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）的核心挑战之一，便是阐释在体型更大具备诸多优势的前提下，动物体型为何未能持续增大。在鸟类中，雏鸟（nestling）体型与产雏量（窝雏数，brood size）偶有研究显示会因大型雏鸟及大窝雏群遭遇的捕食压力升高而受到约束。已知寄生虫对鸟类的生活史性状具有显著调控作用，但寄生虫是否可作为驱动雏鸟体型或窝雏数经历稳定选择（stabilizing selection）的因素，目前仍未明确。本研究以美国内布拉斯加州西部的崖燕（Petrochelidon pyrrhonota）为研究对象，探究了自然巢群与经熏蒸（fumigation）去除吸血外寄生虫（hematophagous ectoparasite）的巢群中，窝雏数与雏鸟体质量对其当年存活率的影响模式。结果显示，来自受寄生虫侵染巢群的崖燕，在最常见的中等窝雏数与中等雏鸟体质量组别中当年存活率最高；而来自未侵染巢群的崖燕，则在雏鸟体质量最大的组别中存活率最高，且其存活率与窝雏数无显著关联。存活率分析结果表明，在存在寄生虫的情况下，窝雏数与雏鸟体质量均受到稳定选择的作用，而在无寄生虫的情况下则无此现象。尽管具体调控机制尚不明确，但寄生虫显然对崖燕的中等体型后代与中等产雏量具有选择偏向性，并塑造了当前观测到的这些性状的分布格局。本研究结果凸显了寄生虫在生活史演化进程中的重要性。