Data from: A brood parasite selects for its own egg traits

Many brood parasitic birds lay eggs that mimic their hosts' eggs in appearance. This typically arises from selection from discriminating hosts that reject eggs which differ from their own. However, selection on parasitic eggs may also arise from parasites themselves, since it should pay a laying parasitic female to detect and destroy another parasitic egg previously laid in the same host nest by a different female. In this study, I experimentally test the source of selection on greater honeyguide egg size and shape, which is correlated with that of its several host species, all of which breed in dark holes. Its commonest host species did not discriminate against experimental eggs that differed from their own in size and shape, but laying female honeyguides preferentially punctured experimental eggs more than host or control eggs. This should improve offspring survival given that multiple parasitism by this species is common, and that honeyguide chicks kill all other nest occupants. Hence, selection on egg size in greater honeyguides parasitising bee-eaters is imposed not by host defences, but by interference competition among parasites themselves.

许多巢寄生鸟类（brood parasitic birds）会产下外观与宿主卵高度相似的鸟卵。这一性状的演化通常源于具有卵识别能力的宿主施加的选择压力：宿主会排斥与自身卵形态存在差异的外来卵。然而，寄生卵所承受的选择压力也可能来自寄生者自身：因为对于正在产卵的雌性寄生鸟类而言，检测并摧毁其他雌性寄生者先前在同一宿主巢内产下的寄生卵，将提升自身的繁殖适合度。本研究通过实验探究了大响蜜鴷（greater honeyguide）卵的大小与形状所承受的选择压力来源——该物种的卵与其数种宿主的卵形态高度相关，而这些宿主均营暗巢繁殖。该物种最主要的宿主并不会排斥大小、形状与自身卵存在差异的实验卵，但正在产卵的雌性响蜜鴷对实验卵的刺穿频率显著高于宿主卵或对照卵。鉴于该物种的多重寄生现象十分普遍，且响蜜鴷雏鸟会杀死巢内所有其他栖息者，这一行为将有效提升其后代的存活率。因此，在寄生蜂虎（bee-eaters）的大响蜜鴷种群中，卵大小所承受的选择压力并非来自宿主的防御行为，而是来自寄生者之间的干扰竞争。