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 sequentially placed these four different model eggs in the nests of Eastern Bluebirds, with each nest receiving all treatments over the course of four days. After watching females enter and leave the nest box just one time after placement of the model egg, we found that speckled eggs were ejected half the time (7 of 14 nests), while no other treatment was ejected more than three times. Thus, Eastern Bluebird females eject eggs based primarily on color patterning (i.e., a speckled pattern) rather than coloration per se, and that they can do so very quickly, as the average female had removed the model egg within six minutes of entering the nest. Because Eastern Bluebirds do not lay speckled eggs, but some brood parasites (e.g., House Sparrows, Brown-headed Cowbirds) do, selection may specifically favor ejection of eggs with a speckled pattern, not just eggs that have within-egg color contrasts.

Collected by observation (behavior) and Photoshop (eggshell color quantifcation). 

Processed in EXCEL and SAS.

Labels in dataset.