Western Bluebirds (Sialia mexicana) Use Public Information for Prospecting but Not Ultimate Nest Site Selection

Public information about conspecific presence and reproductive success, in addition to direct environmental sampling, may allow individuals to efficiently assess breeding site quality. We tested the hypothesis that prospecting Western Bluebirds (Sialia mexicana) use post-breeding public information to select future nest sites in agricultural landscapes. We further examined effects of breeder age and local habitat characteristics to evaluate whether individual experience and environmental context affected site choice/response to public information, and whether these variables influenced reproductive success. We installed 200 nest boxes across 10 vineyards in Napa Valley, California USA concurrent with final broods of the bluebird breeding season. Boxes in five vineyards were assigned an experimental treatment simulating conspecific presence and reproductive success (old nesting material and playback of adult and nestling vocalizations) or near-treatment (within auditory range of playback), while boxes in five additional vineyards served as unmanipulated controls. Prospecting activity was quantified using video monitoring. Nest box selection, reproductive success, and local habitat features were assessed the following breeding season. Prospecting activity was 8.47 times higher at treatment boxes and 2.77 times higher at near-treatment boxes relative to control boxes. This response was mediated by local habitat features: visitation was higher at boxes closer to sparse tree patches and treatment effects declined with increasing nearby tree cover. In contrast, nest box occupancy showed no effect of playback treatment and had a weakly supported but consistent association with similar habitat variables. Breeder age also did not influence settlement responses to public information, and reproductive success was driven primarily by clutch initiation date rather than age/habitat features. Our results suggest that public information may function as a strong post-breeding recruitment cue but do not directly determine nest site selection in this resident songbird population, while final settlement may rely more heavily on direct habitat assessment.