2 
movement of these birds . 
The life cycle of the Sooty 
Tern has been the subject of inten- 
sive study in the central Pacific. 
The first evidence that breeding 
activity is about to take place is 
the presence at night of increas- 
ingly large numbers of Sooty Terns * 
swirling over the island in a loose 
spiral. A highly social species 
like the Sooty Tern requires the 
proper social stimulus in order to 
commence breeding* and in this case 
the social stimulus is provided 
mainly by the dense swirls of birds 
near and v over their intended breed- 
ing island. 
At last* when all conditions 
appear to be right* the first birds 
touch down for a short time during 
the night. We can only speculate 
on what the "proper*' conditions are. 
They probably include the assurance 
of a continued food supply at sea 
within a day’s flying time from the 
island* enough bare ground avail- 
able for nesting* and sufficient 
numbers of Sooty Terns swirling to- 
gether to satisfy the social stim- 
ulus requirements of the colony. 
Gradually the numbers of birds 
touching down increases from plght 
to night and the length of time 
they spend on the ground increases 
too. Finally* some stay through 
the day as well* and immediately 
thereafter eggs are laid. The 
parents take turns incubating their 
egg which they cover with their 
breast on a simple* shallow dish- 
shaped scrape in the sand. They 
are surrounded by other Sooty Terns* 
all incubating their eggs . Their 
nearest neighbors are usually only 
a foot away. In many colonies the 
available nesting space is utilized 
on three levels . Frigatebirds and 
boobies roost in the bushes* Sooty 
Terns on the ground* and shear- 
waters and petrels occupy burrows 
under the ground. 
Sooty Terns incubate their egg 
for about 28 days and about two 
months after the young have hatch- 
ed they are ready to fly. During 
these two months the parents feed 
the chicks daily* and frequently 
several times daily* on small fish 
and squid caught in the ocean near- 
by. 
About five months after the 
first eggs are laid the adult Sooty 
Terns and their newly- fledged young 
head out to sea* leaving behind 
their breeding island. The adults 
will return sometime in the next 
year to breed again. The young 
birds remain at sea for two years* 
but may pay their first call to the 
breeding island in the second year* 
although they do not attempt to 
breed until sometime after their 
fourth, year. By the second year 
the dark gray plumage with white 
speckling on the back of immature 
birds has been replaced by the 
black upperparts and white under- 
parts of adult birds. 
(Cont’d on page b) 
Immature Sooty Tern 
Photo by Philip C. Shelton 
