Understanding foraging strategies remains a central question in behavioural ecology, but studies investigating how foraging of sexes is affected by other individual characteristics, like body size, are still scarce. We investigated how foraging behaviour during chick rearing varies in males and females with brood size, offspring age and individual body mass of parents, in a sexually size-dimorphic waterbird, the Whiskered Tern Chlidonias hybrida. Our study took place at the carp fish ponds in southern Poland, where both invertebrates (dragonflies, a typical prey of females — caught by picking) and small vertebrates (fish, tadpoles, frogs, males typical prey — caught by plunge-diving) are plentiful and available for both sexes during chick-rearing period. In total, 1680 attacks of 29 uniquely marked birds (16 males and 13 females) were observed during chick-rearing period. Foraging techniques were affected by sex of the parent and offspring age, interacting with body mass, and brood size. Males foraged mainly by plunge diving, but avoided this foraging technique if their broods were small and when offspring were young, probably because the chicks were too small to consume vertebrate prey caught by diving. In contrast, females foraged mostly by picking prey from the air, water surface or floating leaves, for most of chick-rearing period, but increased frequency of plunge diving as offspring age increased. A significant interaction between body mass and offspring age suggests that birds differing in body mass foraged differently as their offspring grew. We conclude that despite sex-specific differences in foraging behaviour (and prey type delivered to the chicks), both sexes in the Whiskered Tern alter foraging behaviour in response to both brood and individual birds' attributes.
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Vol. 52 • No. 1