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.诸多巢寄生鸟类（brood parasitic birds）所产的卵会在外观上模拟其宿主的卵。该现象通常源于具有卵识别能力的宿主所施加的选择压力：这类宿主会排斥与自身卵存在差异的外来卵。然而，寄生卵所承受的选择压力也可能来自寄生者自身——由于若同一宿主巢内已有其他雌性寄生者产卵，当前产卵的雌性寄生者通过检测并破坏这些外来寄生卵，将获得演化收益。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：这类卵的大小与形状与其数种宿主物种的卵高度相关，而这些宿主均在黑暗洞穴中筑巢繁殖。该响蜜鴷的最主要宿主物种并不会排斥在大小与形状上存在差异的实验卵，但产卵的雌性响蜜鴷对实验卵的穿刺优先级显著高于宿主卵与对照卵。鉴于该物种的多重寄生现象极为普遍，且响蜜鴷雏鸟会杀死巢内所有其他同住者，这一行为可提升后代的存活概率。因此，在寄生于食蜂鸟的大响蜜鴷种群中，卵大小所承受的选择压力并非来自宿主的防御行为，而是源于寄生者之间的干扰竞争（interference competition）。