The Ontogeny of Diet Selection in Fledgling Ospreys

I studied the ontogeny of diet selection in fledgling Ospreys (Pandion haliaetus) in the 1985 and 1986 postfledging periods to (1) determine if young that forage from the same available fish resource base develop similar diets; (2) examine how diet choice is affected by interactions with conspecifics; (3) examine how within— and between—year differences in fish abundance in two different lake habitats (littoral and limnetic zones) affect patterns of diet selection; and (4) compare diet choice of fledglings with adults who simultaneously were feeding on the same resources base. Eight young in 1985 and 14 young in 1986 were color banded and observed throughout the 170—d postfledging period. Dynamics of the available fish resource base, which consisted of sunfish (Lepomis spp.), shad (Dorosoma spp.), and bass (Micropterus salmoides, Morone saxatilis), were also monitored. Fish abundance differed considerably between years, both by habitat and by fish species. The abundance of all fish was decreasing when young fledged in 1985 and increasing in 1986. Individual differences in diet selection were present in both years, with preference for each fish species in each year exhibited by at least one young. In addition, some young of each year exhibited no preference for any fish species, but instead captured fish in relation to their availability in the lake. Preference patterns of individuals remained constant throughout postfledging in both years, although the mean diet of all young shifted in 1985. Individual preference patterns were strongly related to whichever fish species was captured initially. Siblings, who tended to forage together throughout postfledging, had statistically similar diets. Diets of all young in 1985 and 9 of 14 young in 1986 differed from the mean diet of adults. Although diets of adults shifted during postfledging, diets of young remained constant, suggesting that young were less able than adults to respond to fluctuations in fish availability. Adults also preferentially foraged in the lake's littoral zone whereas young did not exhibit any tendency to choose one habitat over the other. It may be that recognition and use of habitat—related differences in fish abundance represent a skill level not yet developed by recently fledged Ospreys.