ding emphasizes head movements, whereas the parade emphasizes the
position of the wings and head, providing a clear contrast between the
Noddies and the Sterna terns. The displays of Gygis are similar to those
of the dark Noddies (Moynihan, 1962).

Comparisons of behavior help clarify the relationships of various
terns. Moynihan (1959) used comparative behavior as a basis for classi-
fying the Laridae and put all terns into three genera. Larosterna remains
monotypic, Anous includes Gygis and Procelsterna, 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, paradisaea, 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. The high flight
of fuscata is almost identical to that of hirundo and paradisaea. 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 dis-
plays of Gygis and both species of Anous, three species that share the
tropical oceans with Sooties, are clearly different from those of Sooties.
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 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 may feed closer to the breeding colony
than does fuscata. As yet these two are poorly known and further work
on them certainly would be of interest.

Vol. XVI No. 3


Of the 40 species of terns (Sterninae), most feed either on fresh-
water 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 Gygis 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. Several features of Sooty Terns deserve further comment in
this regard.
One of the most puzzling facts about Sooty Terns is that they
rapidly become waterlogged when placed on water (Watson and Lash-
ley, 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) appar-
ently does (Lack, 1956), or do they go without any sleep at all? Perhaps
Sooty Terns have some morphological adaptation that allows them to
fly better than coastal terns, but as yet this remains another unstudied
aspect of Sooty Tern biology.
Being able to feed without landing in the water certainly enhances
the ability of a nonswimming bird to range far from land. Thus Sooties
can exploit food in a large 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 the plankton that form the base of their food chain (Ray-
mont, 1963). Sooties apparently are restricted to feeding on food items
that occur at the very surface of the ocean, or in the air immediately above
it. This food is driven to the surface mainly by large predacious fish (Ash-
mole and Ashmole, 1967: 58) and thus is not evenly distributed through-
out the ocean.
The length of incubation shifts and brooding spells varies consider-
ably between different Sooty Tern 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 As-
cension Island (Ashmole, 1963), and 7 days on Christmas Island (Ash-
mole and Ashmole, 1967). The duration of the shift must depend some-
what 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. The
most reasonable explanation for the great variation between colonies in
time spent foraging is that food is farther away or scarcer for some colonies
than others.
More puzzling is the change in length of brooding spells once the


Usual Food
Feeding Clutch Incubation Fledging carried
Species habitat size period' period' exposed? Reference

Black Tern

Common Tern

Sooty Tern

Least Tern

Sandwich Tern

Brown Noddy

White Tern






Offshore, pelagic

Offshore, pelagic















Goodwin, 1960

Palmer, 1941

This study

Witherby et al., 1941; Hardy, 1957

Witherby et al., 1941

Thompson, 1903; Dorward and
Ashmole, 1963

Dorward, 1963; Ashmole, 1968

1 In days
2 Also carries food in esophagus


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), but detailed information on the rate of feeding is lack-
ing. As the breeding cycle at that colony is nonannual (9.5 months), it
seems unlikely that a regular change in food abundance could occur and
thus account for the differences in foraging between incubation and fledg-
ing 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 an-
nual, perhaps food is more plentiful, closer to the island, or the adults for-
age more diligently when chicks are present. The meager evidence avail-
able 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. However the many terns loafing on the
beaches and soaring in flocks during the time when chicks are being fed
suggest that food supply is not limiting the adults' activities even in that
crucial period.
Table 7 gives some information on the breeding biology of several
representative tern species. I have selected one species each from the gen-
era Chlidonias, Thalasseus, and Anous and three from Sterna, picking
species for which information is available and that seem to be representa-
tive of other similar species. This table is similar to one in Lack (1968:
262). Clutch size or other aspects of the breeding biology of a species may
vary from colony to colony 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 pelagic-
On Bush Key, as at other Sooty Tern colonies, a scrape occasionally
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
that something about the oceanic habitat limits their clutch size.
In contrast, coastal and freshwater feeding terns commonly lay
more than one egg (Table 7). Besides fuscata, only Anous, Gygis, and a
few other Sterna terns (e.g. S. anaethetus and lunata) have a usual clutch
of one. A clutch of two or more presumably is the ancestral condition
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, 1963).
With the smaller clutch size, pelagic terns must have either a high
nesting success, a long life span, or some combination of the two to
maintain their populations. With a clutch of one, partial nesting success
does not exist. Pairs either fledge an offspring or they fail. Among Sooties,
a maximum of 50 percent may renest if the first attempt fails, and the per-
centage declines as the breeding season advances (Ridley and Percy, 1958;
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 7). As storms and
a distant food supply may prevent Sooty Tern chicks from being fed
at regular intervals, a lengthened period of development perhaps allows
them to withstand 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).
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 of
their young about once an hour (Pearson, 1968) and Black Terns feed
young as often as 15 times in an hour (Goodwin, 1960). As a result their
young mature much faster than do young Sooties. The differences be-
tween Sooties and Noddies, both of which raise one chick per pair, have
already been discussed. Noddies seem to forage closer to the colony,
feed their young somewhat more frequently, and their young mature
faster than those of the farther ranging Sooties (Table 7).
As has already been suggested, perhaps the longer prefledging pe-
riod of Sooty Terns produces relatively well-developed young that are
better able to survive once they leave the island, even without a length-
ened period of care by their parents, while shore tern young fledge rapidly
but then depend on a subsequent period of care by their parents.
An important factor favoring the slow development of young Soot-
ies is the scarcity of predators on their breeding islands. 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,
1957) 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 (Stone-
house, 1962; Ashmole, 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, 1960b; Lind, 1963). In contrast, Arctic and Com-
mon 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, again
reducing the time they are vulnerable to predation.
Sooty Terns probably do not first breed until 6 years old (Robertson,
1969). In contrast most Common Terns first breed when 3 or 4 years old
and about 70 percent of their breeding population is made up of birds 6

Vol. XVI No. 3

years old or younger (Austin and Austin, 1956). Thus a cohort of Common
Terns is probably past its peak in the breeding population before the
equivalent cohort of Sooty Terns begins to breed. Deferred maturity
occurs in many other seabirds, including many Larids (Lack, 1968).
Robertson (1969) 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-old birds that visit Bush Key late in the breed-
ing season have little chance of finding a suitable nest site and breeding
successfully. By returning to the colony earlier in successive years, even-
tually they should be able to arrive early enough to find a suitable nest
site and breed successfully. Thus deferred 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 deferred maturity to persist, individuals with such maturity
must raise more offspring in the long run than if they first bred when
younger. As one 32-year-old Sooty Tern is known to have bred on Bush
Key (Robertson, pers. comm.), the adults there clearly have many breed-
ing 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 maximum natality than other similar terns that feed close to their

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. Sooty Terns with a clutch size of one can-
not 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 (Ashmole, 1963). This and the long spells that Ascen-
sion adults spend foraging suggest that at most a pair could raise one
chick. Other colonies that have been studied have not had such failures.
Except for the cats on Ascension Island, adult Sooty Terns suffer
little predation and have a low annual adult mortality, perhaps around
12 to 18 percent at Bush Key (Austin, pers. comm.) versus 25 percent in
adult Common Terns (Austin and Austin, 1956). The Austins' figures
may have been influenced by band loss, as Grosskopf (1964) reports adult
mortality in European hirundo as about 19 percent. Although the food
supply may limit the number of young they can raise and hence deter-
mine the usual clutch size of Sooty Terns, perhaps deferred maturity is a
function of the low predation on and high survivorship of the adults. High
survivorship of adults could result in many birds competing for a limited


number of nest sites, but deferred maturity could reduce this competition
as has already been discussed.
Some pelagic species can raise more than one young (see Harris,
1970), suggesting that food is not entirely limiting, but so far the problem
has been following the survival of these "twins" until they return to breed.
Thus Sooty Terns differ from most other terns. Many of these dif-
ferences 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 ways of carrying food.
Sooties do not have a well-developed low flight display, a display in
which other terns often carry a fish prominently in their bill. Other terns
also may 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 might require a long
flight. I have no information as to whether courtship feeding is important
in providing food resources for the female prior to egg-laying. I had
too few birds marked to obtain good information on whether she spends
some time away from the colony prior to egg-laying, but it is possible
that she does so and thus is not dependent on food from the male.
Comparison of 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 of most other terns, perhaps so their
young can survive lengthy periods when the parents are unable to find
food. The high survivorship of adults and the distant food supply have
led to their delayed maturity and small clutch size.
Thus in their breeding biology, with a long incubation and fledg-
ing period, deferred maturity, and a 1-egg-clutch, Sooty Terns resemble
other pelagic birds such as the albatrosses, shearwaters, and tropicbirds
more closely than most other terns.

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Vol. XVI No. 3


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o-/0. g?^

v, /

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