INFLUENCE OF LANDSCAPE COMPOSITION ON AVIAN COMMUNITY STRUCTURE AND ASSOCIATED MECHANISMS

Many recent studies have addressed the effects of patch size and isolation on bird communities in highly fragmented landscapes, but the importance of landscape composition in more forested landscapes remains poorly understood. The objectives of our study were (1) to determine the effects of two structurally and temporally distinct distur- bance types (agriculture and silviculture) and extent of disturbance (percentage of distur- bance within 1.0 km) on avian community structure within forested landscapes, and (2) to investigate underlying mechanisms responsible for associations between landscape com- position and avian communities. Differences in microhabitat, microclimate, and nesting success among landscapes were examined as possible mechanisms. Breeding bird com- munities in central Pennsylvania (USA) were surveyed twice per year during 1997-1999 at 34 sites within contiguous mature forest, which represented a range of disturbance extent (4-59% nonforest cover within 1.0 km). Each site was in a forested landscape containing predominantly agricultural or silvicultural disturbances (n = 17 each). Our results indicate that type of disturbance within landscapes influenced bird community composition and relative abundance more than extent of disturbance. Compared to forests within landscapes disturbed by silviculture, forests within landscapes disturbed by agriculture, irrespective of the extent of disturbance, had fewer forest-associated species, long-distance migrants, forest-canopy and forest-understory-nesting species, and greater numbers of edge-associated species, including avian nest predators and Brown-headed Cowbirds (Molothrus ater). Few- er species and guilds were associated with the extent of disturbance within a landscape or interactions between disturbance type and extent. Abundances of edge-associated species, residents, and forest-canopy nesters increased with increasing amounts of disturbance within forested landscapes. Local variation in microhabitat and microclimate among landscapes did not explain observed differences in avian community structure. However, nesting suc- cess was greater and numbers of some avian and small mammalian nest predators were lower in stands within forested landscapes disturbed by silviculture than in forested land- scapes disturbed by agriculture. Nesting success was not associated with the extent of a given disturbance type within landscapes. These results demonstrate that, even within for- ested landscapes, the types of disturbance can influence avian community structure and, thus, should be considered in conservation and forest management plans. In particular, agricultural disturbances within forested landscapes seemed to negatively affect bird com- munities in adjacent forests more than silvicultural disturbances. Both species richness and abundance of forest-associated species were greater on sites with higher levels of nesting success. Thus, differences in nesting success resulting from altered interactions between nest predators and nesting birds may be an important underlying mechanism of avian community structure and organization at the landscape scale.