Egg shape and color mediate acceptance thresholds in diverse avian host species with different rates of antiparasitic egg rejection

Eggshell recognition in parental birds is vital for nest management, defense against brood parasitism, optimal embryonic development and minimizing disease and predation risks. This process relies on acceptance thresholds balancing the risk of rejecting own eggs against the benefit of excluding foreign ones, following signal detection theory. We investigated the role of object shape in egg rejection decisions among three host species of the obligate brood parasitic brown-headed cowbird (Molothrus ater), each with a varying known response to parasitic eggs. Following previous studies on the American robin (Turdus migratorius; a robust cowbird-egg rejecter), we presented Eastern bluebirds (Sialia sialis; moderate rejecter) and red-winged blackbirds (Agelaius phoeniceus; an accepter) with 3D-printed blue model eggs varying in width or angularity. To examine the shape–color interaction, we also presented Eastern bluebirds with these series in two colors and maculation: light blue (bluebird mimetic) and white with speckles (more cowbird-like). Both American robins and Eastern bluebirds were less likely to accept blue models as their width decreased. For the blue angularity series, acceptance decreased significantly with increased angularity for the red-winged blackbird, as has been previously seen in the American robin, but not for the Eastern bluebird. For bluebirds with the white-maculated models, these patterns remained but statistical significance reversed: acceptance did not decrease significantly with width, but did decrease significantly with angularity. These results suggest that egg shape variation influences antiparasitic egg rejection behaviors, is modulated by shell color and maculation patterns, and varies among different host species, highlighting the complexity of behavioral defense cues against brood parasitism.