Song Post and Foraging Site Characteristics of Breeding Varied Thrushes in Northwestern California

Abstract

Abstract We used a hierarchical approach to describe habitat characteristics of song posts and foraging sites used by Varied Thrushes (Ixoreus naevius) in coastal redwood (Sequoia sempervirens) forests of northwestern California. We measured mesohabitat (0.04-ha circular plots) and microhabitat (0.5-m radius) scale attributes centered on occupied and random song posts and foraging locations at four study sites during March–August of 1994 and 1995. Ninety-five percent of song posts were in trees or snags. Male thrushes used song posts with low foliage density near the top of large conifers (microhabitat scale), located on steeper slopes, surrounded by a high density of trees, and centered in drainages closer to water (mesohabitat scale) as compared to random locations. Varied Thrushes foraged predominantly on the ground early in the breeding season, then subsequently included fruit in their diet after the young had fledged. Although many variables were correlated with ground foraging locations, microhabitat foliage density had the greatest explanatory power, indicating thrushes selected foraging locations primarily at the microhabitat scale, and emphasizing the importance of measuring habitat characteristics at the appropriate spatial scale. Abrupt forest edges, such as those produced by clearcuts, may reduce habitat suitability for Varied Thrushes possibly explaining their absence from small forest fragments during the breeding season.