Among the most ornate animal traits in nature are the angle-dependent (e.g. iridescent) structural colors of many fishes, damselflies, birds, beetles, and butterflies. Though we now have a solid understanding of the mechanisms that create angle-dependent coloration in several groups, we know little about whether pigmentary colors reflect light in an angle-dependent fashion or if similar or different mechanisms govern angle-dependent reflectance from pigmentary versus structural colors. Here for the first time we describe non-iridescent angle-dependent coloration from the tail and wing feathers of several parrot species (Aves: Psittaciformes). We employed a novel approach—by calculating chromatic and achromatic contrasts (in just noticeable differences, JNDs) of straight and angled measurements of the same feather patch—to test for perceptually relevant angle-dependent changes in coloration on dorsal and ventral feather surfaces. We found, among the 15 parrot species studied, significant angle dependence for seven of our eight feather JND parameters. We then measured micro-scale features on each side of feathers, including size and color of barbs and barbules, to attempt to predict interspecific variation in degree of angle-dependent reflectance. We found that barb height, plumage-color type (e.g. melanin, structural), and differences between barb-barbule coloration (measured using Euclidean distances) were the strongest predictors of angle-dependent coloration. Interestingly, there was no significant phylogenetic signal in any of the angle-dependence models tested. These findings deepen our views on the importance of microscopic feather features in the production of directional animal coloration, especially in tissues that are colored predominantly by pigments and appear to be statically colored.

中文翻译：

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鹦鹉羽毛角度相关颜色变化的羽毛形态预测因子

自然界中最华丽的动物特征是许多鱼类、豆娘、鸟类、甲虫和蝴蝶的角度依赖性（例如彩虹色）结构颜色。尽管我们现在对在几组中产生角度相关着色的机制有了深入的了解，但我们对色素颜色是否以角度相关的方式反射光，或者类似或不同的机制是否控制色素与结构的角度相关反射知之甚少颜色。在这里，我们第一次描述了几种鹦鹉物种（鸟类：鹦鹉形目）的尾巴和翅膀羽毛的非虹彩角度依赖性着色。我们采用了一种新颖的方法——通过计算有色和无色对比度（在明显的差异中，JND）对同一羽毛斑块进行直线和角度测量——以测试背侧和腹侧羽毛表面颜色的感知相关角度相关变化。我们发现，在研究的 15 种鹦鹉物种中，我们的八个羽毛 JND 参数中的七个具有显着的角度依赖性。然后，我们测量了羽毛每一侧的微观特征，包括倒钩和小羽的大小和颜色，以尝试预测角度相关反射度的种间变化。我们发现倒钩高度、羽毛颜色类型（例如黑色素、结构）和倒钩小枝颜色之间的差异（使用欧几里德距离测量）是角度相关颜色的最强预测因子。有趣的是，在任何测试的角度依赖性模型中都没有显着的系统发育信号。