Department of Biology, George Mason University, Fairfax, VA 22030, USA.
Department of Biology, Davidson College, Davidson, NC 28035, USA.
J Exp Biol. 2023 Jul 1;226(14). doi: 10.1242/jeb.245188. Epub 2023 Jul 28.
Hosts of obligate avian brood parasites often evolve defense mechanisms to avoid rearing unrelated young. One common defense is egg rejection, for which hosts often rely on eggshell color. Most research has assumed that hosts respond to perceived color differences between their own eggs and parasite eggs regardless of the particular color; however, recent experiments have found that many hosts respond more strongly to brown foreign eggs than to equally dissimilar blue eggs. Yet, none of these prior studies tested a brown-egg-laying species and, with only one exception, all were conducted in open nests where light levels are considered sufficient for effective color-based egg discrimination. Here, we explored how two cavity-nesting hosts of the parasitic brown-headed cowbird (Molothrus ater) - the blue-egg-laying eastern bluebird (Sialia sialis) and the brown-egg-laying house wren (Troglodytes aedon) - respond to experimental eggs painted six distinct colors ranging from blue to brown. Rejection responses of both hosts were best predicted by perceived differences in color between the model egg and their own eggs. Specifically, we found that house wrens preferentially rejected eggs bluer than their own eggs. However, although we found that bluebirds relied on perceived differences in color for their egg rejection decisions, further tests are needed to determine whether they preferentially rejected brown eggs or simply responded to absolute perceived differences in color. These findings demonstrate that these cavity-nesting birds treat perceived color differences in distinct ways, which has important implications on the coevolutionary arms races and the interpretation of avian-perceived color differences.
专性鸟类巢寄生者的宿主通常会进化出防御机制来避免养育无关的幼鸟。一种常见的防御机制是拒孵，宿主通常依赖蛋壳颜色。大多数研究假设宿主会对自己的蛋和寄生虫的蛋之间感知到的颜色差异做出反应，而不管特定的颜色如何；然而，最近的实验发现，许多宿主对棕色的外来蛋比对同样不相似的蓝色蛋反应更强烈。然而，这些之前的研究都没有测试过产棕色蛋的物种，而且除了一个例外，所有研究都是在开放式巢穴中进行的，在这些巢穴中，光照水平被认为足以进行有效的基于颜色的蛋识别。在这里，我们探讨了寄生的棕色头牛鹂（Molothrus ater）的两种洞穴筑巢宿主——产蓝色蛋的东部蓝鸟（Sialia sialis）和产棕色蛋的褐弯嘴嘲鸫（Troglodytes aedon）——如何对实验中涂有六种不同颜色（从蓝色到棕色）的蛋做出反应。两种宿主的拒孵反应都最好地由模型蛋与它们自己的蛋之间感知到的颜色差异来预测。具体来说，我们发现褐弯嘴嘲鸫优先拒孵比它们自己的蛋更蓝的蛋。然而，尽管我们发现蓝鸟依赖于对颜色的感知差异来做出它们的拒孵决定，但需要进一步的测试来确定它们是否优先拒孵棕色的蛋，或者只是对颜色的绝对感知差异做出反应。这些发现表明，这些洞穴筑巢的鸟类以不同的方式对待感知到的颜色差异，这对协同进化的军备竞赛和鸟类感知到的颜色差异的解释有重要影响。
