Hosts eject conspecific parasitic eggs according to the egg size in a passerine.

Conspecific brood parasitism (CBP) is an alternative reproductive tactic in egg-laying species, where a female lays her eggs in the nest of a conspecific. In a population of spotless starlings (Sturnus unicolor), we observed that some eggs are ejected from the nest during CBP events. It is unclear whether this ejection is a parasitic strategy (a host egg is ejected) or an anti-parasitism strategy (the parasitic egg is ejected). To clarify this, we genotyped the eggs ejected on the ground and found that 100% of them were parasitic. Egg discrimination might be based on tactile or visual cues, and we hypothesized that egg size could be used by hosts to eject parasitic eggs. We conducted experiments in the field using dummy eggs of varying sizes. The results showed that starlings were more likely to eject eggs if they were smaller than their own eggs. In contrast, no significant pattern of egg ejection was observed for larger eggs. Our results suggest that starlings use egg volume recognition as an anti-parasitism strategy to avoid the costs of parasitism. Whether this is a frequency-dependent strategy is worth further studies.