ABSTRACT In all animals, susceptibility to parasites can differ among individuals. Young, nest-bound birds are exposed to a diversity of nest-dwelling ectoparasites that typically feed on their blood. Within broods, hatching asynchrony creates size hierarchies that result in morphological and physiological variation among nest mates, and susceptibility to parasites also may vary predictably with this size hierarchy. Our objective was to use a broad-spectrum, anti-parasite drug, ivermectin (IVM), to treat individual nestling Tree Swallows (Tachycineta bicolor) and assess how nestling susceptibility to parasites varied both within and among broods. Broods were either assigned to an IVM group, where half of the nestlings in a brood received IVM injections and half received control injections of pure sesame oil, or to a control group, where all nestlings received oil injections. We found that the IVM treatment reduced parasite loads for broods as a whole, thereby benefiting all nestlings in IVM broods and suggesting our treatment resulted in herd immunity. Specifically, nestlings from IVM broods had higher hemoglobin concentrations, regardless of whether they received injections with IVM or oil, and greater fledging success, than nestlings from control broods. On the contrary, IVM treatment did not strongly affect nestling morphology, with only marginal effects on the growth rate of ninth primary feathers, and the effects of the treatment on 2 other morphological traits depending on temporal factors. Variation in size within broods, however, influenced the chance of an individual fledging, which increased with relative size within a brood, but only under lower parasite loads (i.e. IVM broods). By experimentally manipulating nestling susceptibility to parasites, we have demonstrated variation in nestling response to an anti-parasite treatment both within and among broods, and future studies should investigate the underlying mechanism for why certain nestlings along the brood size hierarchy are more susceptible to parasites.

中文翻译：

---

抗寄生虫治疗的益处受树燕 (Tachycineta bicolor) 内育雏大小变化的影响

摘要 在所有动物中，对寄生虫的易感性因人而异。幼鸟在巢中会接触到多种栖息在巢中的外寄生虫，这些外寄生虫通常以它们的血液为食。在育雏内，孵化不同步会产生大小等级，导致巢伴侣之间的形态和生理变化，并且对寄生虫的易感性也可能随着这种大小等级而发生可预测的变化。我们的目标是使用广谱抗寄生虫药物伊维菌素 (IVM) 来治疗个体雏鸟树燕 (Tachycineta bicolor)，并评估雏鸟对寄生虫的易感性如何在育雏内和育雏间变化。雏鸟要​​么被分配到 IVM 组，其中一半的雏鸟接受 IVM 注射，另一半接受纯芝麻油的对照注射，或者被分配到一个对照组，所有雏鸟都接受了注油。我们发现 IVM 处理总体上减少了幼鸟的寄生虫负荷，从而使 IVM 幼鸟中的所有雏鸟受益，并表明我们的处理导致了群体免疫。具体而言，与来自对照雏鸟的雏鸟相比，来自 IVM 雏鸟的血红蛋白浓度更高，无论它们是否接受 IVM 或油注射，并且羽翼丰满的成功率更高。相反，IVM 处理对雏鸟形态没有强烈影响，对第九初级羽毛的生长速度只有边际影响，处理对其他 2 个形态特征的影响取决于时间因素。然而，巢内大小的变化影响了个体羽化的机会，随着巢内的相对大小而增加，但仅在较低的寄生虫负载下（即 IVM 育雏）。通过实验操纵雏鸟对寄生虫的易感性，我们已经证明雏鸟对育雏内和育雏间抗寄生虫治疗的反应存在差异，未来的研究应该调查为什么沿着育雏大小等级的某些雏鸟更容易感染寄生虫的潜在机制。