Female starlings eject conspecific parasitic eggs according to egg size

In some bird species, reproduction depends on finding a suitable tree cavity to build a nest. Individuals that fail to obtain this scarce resource must resort to alternative strategies to reproduce. Males, for example, may attempt to mate with females that do have a nest, while females may resort to parasitism—laying their eggs in the nests of other females of the same species. However, raising chicks is costly, and therefore, we expect a selective pressure for breeding females to distinguish their own eggs from those of parasitic females. Nevertheless, it is unclear whether this is always possible, and making a mistake could be a fatal error if a female destroys her own clutch.

In the case of the spotless starling, the severe limitation of nesting sites leads many females to attempt parasitism. In the population we studied in the Sierra de Madrid, nearly 20% of nests are parasitized by females of the same species. Parasitic eggs can often be identified by differences in size or colour, or they may be detected when two new eggs appear in a nest on the same day. Additionally, we have frequently observed fallen eggs beneath the nests, usually broken.

In this study, which is part of Alaïs Lienard PhD thesis, supervised by Raquel Monclús (Université Sorbonne Paris Nord, France), we asked whether these fallen eggs resulted from a parasitic female swapping her own egg for a legitimate one or, conversely, from a breeding female removing a parasitic egg. To answer this question, we conducted genetic analyses.

The results were conclusive: the eggs found beneath the nests did not belong to the legitimate females that bred in those nests. Most likely, they were parasitic eggs that had been expelled. This suggests that some females are indeed capable of distinguishing their own eggs from parasitic ones. But how do they do it?

We conducted an experiment to determine whether females could recognize subtle differences in egg size among different females. To do this, we 3D-printed dummy eggs, slightly varying their size, and placed these fake eggs in starling nests. We found that one in five fake eggs was rejected, with rejection being more frequent when the dummy was smaller than the legitimate egg. In other words, females avoided expelling dummy eggs when they were large.

This pattern may be explained if the cost of making a mistake is worse when the egg is large than when it is small. However, we cannot rule out the possibility that handling and expelling a larger egg is simply more difficult for the females.

This study clearly illustrates the trade-off that females of this species face in optimizing their reproduction—balancing the need to avoid parasitism while minimizing costs to their own reproductive success.