RESPONSES OF CAVITY-NESTING BIRDS TO CHANGES IN AVAILABLE HABITAT REVEAL UNDERLYING DETERMINANTS OF NEST SELECTION

Abstract

Despite an abundance of studies describing nest selection in cavity-nesting birds, few account for the variation in habitat availability across the geographic range of a species and through time. Changes in the frequency of disturbances such as fires, timber harvests, and insect outbreaks can dramatically alter habitat. Responses by cavity-nesting birds to these changes provide an opportunity to evaluate underlying determinants of nest selection. We compared nest site attributes to available resources for 3 species of cavity-nesting birds: the Pileated Woodpecker (Dryocopus pileatus), Williamson's Sapsucker (Sphyrapicus thyroideus), and Black-backed Woodpecker (Picoides arcticus). Data collected in 2003–2004 were compared to data collected in the 1970s in the same study area to identify changes in patterns of selection. Due to severe insect outbreaks, available resources differed greatly between the 2 time periods. In studies separated by 30 y, Pileated Woodpeckers and Black-backed Woodpeckers exhibited remarkable consistency in patterns of nest site selection. Both species still selected Ponderosa Pine (Pinus ponderosa) snags preferentially for nesting despite their decline in availability. Williamson's Sapsuckers were less selective and nested in the most abundant snag species during both time periods. Selection for factors such as tree size and degree of decay remained consistent regardless of differences in habitat availability for the 3 species. By identifying these general patterns of nest site selection, our conclusions are applicable to a wider variety of habitat conditions.