Data from: Parasites favor intermediate nestling masses and brood sizes in cliff swallows

A challenge of life-history theory is to explain why animal body size does not continue to increase, given various advantages of larger size. In birds, body size of nestlings and the number of nestlings produced (brood size) have occasionally been shown to be constrained by higher predation on larger nestlings and those from larger broods. Parasites also are known to have strong effects on life-history traits in birds, but whether parasitism can be a driver for stabilizing selection on nestling body size or brood size is unknown. We studied patterns of first-year survival in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska in relation to brood size and nestling body mass in nests under natural conditions and in those in which hematophagous ectoparasites had been removed by fumigation. Birds from parasitized nests showed highest first-year survival at the most common, intermediate brood-size and nestling-mass categories, but cliff swallows from non-parasitized nests had highest survival at the heaviest nestling masses and no relationship with brood size. A survival analysis suggested stabilizing selection on brood size and nestling mass in the presence (but not in the absence) of parasites. Parasites apparently favor intermediate offspring size and number in cliff swallows and produce the observed distributions of these traits, although the mechanisms are unclear. Our results emphasize the importance of parasites in life-history evolution.生活史理论（life-history theory）面临的一项核心挑战在于：尽管体型更大存在诸多优势，为何动物的体型并未持续增大。在鸟类中，雏鸟体型与每巢雏鸟数量（brood size）偶尔被证实会因大型雏鸟及大窝雏鸟面临更高的捕食压力而受到限制。已知寄生物对鸟类的生活史性状具有显著调控作用，但寄生作用是否能成为驱动雏鸟体型或窝雏数经历稳定选择（stabilizing selection）的因素，目前仍不明确。本研究以美国内布拉斯加州西部的崖燕（Petrochelidon pyrrhonota）为研究对象，分别在自然状态巢以及通过熏蒸处理（fumigation）移除吸血体外寄生虫（hematophagous ectoparasites）的巢中，探究了窝雏数与雏鸟体重对其当年存活率的影响模式。来自寄生巢的崖燕，在最常见的中等窝雏数与雏鸟体重类别中当年存活率最高；而来自未寄生巢的崖燕，则在雏鸟体重最重的组别中存活率最高，且存活率与窝雏数无显著关联。存活率分析结果显示，在存在寄生物的情况下，窝雏数与雏鸟体重受到了稳定选择，而在无寄生物时则未出现该现象。尽管具体调控机制尚不明确，但寄生物显然会促使崖燕的后代体型与数量趋于中等，并塑造了当前观测到的这些性状分布格局。本研究结果凸显了寄生物在生活史演化中的重要作用。