Eggshell texture but not odor treatment affects model egg rejection in American robins (Turdus migratorius)

To curb fitness costs associated with obligate avian brood parasitism, some hosts have evolved to reject foreign eggs in the nest. American robins (Turdus migratorius) are among the few hosts of the brown-headed cowbird (Molothrus ater) that mostly remove parasitic eggs from their nests. With the parasite's eggs looking nothing like their own, American robins likely rely most on visual cues when making rejection decisions. However, we still know little about the roles that tactile and olfactory cues play in robin's or other rejecter hosts' rejection decisions. Here, we conducted a set of experiments to test for the use of tactile or olfactory cues in egg rejection by robins. For the tactile experiment, we found that robins were more likely to reject rough rather than smooth eggs. However, our tactile model egg design was not able to fully discriminate between tactile and visual sensory modalities. In the olfaction experiment, we did not find a significant effect of egg scent treatment on rejection rates. Accordingly, future studies on egg rejection should attempt to fully distinguish between tactile and visual cues, as well as examine olfactory cues in other egg rejecter host species.