Extreme offspring ornamentation in American coots is favored by selection within families, not benefits to conspecific brood parasites

Offspring ornamentation typically occurs in taxa with parental care, suggesting that selection arising from social interactions between parents and offspring may underlie signal evolution. American coot babies are among the most ornamented offspring found in nature, sporting vividly orange-red natal plumage, a bright red beak, and other red parts around the face and pate. Previous plumage manipulation experiments showed that ornamented plumage is favored by strong parental choice for chicks with more extreme ornamentation but left unresolved the question as to why parents show the preference. Here we explore natural patterns of variation in coot chick plumage color, both within and between families, to understand the context of parental preference and to determine whose fitness interests are served by the ornamentation. Conspecific brood parasitism is common in coots and brood parasitic chicks could manipulate hosts by tapping into parental choice for ornamented chicks. However, counter to expectation, parasitic chicks were duller (less red) than nonparasitic chicks. This pattern is explained by color variation within families: Chick coloration increases with position in the egg-laying order, but parasitic eggs are usually the first eggs a female lays. Maternal effects influence chick coloration, but coot females do not use this mechanism to benefit the chicks they lay as parasites. However, within families, chick coloration predicts whether chicks become “favorites” when parents begin control over food distribution, implicating a role for the chick ornamentation in the parental life-history strategy, perhaps as a reliable signal of a chick’s size or age.

子代装饰特征通常出现于存在双亲抚育行为的类群中，这表明亲本与子代间社会互动所产生的选择压力，可能是信号演化的潜在驱动因素。美洲骨顶鸡（American coot）的雏鸟是自然界中装饰特征最显著的子代类群之一，它们拥有鲜亮的橙红色绒羽、鲜红色的喙，以及面部和颅顶的其他红色区域。此前的羽色操控实验表明，亲本对装饰特征更极致的雏鸟存在强烈的选择偏好，从而青睐拥有更华丽装饰羽的个体，但该研究并未阐明亲本产生这种偏好的根本原因。本研究通过分析骨顶鸡雏鸟羽色在巢内家系内部与家系间的自然变异模式，以解析亲本选择偏好的演化背景，并明确该装饰特征究竟服务于哪些个体的适合度利益。同种巢寄生在骨顶鸡类群中十分常见，巢寄生雏鸟本可通过利用亲本对装饰性雏鸟的选择偏好来操控寄主。但与预期相悖的是，巢寄生雏鸟的红色羽色饱和度反而低于非寄生雏鸟。该现象可通过家系内部的羽色变异规律得到解释：雏鸟羽色随产卵顺序的后移而逐渐加深，但巢寄生卵通常为雌鸟产下的首枚卵。母体效应会影响雏鸟的羽色，但骨顶鸡雌鸟并不会通过该机制来提升其作为巢寄生所产卵的子代的适合度。然而在同一家系内部，雏鸟的羽色能够预测当亲本开始调控食物分配时，哪些雏鸟会成为'优先抚育个体'，这表明雏鸟的装饰特征在亲本生活史策略中扮演了重要角色，或可作为雏鸟体型或年龄的可靠信号。