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）面临的核心挑战在于：尽管体型更大具备诸多演化优势，动物体型却并未持续增大。在鸟类中，雏鸟体型与繁殖窝雏数（brood size）这两项性状偶有研究证实，会因大型雏鸟以及大窝雏鸟面临更高捕食压力而受到约束。已知寄生虫对鸟类的生活史性状具有强烈影响，但寄生虫是否能够驱动雏鸟体型或窝雏数的稳定选择（stabilizing selection），目前仍未明确。我们针对内布拉斯加州西部的崖燕（Petrochelidon pyrrhonota）开展研究，分析了自然巢群以及通过熏蒸法去除吸血外寄生动物（hematophagous ectoparasites）的巢群中，窝雏数与雏鸟体质量对其第一年存活率的影响模式。来自被寄生巢的崖燕，其第一年存活率在最常见的中等窝雏数与中等雏鸟体质量组别中达到峰值；而来自未被寄生巢的崖燕，存活率则在雏鸟体质量最重的组别中最高，且与窝雏数无显著关联。存活率分析结果显示：在存在寄生虫的条件下，窝雏数与雏鸟体质量受到稳定选择的作用（而在无寄生虫的条件下则未观察到此现象）。尽管具体作用机制尚未明确，但寄生虫显然对崖燕的后代体型与后代数量的中间表型存在选择偏向，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（life-history evolution）中的关键重要性。