Effects of urbanization and nest-box design on reproduction vary by species in three cavity-nesting passerines in the Okanagan Valley, British Columbia, Canada

Artificial nest boxes provide an important resource for secondary cavity-nesting passerines, whose populations may be limited by the availability of nesting sites. However, previous studies have demonstrated that the design and placement of boxes may affect the reproductive success of the birds that use them. In this study, we asked whether the habitat surrounding a nest box or the type of box influenced reproduction in three cavity-nesting passerines. We studied Western Bluebirds (Sialia mexicana Swainson, 1832), Mountain Bluebirds (Sialia currucoides (Bechstein, 1798)), and Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) breeding in artificial nest boxes at sites across 70 km of the Okanagan Valley of British Columbia, Canada. Sites varied in their degree of urbanization, from relatively undisturbed ranchland, to cultivated vineyards, to frequently disturbed “suburban” habitat, and boxes varied in type of entrance (slot or hole). Western Bluebirds nested earlier in vineyards, and Tree Swallows produced significantly fewer fledglings in suburban habitat. In addition, Tree Swallows nested earlier and produced more fledglings in slot boxes. Our results suggest that conservation actions for cavity-nesting passerines may depend on the target species, which in turn should dictate the appropriate box type and habitat when erecting or replacing nest boxes.