Temporal Variation in Prey Preference Patterns of Adult Ospreys

Abstract

ABSTRAcr.-I examined temporal variation in diet with respect to fish dynamics in a resident population of Ospreys (Pandion haliaetus) in Florida for 18 months. Bass (Micropterus salmoides, Morone saxtilis) were captured in proportion to their abundance. Ospreys exhibited preference for sunfish (Lepomis spp.) from May to August 1985 and from March to July 1986. Shad (Dorosoma spp.) were preferred from September to February. These shifts were closely related to concurrent shifts in sunfish abundance; preference for sunfish was exhibited when they were most abundant. When sunfish abundance declined, Ospreys switched back to shad. Shifts from shad to sunfish also were associated with a switch in foraging habitat. Ospreys hunted preferentially in the littoral zone of the lake during peak abundances of sunfish and foraged in the limnetic zone when concentrating on shad. Changes in preference for sunfish lagged 1.2 months behind changes in sunfish abundance, but lagged less than 0.3 months for shad. Variation in sunfish abundance was greater than that for shad, suggesting that the ability of Ospreys to exhibit strict concordance between abundance and preference may depend on the amplitude of the change in abundance. Received 6 July 1987, accepted 11 November 1987.