Nest-Site Relationships among Cavity-Nesting Birds of Riparian and Snowpocket Aspen Woodlands in the Northwestern Great Basin

Abstract

We examined nest-site and nest-cavity characteristics for six species of cavity-nesting birds in montane riparian and snowpocket aspen (Populus tremuloides) woodlands in the northwestern Great Basin. Live trees and snags with DBH >24 cm were favored as nest sites by all species. Red-naped Sapsuckers (Sphyrapicus nuchalis) and Northern Flickers (Colaptes auratus) provided different sizes of nest cavities for a suite ofnonexcavator species. Flickers preferentially nested in snags; sapsuckers nested primarily in live trees, but used live trees and snags in proportion to their availabilities. Relative abundances of excavators and nonexcavators were associated positively with numbers of cavities. Nest-site variables overlapped extensively among species; Tree Swallows (Tachycineta bicolor) relied heavily on sapsuckers for provision of nest cavities, European Starlings (Sturnus vulgaris) and Mountain Bluebirds (Sialia currucoides) primarily used flicker-excavated cavities, and House Wrens (Troglodytes aedon) used nest cavities across the broadest range of nest-site characteristics. Compass orientation of nest-cavity entrances was strongly bimodal, with most facing east or southwest. Cavity entrances of species that foraged largely outside of riparian woodlands were oriented toward woodland edge, in contrast to nest cavities used by species that foraged largely within riparian woodlands. Snowpocket woodlands were much more extensive than riparian aspen, but birds strongly preferred riparian aspen stands as nesting habitats, presumably due to the scarcity of large aspen in snowpockets. Nest cavities appear to be a limiting resource with high potential for interspecific nest-site competition in these woodlands. Decades of livestock overuse and fire suppression have greatly diminished the availability of large aspen in riparian habitats throughout the region.