21st January 2026, Brno
Swallows can remove foreign eggs from their nests even without a “template” of what their own eggs should look like. A new study by researchers from the Institute of Vertebrate Biology of the Czech Academy of Sciences, published in the prestigious journal Biology Letters, offers fresh insights into bird behaviour related to nest hygiene and defence against brood parasitism.
The researchers tested the behaviour of female barn swallows (Hirundo rustica) before egg-laying began. “Several days before the first egg was laid, we placed an artificial egg either mimicking a swallow egg (a so-called mimetic egg) or of a conspicuously different blue colour (a non-mimetic egg) into empty nests and observed the swallows’ reactions. Thanks to long-term research on swallows in the Třeboň region, we were able to compare the behaviour of experienced females that had nested in previous years with those nesting for the first time,” explains Dr. Michal Šulc from the Institute of Vertebrate Biology of the Czech Academy of Sciences.
Recognising one’s own eggs is crucial for many bird species. For example, in species nesting in dense colonies, where nests are only tens of centimetres apart, egg appearance may help parents identify their own clutch. For hosts of brood parasites, this ability is even more important, as foreign eggs can threaten the survival of their own offspring. Until now, however, it was unclear whether birds possess an innate ability to recognise their eggs or acquire it through experience.
Recent experience makes the difference
The results showed that neither first-time breeding females nor experienced females distinguished between the two types of eggs offered. Instead, the difference in behaviour was evident in the overall willingness to remove an egg from the nest, with females having bred recently responding most frequently.
“We found that swallows do not have an innate image of their own eggs, nor do they seem to learn it over time,” explains Dr. Lisandrina Mari, the study’s first author from the Institute of Vertebrate Biology of the Czech Academy of Sciences and the University of Jyväskylä in Finland. “What matters most is recent breeding experience. Swallows in the Czech Republic often breed twice a year, and fresh experience increases their attentiveness to the nest contents.”
According to the authors, awareness of having laid eggs is more important than the appearance of the eggs themselves. If an egg appears in the nest before the female begins laying, it is more likely to be removed, and this response is further strengthened by recent breeding experience.
Nest hygiene supports successful breeding
“Our results suggest that this behaviour is a general nest hygiene response rather than a specialised defence against brood parasites,” says Michal Šulc. “Swallows commonly reuse old nests, in which unhatched eggs from previous breeding attempts may remain. Removing them is important for the successful incubation of new eggs.”
The study also shows that experience gained within a single breeding season has a stronger effect than older experience. Short-term physiological or hormonal changes associated with reproduction are likely to influence behaviour as well. “Females may undergo changes during the season that enhance their ability to keep the nest clean and ready for egg-laying,” adds Šulc.
Overall, the study contributes to a better understanding of how individual experience improves nest care and defence against brood parasitism, and it demonstrates that even seemingly complex behaviours can be governed by relatively simple mechanisms.
Publication: Mari L. et al. (2025): Recent breeding experience improves egg ejection behaviour. Biology Letters. https://doi.org/10.1098/rsbl.2025.0615
