Carotenoid plumage coloration is an important signal of quality, and plays an important role in mate choice in many bird species. However, it remains unclear what mechanism makes carotenoids an honest signal. Here, we test the hypothesis that carotenoid plumage coloration might indicate gut health. Parasitic and microbial infections can affect nutrient absorption due to decreased gut surface or by altered gut microbiome. We took an advantage of a naturally occurring coinfection of parasites inhabiting the upper and lower portions of the digestive track to distinguish between the direct and indirect effects of parasites on carotenoid acquisition. Protozoan coccidian intestinal parasites are widespread in greenfinches (Chloris chloris) and the majority of greenfinches are infected in nature. Trichomonosis is an emerging disease of the upper digestive track that causes high mortality among greenfinches. We captured wild greenfinches (N=71) and administered anticoccidial medication toltrazuril (TOLTRA) to one group, antibiotic metronidazole (METRO) that is also effective for treating Trichomonas gallinea, to the second group, and third group received no medication. In the METRO group, feathers grown during the experiment had significantly higher chroma of yellow parts, but there was no effect of TOLTRA on feather chroma. These results suggest that METRO increased the efficiency of carotenoid modification or deposition to the feathers rather than nutrient acquisition, and/or freed energy resources that could be invested in coloration. Alternatively, in accordance with shared pathway hypothesis, increase in efficiency of vital cellular processes might have occurred, as many microbial metabolites can modulate mitochondrial and immune function.

中文翻译：

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抗生素处理可增加绿翅雀（Chloris chloris）中类胡萝卜素羽毛的发黄

类胡萝卜素的羽毛着色是质量的重要信号，并且在许多鸟类物种的配偶选择中起着重要作用。但是，目前尚不清楚什么机制使类胡萝卜素成为诚实信号。在这里，我们测试了类胡萝卜素羽毛颜色可能表明肠道健康的假设。由于肠道表面减少或肠道微生物组改变，寄生虫和微生物感染会影响营养吸收。我们利用寄生在消化道上部和下部的寄生虫的自然共感染优势来区分寄生虫对类胡萝卜素获取的直接和间接作用。原生动物球虫肠道寄生虫广泛存在于绿鳍金枪鱼（Chloris chloris）中，大多数绿鳍金枪鱼是自然界感染的。毛滴虫病是一种新兴的上消化道疾病，导致青鳍金枪鱼的高死亡率。我们捕获了野生绿鳍金枪鱼（N = 71），并向一组施用抗球虫药物托特雷利（TOLTRA），对甲虫滴虫也有效的抗生素甲硝唑（METRO），向第二组施用，第三组未接受任何药物治疗。在METRO组中，在实验过程中生长的羽毛的黄色部分的色度明显更高，但TOLTRA对羽毛色度没有影响。这些结果表明，麦德龙增加了类胡萝卜素修饰或沉积在羽毛上的效率，而不是养分的获取，和/或释放了可用于着色的能量资源。或者，根据共享途径假设，