Influence of Vegetation on Occupation of Nest Boxes by Chestnut-Backed and Mountain Chickadees

Abstract

We analyzed how vegetation structure and nest box orientation influenced patterns of nest box occupancy by chestnut-backed (Poecile rufescens) and mountain (R gambeli) chickadees in the western Sierra Nevada for 3 breeding seasons. The range expansion by the chestnut-backed chickadee during the past 50 years into forests where the mountain chickadee was formerly the only resident parid provided an opportunity to examine how these 2 closely related species use forest resources in a zone of recent sympatry. Our study was conducted about 35 to 40 years after chestnut-backed chickadees colonized the western Sierra Nevada. Although there was broad overlap in the habitat structure around the boxes occupied by the 2 species, chestnut-backed chickadees tended to occupy boxes that were in denser, more closedcanopy stands and mountain chickadees tended to occupy boxes in stands with a more open, park-like structure. Both species clearly avoided nest boxes and habitat plots with a western orientation and consistently used boxes in patches of habitat that were oriented north, east, and south.