Breeding Biology of the Hooded Crow Corvus corone cornix in Warta River Valley (W Poland)

Abstract

The study area (16 km2) in “Ujście Warty” National Park, W Poland — was the valley of a lowland river at its confluence with the River Odra, covered by a mosaic of grassy vegetation and willow scrub. 111 breeding attempts were recorded during 2000–2002. The mean nest density (3.2 nests/km2) was higher than that recorded by other authors in agricultural landscapes, but lower than in urban areas. The nest construction was adapted to fit young willow trees. The mean clutch size was similar to that recorded in other populations (4.43), but eggs were smaller (41.2 mm x 29.1 mm). The hatching success was lower (76%) in comparison with other studies, but the mean number of fledglings (2.15 per nest and 2.96 per nest in successful broods) was relatively high. The main reasons for losses were unhatched eggs, predators, starved nestlings and poor nest construction. We hypothesise that the smaller egg size and lower hatching success recorded in this population was due to unfavourable and unpredictable feeding conditions (floods) during the period of egg formation and egg laying. Later in the season, receding floodwaters laid bare areas suitable for foraging on invertebrates; waterfowl eggs also became readily available. Predation was low (lack of nonbreeding stock of Hooded Crow). As a result of good conditions during chick rearing, the overall reproductive output was relatively high in comparison with other populations.