Assessing the Influence of Spatial Scale on the Relationship between Avian Nesting Success and Forest Fragmentation

Ecological processes are dependent on the spatial and temporal scale at which they are viewed, and a process at any one scale may be influenced by factors at other scales. Thus, an ecological process at a broader scale may act to constrain processes at finer scales (Allen and Starr 1982, Thompson et al. 2000). Developing a full understanding of the spatial scales at which habitat conditions impinge on ecological processes therefore demands a multi-scale approach (Wiens 1989). The continuum of possible spatial scales can be broken into: (1) the space occupied by an individual, (2) the patch scale – the habitat patch occupied by many individuals and species, (3) the landscape scale – the collection of different habitat patches occupied by local populations, and (4) a biogeographic scale that encompasses different climates, vegetation formations, and assemblages of species (adapted from Wiens et al. 1986). Habitat fragmentation alters the spatial arrangement, shape and relative proportions of different habitat patches. These changes have a profound influence on ecological processes that are sensitive to alteration of the composition of environments, particularly at spatial scales 2-4 above. Two ecological processes, nest predation and brood parasitism by the Brown-headed Cowbird (Molothrus ater), are the primary influences on nesting success of most North American land birds (Martin 1992). Many studies that have investigated the relationship between habitat and nesting success have focused on the question of how vegetation characteristics of the nest micro-environment influence nest success (e.g., Martin 1992, Larison et al. 2001). This focus is at the scale of the space occupied by an individual bird (Wiens et al. 1986). In contrast, studies at broader scales are mostly concerned with how variation in predator/parasite density or