Examining causes and characteristics of egg ejection behaviour in the American robin (Turdus migratorius)

American robins (Turdus migratorius) are among few hosts of parasitic brown-headed cowbirds (Molothrus ater) that frequently eject foreign eggs from their nests. However, we know little about some characteristics of the robins’ egg ejection process including its distance and direction. We used a novel technique (i.e., radio transmitters inserted into 3D-printed eggshells) to examine egg rejection in female robins as a function of model egg coloration (i.e., robin-mimetic blue, non-mimetic deep blue, and non-mimetic beige). Based on prior work, we predicted that female robins’ egg ejection decisions would be dependent on egg coloration. Accordingly, we found a significant effect of female identity (using nest identity as a proxy) and model egg colour on egg ejection, but neither predicted ejection distance or direction. Deep blue model eggs had shorter ejection latencies than beige eggs. Fully characterizing the egg ejection process will allow us to further understand the mechanisms and outcomes of egg ejection behaviours in host-parasite interactions.