The study was conducted in the Lake Karaś reserve (N Poland), where the density of Willow Warblers in osier and birch thickets reached 28.7–33.3 p/10 ha. Collar rings were used to collect the food brought to the nestlings. 487 food samples were collected from 26 nests, comprising 2573 food items. 167 of the samples were from females (652 individual food items), 95 from males (616 food items). The nestlings were fed mainly on Arthropoda (94.6%) and Mollusca (5.4%). The dominant food items were Diptera (29.8%), Homoptera (28.7%), Ephemeroptera (10.5%), Araneida (7.6%) and Trichoptera (6.5%). The composition of the dominant food items and the average mass of food deliveries changed markedly with nestling growth. The types of food brought by each sex also varied significantly. Males brought heavier portions of food consisting of larger numbers of lighter prey. However, the composition of these portions was not as diverse as those brought by females. This may be related to differences between males and females in their respective territories and foraging techniques.
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