Factors affecting the breeding density of Northern Goshawks (Accipiter gentilis).

Nesting locations of the Northern Goshawks(Accipiter gentiles) were investigated in two areas of about 348km² in Tochigi prefecture, about 50km north of Tokyo. A total of 22 nesting locations were found in the area. Their food items were also monitored. A multiple regression model was constructed to predict the breeding density (measured as the total area covered by 1.8km buffers generated from the location of each nests within a 3x3km grid) from several environmental factors such as vegetation and human population size. The resulted model could explain about 63% of the variation in the breeding density r²=0.63. Factors used in the model were: length of forest edge facing grassland (positive factor), area of bare land, construction sites etc. and human population size within each grid (negative factors). In this area, Goshawks were feeding mainly on small or medium sized birds, but the abundance, or biomass, of the birds was not an important factor that affected the breeding density of the Goshawks. The reason for this might be either that the food (amount of birds) is generally abundant in the area and therefore not an important limiting factor, or that availability of the suitable hunting area such as the forest edge facing the grassland is more important than the availability of the food itself.