Barn Swallows With Recent Breeding Experience Were More Likely to Remove Clearly Different Eggs

January 21, 2026

Barn swallow feeding three chicks in a nest at Springe, Germany

Research · Behaviour

Female barn swallows (Hirundo rustica) with recent breeding experience were the most likely to eject artificial eggs from their nests. The experiment tested whether the birds recognise foreign eggs by comparing them with an innate or learned visual image of their own eggs.

Across four breeding seasons from 2021 to 2024 on four farms in southern Bohemia, Czech Republic, the researchers conducted 187 experiments before females began laying. They placed either a mimetic artificial egg or a clearly different blue non-mimetic egg in the nest, allowing egg recognition to be tested before a first-time breeder had seen her own eggs. Individually marked birds made it possible to compare first-time breeders with experienced females; experienced birds were further separated into those whose previous breeding attempt was in an earlier year and those that had already laid a clutch during the same season.

Egg appearance did not explain rejection

Neither naive nor experienced females removed the non-mimetic eggs more often than the mimetic eggs. This result did not support the idea that barn swallows use a fixed visual template of their own eggs, or that such a template is learned through earlier breeding.

Recent breeding experience mattered most

Females with recent experience from an earlier breeding attempt in the same season were the most likely to eject the artificial eggs, followed by females with experience from a previous year and then first-time breeders. The difference between recently experienced and first-time females was weak when both egg types were considered together, but clear for the blue non-mimetic eggs. The study found no clear difference between recent and older experience, or between older experience and first-time breeders.

The authors therefore propose that egg ejection may depend more on a mechanism linked to the onset of a female’s own laying than on recognition of a particular egg colour or pattern. Such a mechanism could support nest sanitation by helping females remove old, unhatched eggs from reused nests. They discuss endocrine and cognitive changes as possible explanations for why recent breeding experience strengthens the response, but these were not measured in the experiment.

Sources and image credit

Mari, L., Hughes, A. E., Troscianko, J., Tomášek, O., Albrecht, T., Jelínek, V., & Šulc, M. (2026). Recent breeding experience improves egg ejection behaviour. Biology Letters, 22(1), 20250615.Primary peer-reviewed article published 21 January 2026 under CC BY 4.0. The study used 187 pre-laying egg-insertion experiments in an individually marked barn-swallow population in southern Bohemia. Recent-experience females showed the highest ejection tendency; evidence for recent versus naive females was weak overall but supported for non-mimetic eggs. Neither naive nor experienced females preferentially rejected non-mimetic over mimetic eggs, arguing against a fixed or learned visual template.

Image: © Michael Gäbler / Wikimedia Commons / CC BY-SA 3.0 — barn swallow (Hirundo rustica) feeding chicks at Wisentgehege Springe near Hanover, Germany, on 26 August 2015. Illustrative photograph, not from the study. · https://creativecommons.org/licenses/by-sa/3.0/ · Source