Long-term spatio-temporal dynamics of corvids wintering in urban parks of Warsaw, Poland

Abstract

Wintering in urban habitats seems to be a profitable and an increasingly important strategy for many bird species. In central Europe, the corvids Rook, Jackdaw, Magpie and Hooded Crow constitute one of the most abundant groups of wild vertebrates in cities. Counts of corvids wintering in Warsaw, central Poland were conducted in three parks from October to December during 18 seasons (1977-2003; 258 separate daily counts, 26,166 records). We tested the effects of date, year, park identity and snow cover on the observed densities, and that of snow depth on the vertical distribution, of corvids. From October to December, the densities of Rook and Jackdaw increased while those of Magpie and Hooded Crow decreased. The density of Rook decreased, and that of the other species increased, in the long-term. Apart from the Rook, the densities of these species also varied among parks. Snow occurrence negatively affected the densities of Rook and Jackdaw, but it had no significant effect on the densities of Magpie and Hooded Crow. The vertical distribution of Rooks and Jackdaws depended on the depth ofsnow cover: Almost all individuals were observed in trees when the cover was deep, suggesting a change in foraging strategy during snowy periods. After excluding the factors snow, year and park from the models, corvid densities correlated with each other, suggesting that factors other than competition, such as food availability, may be responsible for synchronizing the dynamics of these species in urban parks.