Abstract
Although migratory forest birds have declined at many sites in Japan, the negative effect of forest fragmentation on bird populations has not been intensively studied. We sought to ( 1 ) determine whether there are particular bird species or groups of species that are sensitive to the area and isolation of forests in Japan and ( 2 ) compare the types of species that are area‐sensitive in Japan, Western Europe, and eastern North America. We estimated the abundance of breeding birds by using the fixed‐radius, point‐count method in 53 deciduous forests ( 1–5045 ha ) in southern Hokkaido. Forest area was positively related to the abundance ( average number of individuals per survey plot ) and species richness ( average number of species per survey plot ) of forest‐interior birds and was negatively related to the abundance and species richness of forest‐edge birds. As in eastern North America, forest area was the best predictor of the abundance of several species of forest birds. Also, some species were restricted to large forests, with minimum areas larger than expected by chance. Forest area was not significantly related to the total abundance of tropical migrants, but five of the nine potentially area‐sensitive species were tropical migrants. Jungle Crows ( Corvus macrorhynchos ), which are nest predators, were more abundant in small forests than in large forests, but Oriental Cuckoos ( Cuculus saturatus ), which were the most common brood parasites, were more abundant in large forests. To conserve area‐sensitive species in deciduous forests in northern Japan, it will be necessary to preserve large areas of contiguous forest and monitor the effect of predators on nest success.