Associations between parasite loads and sexual signal expression have long been the focus of research. However, our understanding of how sexual selection operates in the context of multiple parasite infections within the same host is still quite limited. We examined the expression of plumage coloration, which is sexually selected in North American barn swallows, Hirundo rustica erythrogaster, and tail streamer length, which is sexually selected in other barn swallow subspecies, in the context of eight different parasite infections. We found that two parasites, nest mites and Plasmodium, were associated with colour expression, but in opposite directions. Attractive males were less likely to have mites in their nests, but more likely to be infected with Plasmodium. We found that different underlying mechanisms were generating the links between these different parasites and the same colour signal. Males that invested more in colour expression were less likely to survive when infected with Plasmodium, suggesting a physiological mechanism linking colour to Plasmodium infections. However, we found no clear cost to males when examining nest mites, which primarily infect offspring. Instead, using experimental mite manipulations, we provide evidence for a behavioural mechanism, where territorial behaviour and male–male competition likely drive the association between colour expression and nest mite infections. We did not find associations between parasites and streamer length in this subspecies, suggesting that the links between divergent sexual signals and parasites may be quite flexible. These results suggest that the evolution of a single sexual signal can be shaped by several, even opposing, links with different parasite infections.

长期以来，寄生虫负荷与性信号表达之间的关联一直是研究的重点。然而，我们对于在同一宿主内发生多种寄生虫感染的情况下如何进行性选择的理解仍然十分有限。在八种不同的寄生虫感染的背景下，我们检查了羽毛着色的表达，该着色是在北美家燕，Hirundo Rusta erythrogaster中进行性选择的，以及在其他家燕亚种中进行性选择的尾stream长度。我们发现两种寄生虫，巢螨和疟原虫与颜色表达相关，但方向相反。有吸引力的雄性鸟巢中的螨虫可能性较小，但更容易感染疟原虫。我们发现，不同的潜在机制在这些不同的寄生虫和相同的颜色信号之间产生了联系。在色彩表现上投资更多的男性感染疟原虫后存活的可能性较小，这表明了将色彩与疟原虫联系起来的生理机制。感染。但是，当我们检查巢螨时，我们发现雄性没有明显的成本，巢螨主要感染后代。取而代之的是，使用实验性的螨虫操作，我们提供了一种行为机制的证据，在这种行为机制中，地域行为和雄性-雄性竞争可能驱动颜色表达与巢螨感染之间的关联。在该亚种中，我们未发现寄生虫与流光长度之间的关联，这表明不同性信号与寄生虫之间的联系可能非常灵活。这些结果表明，单个性信号的进化可以通过与不同寄生虫感染的几个甚至相反的链接来塑造。