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Longer-Tailed Female Barn Swallows Were More Likely to Have Extra-Pair Mates

Female North American barn swallow gathering nest material in Washington State
Research · Behaviour

Female North American barn swallows (Hirundo rustica erythrogaster) with longer tails were more likely to mate with males other than their social partner. In a study of 47 social pairs, traits of females were more strongly associated with extra-pair fertilizations than traits of males.

Barn swallows are socially monogamous, but some offspring in a nest may be sired by a male other than the female’s social mate. The researchers examined whether these extra-pair fertilizations were associated with plumage traits and with the reproductive results of both members of the social pair.

Longer-tailed females were more likely to have extra-pair mates

Females that engaged in extra-pair mating tended to fledge more offspring during the breeding season. Females with longer tails also tended to have offspring sired by a greater number of males. Across the analyses, female traits were more informative than male traits when predicting extra-pair outcomes for both females and males.

Female throat colour was also associated with the extra-pair fertilization success of her social partner. Males paired with darker-throated females had more extra-pair offspring and sired offspring with more females outside the social pair, while the males’ own measured plumage traits did not significantly predict their extra-pair success.

The pattern was not explained by females laying more eggs

The pattern was not explained by females simply laying more eggs. Female plumage traits were not correlated with fecundity, measured as the total number of eggs laid, and fecundity itself was not associated with any measure of extra-pair fertilization. Fecundity did, however, correlate positively with the total number of offspring a female fledged, showing that egg production still contributed to annual reproductive success through a separate pathway.

The authors interpret the findings as evidence that extra-pair fertilizations may contribute to sexual selection on female plumage in this population. The study does not establish that longer tails themselves cause females to have more genetic sires. Tail length can increase with age, and exact age was unknown for about one-third of the sampled birds; among females of known age, older birds had longer tails. The authors therefore note that age and breeding experience, male preferences or other linked traits could contribute to the observed pattern.

Sources and image credit

  1. Kenny-Duddela, H. V., Schield, D. R., Laubach, Z. M., Levin, I. I., Keller, K. P., & Safran, R. J. (2026). Extra-pair fertilizations are more strongly associated with female traits than male traits or fecundity in Hirundo rustica erythrogaster (North American Barn Swallow). Ornithology, 143(3), 1–15. https://doi.org/10.1093/ornithology/ukag013Primary peer-reviewed study published 15 March 2026. In 47 social pairs, female traits—especially tail length—were more strongly associated with extra-pair fertilization outcomes than male traits or fecundity.

Image: VJAnderson — female North American barn swallow (Hirundo rustica erythrogaster) gathering nest material in western Washington State, USA, photographed 18 June 2019; Wikimedia Commons; representative female of the same subspecies, not a bird from the study; cropped to 16:9 · CC BY-SA 4.0 · Source

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