Our understanding of the proximate and ultimate mechanisms shaping competitive phenotypes primarily stems from research on male-male competition for mates, even though female-female competition is also widespread. Obligate secondary cavity-nesting has evolved repeatedly across avian lineages, providing a useful comparative context to explore how competition over limited nest cavities shapes aggression and its underlying mechanisms across species. Although evidence from one or another cavity-nesting species suggests that territorial aggression is adaptive in both females and males, this has not yet been tested in a comparative framework. We tested the hypothesis that cavity-nesting generates more robust territorial aggression, in comparison to close relatives with less restrictive nesting strategies. Our focal species were two obligate secondary cavity-nesting species and two related species with more flexible nesting strategies in the same avian family: tree swallow (Tachycineta bicolor) vs. barn swallow (Hirundo rustica); Eastern bluebird (Sialia sialis) vs. American robin (Turdus migratorius). We assayed conspecific territorial aggression, and found that cavity-nesting species physically attacked a simulated intruder more often than their close relatives. This pattern held for both females and males. Because territorial aggression is often associated with elevated testosterone, we also hypothesized that cavity-nesting species would exhibit higher testosterone levels in circulation. However, cavity-nesting species did not have higher testosterone in circulation for either sex, despite some correlative evidence that testosterone is associated with higher rates of physical attack in female tree swallows. Our focus on a competitive context that is relevant to both sexes - competition over essential breeding resources - provides a useful comparative framework for co-consideration of proximate and ultimate drivers of reproductive competition in females and males.

中文翻译：

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筑巢策略影响领土侵略而不影响睾丸激素：雌性和雄性鸟类的比较方法

我们对形成竞争表型的最接近和最终机制的理解主要源于对伴侣的男女竞争的研究，即使男女竞争也很普遍。专性次要巢穴在禽类世系中反复进化，提供了有用的比较背景，以探索有限巢穴间的竞争如何形成侵略性及其在物种间的潜在机制。尽管来自一个或另一个洞巢物种的证据表明，领土侵略在雌性和雄性中均具有适应性，但尚未在比较框架中进行过测试。我们测试了这样的假说，即与嵌套限制策略较少的近亲相比，腔嵌套可以产生更强大的领土侵略性。我们的重点物种是在同一禽类中有两个专性次要巢穴物种和两个具有更灵活套巢策略的相关物种：树燕子（Tachycineta bicolor）与燕子燕子（Hirundo Rustica）；东部蓝鸟（Sialia sialis）对美洲知更鸟（Turdus migratorius）。我们分析了同种的领土侵略，发现与其他近亲相比，巢穴物种更容易物理地攻击模拟入侵者。这种模式对女性和男性均适用。因为领土侵略通常与睾丸激素升高有关，所以我们还假设，巢穴物种在循环中会表现出较高的睾丸激素水平。但是，无论哪种性别，巢穴物种的循环睾丸激素含量都不高，尽管有一些相关证据表明，雌性树燕子中睾丸激素与更高的身体攻击率有关。我们关注与性别相关的竞争环境-对基本育种资源的竞争-为共同考虑男女生殖竞争的最终驱动因素提供了有用的比较框架。