Reproductive limitation in coastal ospreys (Pandion haliaetus): an ecological and an evolutionary perspective

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.

Grantor dc:publisher

Boston University

Year dc:date.issued

1984

Author dc:creator

• Poole, Alan Forsyth

• Biology

Statement dc:rights

• This work is being made available in OpenBU by permission of its author, and is available for research purposes only. All rights are reserved to the author.

Language dc:language.iso

en_US

Handle dc:identifier.uri

https://hdl.handle.net/2144/48220

OAI identifier oai:identifier

oai:open.bu.edu:2144/48220