Spottiswoode Claire N
Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK.
Oecologia. 2007 Dec;154(3):589-600. doi: 10.1007/s00442-007-0844-6. Epub 2007 Sep 9.
Colony sizes in birds can vary by orders of magnitude within species, and many studies have shown that selection pressures differ dramatically among small and large colonies. Does such selection result in phenotypic sorting at the level of individuals? This study describes inter-colony differences in morphology and reproductive investment in a population of a highly colonial, communal and sedentary African passerine bird, the sociable weaver Philetairus socius. Relative colony sizes were fairly stable over a 10-year period. Adults differed among colonies in terms of bill morphology, condition, body size and degree of ectoparasite infestation, and the last two declined consistently with colony size. In larger colonies, smaller eggs were laid, and nestlings were more parasite-infested, showed weaker cell-mediated immune responses, and experienced higher levels of brood reduction and snake predation. Taken together with another study showing that adult survival is higher in larger colonies, these results suggest that patterns of age-specific mortality are consistently related to colony size in the sociable weaver. Based on these observations I suggest two hypotheses that might account for the observed phenotypic sorting, involving colony size-dependent patterns in (1) density-dependent competition for food and (2) adaptive life-history adjustment.
鸟类群体的规模在物种内部可能相差几个数量级，许多研究表明，大小不同的群体所面临的选择压力存在显著差异。这种选择是否会在个体层面导致表型分选呢？本研究描述了一种高度群居、集群营巢且定居的非洲雀形目鸟类——群居织雀（Philetairus socius）群体间在形态和繁殖投入方面的差异。在10年的时间里，相对群体规模相当稳定。不同群体中的成年个体在喙的形态、身体状况、体型以及体外寄生虫感染程度方面存在差异，且后两者随群体规模的增大而持续下降。在较大的群体中，所产的卵较小，雏鸟感染寄生虫的情况更严重，细胞介导的免疫反应较弱，并且育雏减少和蛇类捕食的水平更高。结合另一项表明较大群体中成年个体存活率更高的研究，这些结果表明，群居织雀中特定年龄死亡率的模式与群体规模始终相关。基于这些观察结果，我提出了两个可能解释所观察到的表型分选的假设，涉及（1）食物的密度依赖性竞争和（2）适应性生活史调整中与群体规模相关的模式。
