Data from: The relationship between blood parasites and ornamentation depends on the level of analysis in the common yellowthroat

The Hamilton-Zuk hypothesis predicts that ornament expression is a signal of the ability of individuals to resist parasite infection. Thus, across a population (i.e., between-individuals) more ornamented individuals should have lower levels of parasitism. Numerous studies have tested this prediction and the results are mixed. One reason for these conflicting results may be that many studies have examined this relationship at the between-individual level, which may be affected by confounding factors such as selective mortality. Using within-subject centering we examined the relationship between male ornamentation and avian blood parasites at both the between- and the within-individual level. These relationships focus on differences in genetically-based resistance to parasites and the trade-off in resource allocation between parasite resistance and ornament expression within an individual, respectively. We studied male common yellowthroats (Geothlypis trichas), which have two plumage ornaments, a yellow, carotenoid-based bib (throat and chest) and a black, melanin-based facial mask. Surprisingly, within-individuals, an increase in parasitism between years was associated with an increase in mask size and, potentially, greater concentration of carotenoids in the yellow feathers. This suggests that males may be able to tolerate an increase in parasitism and still increase ornament expression. In contrast, ornamentation was not related to parasitism at the between-individual level. Thus, our study revealed relationships between ornaments and parasitism at the within-individual level that were not present at the between-individual level. Our results highlight the importance of examining both within- and between-individual relationships as correlations between variables, such as ornaments and parasites, may depend on the level of analysis (i.e., within- or between- individuals).

汉密尔顿-祖克假说（Hamilton-Zuk hypothesis）提出，装饰性特征的表达是个体抵御寄生虫感染能力的信号。因此，在种群层面（即个体间层面），装饰更华丽的个体其寄生虫感染水平应当更低。大量研究已对这一预测展开验证，但结果莫衷一是。造成这些相悖结果的原因之一可能是，诸多研究均在个体间层面检验了该关联，而这一分析可能受到选择性死亡等混杂因素的影响。本研究采用个体内中心化（within-subject centering）方法，分别在个体间与个体内层面检验了雄性鸟类装饰特征与血液寄生虫的关联。这两种关联分别聚焦于基于遗传的寄生虫抗性差异，以及个体内寄生虫抗性与装饰特征表达之间的资源分配权衡。我们以普通黄喉鹀（Geothlypis trichas）为研究对象，该物种拥有两处羽饰特征：基于类胡萝卜素（carotenoid）的黄色喉胸围脖，以及基于黑色素（melanin）的黑色面部面罩。令人意外的是，在个体内层面，年度间寄生虫感染水平的提升与面罩尺寸的增大相关，同时可能伴随黄色羽毛中类胡萝卜素浓度的升高。这表明雄性个体或许能够耐受寄生虫感染水平的上升，同时仍能提升装饰特征的表达。相反，在个体间层面，装饰特征与寄生虫感染并无关联。因此，本研究揭示了仅存在于个体内层面的装饰特征与寄生虫感染的关联，而该关联在个体间层面并不存在。本研究结果凸显了同时检验个体内与个体间关联的重要性——诸如装饰特征与寄生虫感染这类变量间的相关性，可能取决于分析的层面（即个体内或个体间层面）。