Departamento de Ecología, Genética y Evolución and IEGEBA-CONICET, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, , Buenos Aires C1428EGA, Argentina.
Biol Lett. 2013 Mar 13;9(3):20130076. doi: 10.1098/rsbl.2013.0076. Print 2013 Jun 23.
The hosts of brood parasitic birds are under strong selection pressure to recognize and remove foreign eggs from their nests, but parasite eggs may be too large to be grasped whole and too strong to be readily pierced by the host's bill. Such operating constraints on egg removal are proposed to force some hosts to accept parasite eggs, as the costs of deserting parasitized clutches can outweigh the cost of rearing parasites. By fitting microcameras inside nests, we reveal that the Neotropical baywing (Agelaioides badius), a host of the screaming cowbird (Molothrus rufoaxillaris) and shiny cowbird (Molothrus bonariensis), instead circumvents such constraints by kicking parasite eggs out of the nest. To our knowledge, this is the first report of a passerine bird using its feet to remove objects from the nest. Kick-ejection was an all-or-nothing response. Baywings kick-ejected parasite eggs laid before their own first egg and, if heavily parasitized, they ejected entire clutches and began again in the same nest. Few baywings were able to rid their nests of every parasite egg, but their novel ejection method allowed them to reduce the median parasitism intensity by 75 per cent (from four to one cowbird eggs per nest), providing an effective anti-parasite defence.
巢寄生鸟类的宿主面临着强烈的选择压力，需要识别并从巢中移除外来的鸟蛋，但寄生虫的鸟蛋可能太大而无法整个抓住，也可能太硬而难以被宿主的喙刺穿。这种对移除鸟蛋的操作限制被认为是迫使一些宿主接受寄生虫鸟蛋的原因，因为放弃寄生的鸟窝的成本可能超过了饲养寄生虫的成本。通过在巢内安装微型摄像机，我们发现新热带凤头鹃（Agelaioides badius），一种尖叫牛鹂（Molothrus rufoaxillaris）和闪亮牛鹂（Molothrus bonariensis）的宿主，通过将寄生虫鸟蛋踢出巢外来规避这种限制。据我们所知，这是第一份关于雀形目鸟类用脚将巢内物体踢出的报告。踢蛋是一种全有或全无的反应。凤头鹃会将自己的第一枚蛋之前产下的寄生虫蛋踢出去，如果被严重寄生，它们会将整个鸟窝一起踢出去，并在同一个巢里重新开始。很少有凤头鹃能够将每一个寄生虫蛋从巢中清除，但它们新颖的排泄方法使它们将寄生虫的平均寄生强度降低了 75%（从每个巢中的四个寄生虫蛋减少到一个），提供了一种有效的抗寄生虫防御。
