Uropygial gland size increases isometrically with body size in 35 North American bird species

The uropygial gland and the oils it produces have been shown to serve important functions in many bird species including plumage maintenance and chemical communication. Previous species comparisons of uropygial gland size have only been conducted in South American and European birds and have found little to no phylogenetic signal. Here, we document uropygial gland measurements of 34 different passerines, 1 hybrid chickadee population, and 1 piciform in eastern Pennsylvania, most of which are reported for the first time. Uropygial gland size is positively related to overall body size, controlling for phylogenetic relatedness. We show a significant phylogenetic signal of relative uropygial gland size and many of the species with the largest gland sizes belong to the Paridae. Overall, females show nonsignificant but slight trend of larger relative uropygial gland sizes than males. This effect is stronger in some species than others and can also be seasonally dependent. We found no effect of total eggshell surface area or nest location on relative uropygial gland size. Together our results provide the first documentation of uropygial gland sizes in many of these North American species and provide interesting insight into factors influencing relative gland size including sex, season, and species.

尾脂腺（uropygial gland）及其分泌的油脂已被证实于众多鸟类类群中发挥重要功能，涵盖羽毛维护与化学通讯两大方面。此前针对尾脂腺大小的跨物种比较研究仅覆盖南美与欧洲鸟类，且几乎未检出系统发育信号。本研究对宾夕法尼亚州东部的34种雀形目鸟类、1个山雀杂交种群以及1种鴷形目鸟类的尾脂腺进行了测量，其中多数类群的相关数据尚属首次报道。在控制系统发育亲缘关系的前提下，尾脂腺大小与鸟类整体体型呈正相关。本研究证实相对尾脂腺大小存在显著的系统发育信号，且诸多尾脂腺相对尺寸最大的物种均隶属于山雀科。整体而言，雌性个体的相对尾脂腺尺寸虽未达到显著水平，但呈现出略大于雄性个体的趋势。该性别差异效应在部分物种中更为显著，且还会受到季节因素的影响。本研究未发现总蛋壳表面积或营巢位置对相对尾脂腺大小存在显著影响。综上，本研究首次报道了诸多北美鸟类类群的尾脂腺大小数据，并为影响相对尾脂腺大小的相关因素（包括性别、季节及物种本身）提供了富有价值的见解。