Avian brood parasites lay their eggs in other birds' nests, and hosts can mitigate the fitness cost of raising unrelated offspring by rejecting parasitic eggs. A visually-based cognitive mechanism often thought to be used by hosts to discriminate the foreign egg is to compare it against the hosts' own eggshell by size, shape, maculation, and/or ground coloration (i.e., absolute chromatic contrast). However, hosts may instead discriminate eggs based on their colors along a scale of natural avian eggshell coloration (i.e., directional chromatic contrast). In support of this latter visual process, recent research has found that directional chromatic contrasts can explain some host species' rejection behavior better than absolute chromatic or achromatic contrasts. Here, for the first time, we conducted an experiment in a cavity-nesting host species to test the predictions of these different visual mechanisms. We experimentally parasitized nests of the Common Redstart Phoenicurus phoenicurus, a regular host of a mimetic-egg laying Common Cuckoo Cuculus canorus host-race, using painted, immaculate 3D-printed model eggs in two geographically distant areas (Finland and Czech Republic). We found that directional chromatic contrasts better explained rejection behaviors in both parasitized (Finland) and non-parasitized (Czech Republic) host populations, as hosts rejected eggs that were noticeably browner, but not eggs that were noticeably bluer, than redstart eggs. These results support the paradigm of a single rejection threshold predicted by the directional chromatic contrast model and contribute to a growing generality of these patterns across diverse avian host-brood parasite systems.

中文翻译：

---

使不同的视觉模型适应沿着巢穴宿主物种的自然卵颜色梯度发生的寄生卵排斥行为模式。

禽类寄生虫会将卵产在其他鸟巢中，寄主可以通过拒绝寄生卵来减轻饲养无关的后代的健身成本。通常认为宿主会使用基于视觉的认知机制来区分异卵，是通过大小，形状，斑点，和/或底色（即绝对色差）将其与宿主自身的蛋壳进行比较。但是，寄主可能会根据其颜色沿着自然的禽蛋壳着色（即方向色对比度）比例来区分鸡蛋。为了支持后者的视觉过程，最近的研究发现定向色差比绝对色差或消色差可以更好地解释某些宿主物种的排斥行为。在这里，第一次 我们在腔巢宿主物种中进行了一项实验，以测试对这些不同视觉机制的预测。我们在两个地理上遥远的地区（芬兰和捷克共和国）使用彩绘的，完美无缺的3D打印模型卵，对普通的Redstart Phoenicurus phoenicurus巢进行了实验性寄生，该巢是模拟蛋产下的普通杜鹃Cuculus canorus宿主的常规宿主。我们发现，定向色差可以更好地解释寄生虫（芬兰）和非寄生虫（捷克）寄主群体的拒绝行为，因为寄主拒绝的卵比重启动卵明显棕色，而没有明显变蓝的卵。