Data from: 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),
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.