Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, C1428EGA, Buenos Aires, Argentina.
Proc Biol Sci. 2012 Sep 7;279(1742):3401-8. doi: 10.1098/rspb.2012.0612. Epub 2012 May 30.
Egg mimicry by obligate avian brood parasites and host rejection of non-mimetic eggs are well-known textbook examples of host-parasite coevolution. By contrast, reciprocal adaptations and counteradaptations beyond the egg stage in brood parasites and their hosts have received less attention. The screaming cowbird (Molothrus rufoaxillaris) is a specialist obligate brood parasite whose fledglings look identical to those of its primary host, the baywing (Agelaioides badius). Such a resemblance has been proposed as an adaptation in response to host discrimination against odd-looking young, but evidence supporting this idea is scarce. Here, we examined this hypothesis by comparing the survival rates of young screaming cowbirds and non-mimetic shiny cowbirds (Molothrus bonariensis) cross-fostered to baywing nests and quantifying the similarity in plumage colour and begging calls between host and cowbird fledglings. Shiny cowbirds suffered higher post-fledging mortality rates (83%) than screaming cowbirds (0%) owing to host rejection. Visual modelling revealed that screaming cowbirds, but not shiny cowbirds, were indistinguishable from host young in plumage colour. Similarly, screaming cowbirds matched baywings' begging calls more closely than shiny cowbirds. Our results strongly support the occurrence of host fledgling mimicry in screaming cowbirds and suggest a role of visual and vocal cues in fledgling discrimination by baywings.
强制性鸟类寄生雏鸟的卵拟态和宿主对非拟态卵的排斥是宿主-寄生虫共同进化的著名教科书范例。相比之下，寄生雏鸟及其宿主在卵期之外的相互适应和反适应受到的关注较少。尖叫牛鹂（Molothrus rufoaxillaris）是一种专门的强制性寄生雏鸟，其幼鸟与主要宿主湾翅牛鹂（Agelaioides badius）的幼鸟完全相同。这种相似性被认为是对宿主对奇怪外观幼鸟的歧视的一种适应，但支持这一观点的证据很少。在这里，我们通过比较寄养在湾翅牛鹂巢中的尖叫牛鹂和非拟态闪亮牛鹂（Molothrus bonariensis）幼鸟的存活率，并量化宿主和牛鹂幼鸟之间的羽毛颜色和乞食叫声的相似性，来检验这一假设。由于宿主的排斥，闪亮牛鹂的离巢后死亡率（83%）高于尖叫牛鹂（0%）。视觉建模表明，尖叫牛鹂在羽毛颜色上与宿主幼鸟无法区分，而闪亮牛鹂则不然。同样，尖叫牛鹂的乞食叫声与湾翅牛鹂的乞食叫声更为相似，而闪亮牛鹂则不然。我们的研究结果强烈支持尖叫牛鹂中存在宿主幼鸟拟态，并表明视觉和声音线索在湾翅牛鹂对幼鸟的识别中起作用。
