Changes in density distribution of the Hooded Crow Corvus corone cornix and the Magpie Pica pica in Northern Italy
FASOLA M., CACCIAVILLANI S., MOVALLI C. AND VIGORITA V.Abstract:
We describe the density distribution ofbreeding Hooded Crows and Magpies over 12,827 km of planitial landscape, and we quantify the changes that have occurred since 1980. Nest were censused by means of winter roadside counts. We tested this technique by applying it to 12 sample zones where the breeding pairs had been censused during the preceding reproductive season, and we caIculated a conversion index from winter nest counts to breeding densities. The breeding populations for the entire study area in 1994 increased by 107% of the 1980 population for the Hooded CTOWand by 27% for the Magpie. Compared to 1980, the areas with high density of Hooded Crows in 1994 expanded north- and east-ward, while the distribution of the Magpie was similar to that in 1980. The density distributions of both corvids showed a clear structure with centers of abundance and with concentric bands of decreasing abundance, a pattern probably determined by gradients of environmental factors. However, the planitiallandscape of our study area is very uniform, and there is no noticeable gradient to match the density variations between the two corvids; the variations therefore remain unexplained. The 1980 distribution patterns had suggested that predation or competition by Hooded Crows could limit Magpie distribution at a geographic scale, but the 1994 data do not confirm this hypothesis.