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）预测，装饰特征的表达是个体抵抗寄生虫感染能力的信号。因此，在种群的个体间层面，装饰更为艳丽的个体其寄生虫感染水平理应更低。已有大量研究对该预测展开检验，但结果莫衷一是。导致这些研究结论相悖的原因之一，或许是诸多研究均在个体间层面考察这一关联，而该层面易受选择性死亡等混杂因素的干扰。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分析了雄性装饰特征与鸟类血液寄生虫之间的关联。这两类关联分别聚焦于个体基于遗传的寄生虫抵抗能力差异，以及个体内寄生虫抵抗与装饰特征表达之间的资源分配权衡。我们以雄性普通黄喉地莺（Geothlypis trichas）为研究对象，该物种具有两处羽毛装饰：一处为基于类胡萝卜素的黄色喉胸围脖，另一处为基于黑色素的黑色面部面罩。令人意外的是，在个体内层面，年度间寄生虫感染水平的上升与面罩尺寸增大存在关联，同时黄色羽毛的类胡萝卜素浓度或也有所提升。这表明雄性个体或许能够耐受寄生虫感染水平的上升，同时仍可增强装饰特征的表达。与之相反，个体间层面的装饰特征与寄生虫感染水平并无关联。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之间的关联，此类关联在个体间层面并未显现。我们的研究结果凸显了同时考察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，或许取决于分析的层级（即个体内或个体间层面）。