Predation of eggs or nestlings is generally believed to be the most influential factor limiting passerine reproductive success. Thus, there should be strong selective pressures for birds to place their nests in sites that are inaccessible to predators or that are less likely to be discovered by them. We found and monitored 231 nests of 4 species of arboreal, cup-nesting birds: Western Wood-Pewee (Contopus sordidulus), Warbling Vireo (Vireo gilvus), American Robin (Turdus migratorius), and Yellow-rumped Warbler (Dendroica coronata). We determined strength of nest tree species selection by comparing nest trees and tree species availability. Western Wood-Pewees and Warbling Vireos demonstrated strong preference for placing nests in quaking aspen (Populus tremuloides). Yellow-rumped Warblers showed a weak preference for aspen, and American Robins demonstrated no preference. We designed a series of experiments to determine whether yellow pine chipmunks (Tamias amoenus), an abundant nest predator, could climb aspen trees and, if so, what factors might prevent them from doing so. Yellow pine chipmunks were unable to climb aspen but showed no difficulty in climbing and maneuvering on lodgepole pine (Pinus contorta) boles, which were identical in diameter to aspen boles. Refuge from chipmunks as potential nest predators is likely contributing to nest site selection for a few arboreal cup-nesting bird species where aspen trees are available.