Model egg rejection by Eastern Bluebirds

Brood parasitism results in substantial costs to hosts, yet not all species eject foreign eggs. Because the costs of mistakenly ejecting one’s own eggs are high, selection may favor ejection behavior only if it is unlikely a host will incorrectly eject her own eggs. Eastern Bluebirds are currently subject to relatively low levels of interspecific brood parasitism but still sometimes eject parasitic eggs. Therefore, we tested which visual cues they use to eject foreign eggs with the prediction that only the most dissimilar eggs would be ejected, reducing the likelihood of a female’s making a mistake. House Sparrows, which occasionally parasitize bluebirds, lay eggs that have an off-white ground color with brown speckling. Therefore, to test which colors or patterns allow for discrimination of parasitic eggs, we generated 3D-printed model House Sparrow eggs and painted them entirely off-white, entirely brown, half off-white and half brown, or off-white with brown speckling. We then sequen...