Recent breeding experience improves egg ejection behaviour

Recognizing one's own eggs is crucial for birds, especially for hosts of brood parasites that must identify and reject different-looking parasitic eggs. While birds seem to possess a 'template image' of their eggs, whether it is innate or refined over time remains unclear. We addressed this question by experimentally inserting either artificial mimetic eggs (ME) or non-mimetic eggs (NME) into the nests of barn swallows (Hirundo rustica) during the pre-laying stage. We harnessed the potential of our individually marked population to perform a unique comparison between naive first-time breeders and experienced females, as well as females with 'old' (from a previous season) and 'recent' (from the previous breeding attempt within the same season) experience, allowing us to investigate the role of memory. We found that neither naive nor experienced females ejected NME more often than ME, suggesting that the template image does not play a primary role in egg recognition. Instead, awareness of own egg-laying might be the crucial mechanism at play, facilitating nest sanitation behaviour. Lastly, we provide the first evidence that this mechanism improves with recent breeding experience.