Spatial extent and relative influence of landscape-level factors on wintering bird populations

The influences of the landscape matrix (complex of habitats surrounding a study plot) and within-patch vegetation were studied in bird communities wintering in the piedmont of Georgia, USA. Variation at the landscape and within-patch levels was controlled to reduce the likelihood of confounding and spurious relationships. The landscape matrix within 500 m of each study plot was quantified from aerial photographs. Statistical models using landscape matrix and within-patch vegetation variables explained 73-84% of variation in bird abundance and diversity among sites with landscape matrix variables accounting for 30-90% of the variation. Variation in bird species richness and diversity was explained solely by landscape variables. Models for individual species such as Carolina wrens (Thryothorus ludovicianus) and rufous-sided towhees (Pipilo erythrophthalmus) had r² > 0.80, with the landscape matrix variables accounting for the majority of this variation. However, other species like northern cardinals (Cardinalis cardinalis) and white-throated sparrows (Zonotrichia albicollis) were most strongly influenced by within-plot vegetation. The landscape influence extended beyond habitats immediately adjacent to the study plots as indicated by significant variables describing variation in more distant habitat patches. These analyses illustrate a technique for comparing the strength of within-patch versus landscape influences and measuring the spatial extent of the landscape influence in fine-grained landscapes.