Abstract:
Fresh egg-weights and feeding rates to chicks were related to chick
survival as one means of quantifying apportionment of parental investment
wi thin broods of Caspian Terns (SterDI casRla) at a colony in
Georgian Bay. Lake Huron, during 1978 and 1979. Ftrst-laid eggs from
2-egg clutches were Significantly heavier and usually hatched one to
three days earlier than second-laid eggs in both years of the study.
In both years, first-hatched chicks were larger and generally
better fed than second-hatched siblings. The disparity between feedIng
rates of first- and second-hatched ehicks was greater in 1979.
Brood feeding I rates correlated positively with the percentage of food
fed to the least-fed sibUng through the period of B-chick ages zero
to 10 days in 1978. I suggest that after this age period, parental
control over whlcb cbick was fed diminished. In 1978, 10 of 16 secondhatched
chicks were fed more than their older siblings during their
first 5 days. 'lb.is is interpreted as a parental response to reduce
the competitive advantage of the larger first-hatched chicks.
Most chick losses were apparently caused by starvation or preda.
tion. In 1979, seeorvl-hatched chick disappearance (due to predation)
was -related to low feeding rates, whereas first-hatched chick disappearance
was related to low fresh egg-weights.. First-hatched chicks
survived better than second-hatched chicks both years, and more pairs
fledged two chicks in 1978. Maximum estimated feeding rates at the
nest and fledging ages suggested that food was more avatlable in 1978
than in 1979.
In 1979, second eggs apparently functioned as "insurance" eggs.
When the first-laid egg falled to hatch, or the first-hatched chick
died, the second-hatched chick was often successfully fledged. When
first-hatched chicks survived, the second-hatched chick usually starved
or was preyed upon, reducing the brood to one chick.
Parental investment patterns favored first-hatched chicks. Brood
reduction, when employed, discouraged total nest failure, however, under
appropriate conditions, brood reduction was avoided and full broods
(or two chicks) were fledged.
