Context-dependent response to eggs: egg retrieval versus egg rejection in a conspecific brood parasite

Birds often need to distinguish their own eggs from those of others or from other objects that could be confused with their eggs. Egg recognition occurs in a variety of birds that retrieve eggs displaced from the nest. Egg recognition and rejection is also a particularly widespread defence against brood parasites. We studied egg retrieval and rejection in the American coot, Fulica americana, a species with high levels of conspecific brood parasitism. Previous work revealed that hosts recognize and reject many parasitic eggs. We conducted experiments to determine whether coots also show egg retrieval behaviour and, if they do, whether the same cues trigger retrieval and rejection. If these two responses share the same general cognitive mechanism, a given egg phenotype should elicit the same retrieval and rejection response (with the realization that failure to retrieve is analogous to rejecting eggs). Coots retrieved many eggs and objects placed on their nest rims. All coot eggs were retrieved, including eggs of other conspecific females, and most chicken eggs were also retrieved. The retrieval of a moderate proportion of non-egg-shaped objects like cubes and cylinders shows that an egg shape is not essential for retrieval. Two observations suggest that egg retrieval and rejection are triggered by different cues. The nonretrieval rates of parasitic eggs differed significantly from the corresponding egg rejection rates obtained in an earlier study. Moreover, a moderate fraction of retrieved eggs and objects were subsequently rejected soon after being retrieved. The rejection of the same eggs that were previously retrieved into the nest underscores the remarkable sensitivity of retrieval and rejection decisions to slight changes in an egg's location; a difference of a few centimetres triggers a very different response mechanism. Overall, our findings suggest that selection from brood parasitism has not shaped the evolution of egg retrieval in coots.