Direct evidence of infanticide is still scarce, which prevents understanding the evolutionary and ecological importance of infanticide. In Barn Swallows (Hirundo rustica) all direct evidence of infanticide has been gained from nestlings a few (≤4) days after hatching and mostly from colonial breeders. Here, we report the infanticide of a 17-day-old nestling by a mated male Barn Swallow in a sparse Japanese population, demonstrating that infanticide is not confined only to small nestlings. In sparse populations where preferable nest sites are limited, selection may favor infanticide to occupy good nest sites even if it entails substantial handling costs.

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