Abstract
We analyzed how the composition of mixed-species flocks occupied by chestnut- backed (Poecile rufescens) and / or mountain (P gambeli) chickadees varied during the 1987 to 1988 and 1988 to 1989 nonbreeding seasons (October-March) in the western Sierra Nevada. These 2 chickadee species foraged during the nonbreeding season with -15 other species of birds; av- erage number of species per flock was 3.1. Flocks consisting of only chestnut-backed or moun- tain chickadees made up 20% (42 of 209) all flocks. Average flock size and number of mountain chickadees per flock increased significantly from early (August through September) through middle (October through December) of the nonbreeding season. These results contrast with other studies of mixed-species flocking by birds which indicated that flock size typically de- creases as the nonbreeding season progresses through the fall and winter months.
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