Reproductive limitation in coastal ospreys (Pandion haliaetus): an ecological and an evolutionary perspective
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Abstract
Field studies of breeding ospreys were conducted in coastal
regions of Massachusetts, New York, and south Florida from 1978-1983
to determine: ( i) the proximate control that local food supply exerts
on reproductive performance in these populations; (ii) how food
limitation has ultimately shaped life-history patterns in this
species, especially clutch-size and laying date.
Reduced food delivery to nests increased nestling loss.
Reproductive loss was especially high in Florida where latitude and
season restricted daylight foraging time for the providing male
parent. Starvation of smaller, later-hatching chicks within broods
was the primary cause of nestling death. Sibling aggression accounted
for the preferential feeding of older nestmates, but only in
food-stressed colonies.
Food consumption and weight gain were measured for 18 female ospreys prior to egg-laying. Females gained little weight during this
courtship period, and rates of food intake had no strong relationship
to clutch or egg size in these birds. Females that laid early were
better fed than those laying late, but the ages and breeding
experience of pairs explained most of the variability in
their laying dates. Reproductive success within local populations of
ospreys declined steadily as the breeding season progressed. This
pattern could not be explained by changes in clutch size or food
availability and was only partially explained by changes in the age
and experience of pairs. Reduced reproductive effort is discussed as
an alternative explanation.
Females laying modal 3-egg clutches fledged nearly as many young
as those laying 4 eggs. Pairs with 4-egg clutches showed more yearly
variability in reproductive success than those with clutches of 3
eggs; rarely did pairs fledge 4 young. Males in pairs given enlarged
broods (5 young) increased food delivery to their nests, but the
amount of food available per chick was still less than in normal
3-young broods. Females in pairs given enlarged broods ate less food
and suffered potentially drastic weight loss.
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