Evidence for Nest-Site Preference in the Western Bluebird (Sialia mexicana) when Availability is High



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For tree-dwelling birds that cannot excavate their own holes (secondary cavity nesters), nest-site selection is the primary means of maximizing reproductive success. Choice, however, is limited by abundance. Nestboxes effectively remove this constraint - possibly revealing preference. Previous research has shown that birds select box orientations relative to the sun that maximize offspring survivorship, but competitive interactions for limited numbers of boxes can obscure preference. In this study, I investigated the effect of nestbox orientation on occupancy rates of the Western bluebird (Sialia mexicana) in a network of nestboxes with 14 years’ worth of data. I detected orientation preferences, but habitat and year were stronger predictors of occupancy. Furthermore, orientation preference was maintained within only one habitat. Year and habitat differentially affected reproductive success: year influencing clutch size and fledging rates, and habitat influencing hatching rates and fledging rates. No reproductive benefit was observed with respect to orientation in the larger data set, or within the where preference was maintained. To test whether that offspring fitness was affected, I tested and found no difference in fledgling weights between preferred and least-preferred orientations. Overall, these results reflect changing risks to survival over development, spatial variability across the network, and more subtle climactic interactions than initially assumed.



avain, ecology, niche, theory, conservation