Abstract

Brood parasitism results in substantial costs to hosts, yet not all species eject foreign eggs. Because the costs of mistakenly ejecting one’s own eggs are high, selection may favor ejection behavior only if it is unlikely a host will incorrectly eject her own eggs. Eastern Bluebirds (Sialia sialis) are currently subject to relatively low levels of interspecific brood parasitism but still sometimes eject parasitic eggs. Therefore, we tested which visual cues they use to eject foreign eggs with the prediction that only the most dissimilar eggs would be ejected, reducing the likelihood of a female making a mistake. House Sparrows (Passer domesticus), which occasionally parasitize bluebirds, lay eggs that have an off-white ground color with brown speckling. Therefore, to test which colors or patterns allow for discrimination of parasitic eggs, we generated 3-dimensional (3D)-printed model House Sparrow eggs and painted them entirely off-white, entirely brown, half off-white and half brown, or off-white with brown speckling. We then sequentially placed these 4 different model eggs in the nests of Eastern Bluebirds, with each nest receiving all treatments over the course of 4 days. After watching females enter and leave the nest box just one time after placement of the model egg, we found that speckled eggs were ejected half the time (7 of 14 nests), while no other treatment was ejected more than 3 times. Thus, Eastern Bluebird females eject eggs based primarily on color patterning (i.e. a speckled pattern) rather than coloration per se, and that they can do so quickly, as the average female had removed the model egg within 6 min of entering the nest. Because Eastern Bluebirds do not lay speckled eggs, but some brood parasites do (e.g., House Sparrows, Brown-headed Cowbirds [Molothrus ater]), selection may specifically favor ejection of eggs with a speckled pattern, not just eggs that have within-egg color contrasts.

中文翻译：

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东部蓝鸟（Sialia sialis）使用颜色图案，但不使用颜色本身作为提示，以弹出种间寄生虫卵

摘要

繁殖寄生会给寄主造成沉重的代价，但并非所有物种都会抛出外来卵。由于错误地抛出自己的卵的代价很高，因此，只有在主机不太可能错误地抛出自己的卵的情况下，选择才可能有利于抛出行为。目前，东部蓝鸟（Sialia sialis）的种间亲鱼寄生性相对较低，但有时仍会弹出寄生卵。因此，我们测试了它们会使用哪种视觉提示来喷射异卵，并预测只会喷射出最不相似的卵，从而降低了雌性犯错的可能性。麻雀（Passer domesticus）（有时会寄生于蓝鸟），产卵的颜色为米白色，褐色斑点。因此，为了测试哪些颜色或图案可以区分寄生虫卵，我们生成了3维（3D）打印的模型麻雀卵，并将它们涂成完全白色，完全棕色，一半白色和一半棕色或不透明。 -白色，有棕色斑点。然后，我们将这4个不同的模型鸡蛋依次放入“东部蓝鸟”的巢中，每个巢在4天的时间内接受所有处理。观察雌性在放置模型卵后仅一次进出巢箱后，我们发现有斑点的卵被排出一半时间（14个巢中的7个），而没有其他处理被排出超过3次。因此，东部蓝鸟雌鸟主要根据颜色图案来射出卵（即 （有斑点的图案）本身而不是着色，并且它们可以很快完成，因为普通雌性在进入巢穴后6分钟内就将模型卵移出。因为东部蓝鸟不会生有斑点的卵，但是有些卵寄生虫会生（例如，麻雀，棕头牛Cow [Molothrus ater ]），选择可能特别倾向于弹出带有斑点图案的鸡蛋，而不仅仅是具有蛋内颜色对比的鸡蛋。