Nest cavity availability and selection in aspen–conifer groves in a grassland landscape

Abstract

Cavity-nesting vertebrates, a significant component of forest wildlife communities, are connected ecologically in nest webs arranged in guilds of cavity producers (strong and weak excavators) and cavity consumers. The availability of tree cavities for nesting and roosting is considered critical to maintaining communities of cavity nesters. Using univariate tests and resource selection indices, we examined nesting use of both natural and excavated cavities in small aspen–conifer groves of interior British Columbia in relation to availability of cavity, tree, and habitat features. In a 2-year study, we monitored 197 cavities for use, 8% of which were lost between years. Of those that survived, 26% were occupied in both years, 29% were used in only one year, and 45% were not used in either year. Occupied cavities were larger internally than those not occupied in either year and were more likely to be in trees with only one cavity. In species-level resource selection analyses, tree swallows (Tachycineta bicolor Vieillot), European starlings (Sturnus vulgaris L.), and chickadees (Poecile spp.) used live, heart rot infected trees more than expected from their proportional abundance and used dead trees less than expected, while northern flickers (Colaptes auratus L.), mountain bluebirds (Sialia currucoides Bechstein), and red squirrels (Tamiasciurus hudsonicus Erxleben) used live and dead trees in proportion to their availability. Managed, open landscapes with small mixed or deciduous forest groves can sustain healthy communities of cavity nesters, if there is a range of suitable trees for excavators and open areas for foraging.