Composition and Temporal Variation of Flocks in the Sierra Nevada

Abstract

The size and species composition of bird flocks in a mixed-conifer forest of the western Sierra Nevada was nearly identical over three winters. Golden-crowned Kinglets (Regulus satrapa), Chestnut-backed Chickadees (Parus rufescens), and Red-breasted Nuthatches (Sitta canadensis) had the highest number of individuals/flock. Kinglets and nuthatches were the most frequent flock members, although they occurred in only 28% and 20% of all flocks, respectively. Most species formed monospecific flocks, although they more often occurred in mixed-species flocks. Although flock size was significantly greater in early winter than in middle and late winter, the number of species/flock did not vary temporally. No meaningful correlations between flock composition and weather conditions were evident. Differences in flock composition between this and other studies resulted from the relatively high frequency of nuthatches found in our flocks. In addition, flock composition was apparently influenced greatly by habitat. The anti-predator and foraging efficiency models emphasized in the literature are evaluated, and a framework for a more complete testing of causes of flocking discussed.