In winter 2004/2005, 1532 Water Pipits were recorded during 37 censuses carried out along an established route on a sewage farm flooded with wastewater (Wrocław, SW Poland). Single birds were seen in nearly 39% of all 299 encounters, while the largest concentrations, between 16–28 individuals, accounted for 9%. 78% of all birds were observed on meadows flooded with communal wastewater. The remaining ones stayed around irrigation ditches (n = 172, 11.5%), sedimentation basins (n = 88, 5.9%) and reedbeds (n = 72, 4.8%). The mean size of the Water Pipit concentration was largest on the meadows (mean ± SE = 6.54 ± 0.50 individuals) and smallest at the sedimentation basins (mean = 1.44 ± 0.14). In this winter season (December-first half of March), rainfall enlarged numbers of birds to forage on the meadows, and the thickness of the snow cover was positively correlated with bird abundance at the sedimentation basins. The dominant available prey items inhabiting the warm wastewater were Diptera larvae (96%), 88% of which belonged to the genus Eristalis. The mean (± SD) invertebrate biomass was highest in the basin sediments (1.03 ± 1.14 g/dm3 of deposits), and lowest on the flooded meadows (0.20 ± 0.37 g/dm3 of deposits). The results point to the significance of the artificial environmental conditions created by warm sewage water, which enable the birds to remain largely independent of the weather and thus to overwinter in a cold region of central Europe.
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Vol. 41 • No. 1