DIVERSITY AND ABUNDANCE OF BIRDS IN RELATION TO FOREST FRAGMENTATION,HABITAT QUALITY AND HETEROGENEITY

This study investigated the importance of habitat quality and habitat heterogeneity for the abundance and diversity of breeding birds in con tinuous forest and in forest fragments surrounded by farmland in centr al Sweden. Positive correlations were found between species number and area, volume of Aspen Populus tremula and habitat heterogeneity. Spat ial segregation of habitats at a relatively fine-grained scale is sugg ested to allow for the co-occurrence of more species. The abundance of at least 18 of the species in this study was influenced by fragmentat ion, and nine of these species preferred fragments to forest sites. Th e total density of virus was higher in fragments than in forest sites, probably because several fragment species forage in farmland surround ing the sites and a few also forage at edges. Nine species were more c ommon in forest sites than in fragments, but only one species was rest ricted to continuous forest. However, several fragments were relativel y close to forests (150 m) and forest was common in larger scale conte xts. The abundance of most species (25 of 33 species) in this study wa s correlated with habitat quality variables (i.e. variables measuring the size, volume and diversity of 'tree species'). Among these habitat variables the most important tons the occurrence of deciduous trees w hich seemed to be important for 14 species. The second most important habitat factor seemed to be the diameter of trees, which was positivel y correlated with the abundance of eight species of which five are hol e-nesters. Among coniferous trees, six species were positively correla ted with the volume of Norway Spruce Picea abies, whereas no species s eemed to be correlated with the volume of Pine Pinus sylvestris.