Grey Herons foraged in a high number only in one of three preferred foraging areas in the Dobczyce Reservoir, S. Poland: (1) the backwater area at the main tributary inlet (MTI), (2) the shore of the near-dam pool (NDP), and (3) shallow lateral bay (SLB). Median value of the fraction of herons foraging in MTI was 64% of all counted in the reservoir in 2001-2002 despite the smallest fish density there (2 times smaller than in NDP, and by one order of magnitude than in SLB). Roach (Rutilus rutilus) was the most abundant both in the heron diet (13 species) and in littoral fish communities (8 species). Total length of prey ranged within 4.2-26.5 cm. The individual size range of littoral fish was similar (4.6-31.5 cm). However, the size distributions in three foraging areas and in heron diet were different. Fish longer than the median total length of heron prey, i.e. ≥8 cm in total length constituted 51.9% of heron diet, 34.7% of the fish community in MTI, 8.5% in the psammolittoral of NDP, and only 5.2% in the phytolittoral of SLB. The strategy of selective choice of longer fish from those occurring at foraging sites allowed greater reward with roughly unchanged foraging cost. This may explain why Grey Heron foraged mainly in the habitat with the lowest fish abundance and highest water turbidity but with the largest prey size.
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Vol. 29 • No. 2