North American barn swallows pair, mate, and interact assortatively

While female mate choice is well established, mutual choice may play a larger role in mate selection than currently recognized. Assortative mating is a common form of non-random mating in animals that can result from mutual choice. However, few studies address assortative patterns beyond the social pair, potentially overlooking assortativity in the mating pair and in the social environment that shapes reproductive decisions. We asked whether North American barn swallows (Hirundo rustica erythrogaster) breeding in a large colony form pairs, mate (through both within-pair and extra-pair fertilizations), and interact assortatively by ventral plumage color, wing length, and age. Social interactions were tracked using proximity loggers, which recorded close contact between tagged individuals when birds were mating and laying eggs. Barn swallows paired and mated assortatively by their ventral plumage color; however, the assortative patterns in mating pairs were not as strong as they were in social pairs. Barn swallows also interacted assortatively, associating more often with individuals of both sexes who had similar phenotypes relative to the other birds in the colony. Finally, older males and females with darker ventral plumage achieved the highest reproductive success. Investigation of assortative behavior beyond the level of the social pair provides a more complete understanding of mate choice and suggests a mechanism that may maintain the large variation in ventral plumage color in North American barn swallows.

尽管雌性配偶选择已得到广泛证实，但相互选择在配偶选择中所发挥的作用可能远超当前认知。选型交配（Assortative Mating）是动物界一类常见的非随机交配模式，其可源于相互选择。然而，当前鲜有研究关注社会配对层级之外的选型交配模式，这可能会遗漏交配配对以及影响繁殖决策的社会环境中的选型关联性（Assortativity）。本研究旨在探究大种群繁殖的北美家燕（*Hirundo rustica erythrogaster*）是否会基于腹部羽色、翅长与年龄形成配对、进行交配（包括配对内受精与配对外受精），并发生选型社交互动。本研究借助近距离记录器（proximity logger）追踪社交互动，该设备可记录家燕在交配与产卵期间，佩戴标记的个体间的近距离接触情况。研究结果显示，北美家燕会基于腹部羽色进行选型配对与交配；但交配配对中的选型模式强度显著弱于社会配对。此外，北美家燕同样存在选型社交行为：相较于种群内其他个体，它们更频繁地与表型相似的雌雄个体建立社交关联。最终，年龄更大且腹部羽色更深的雌雄个体获得了最高的繁殖成功率。对社会配对层级之外的选型行为开展研究，能够更全面地阐释配偶选择机制，并提出了一种可维持北美家燕腹部羽色显著变异的潜在作用路径。