Photobleaching shapes the expression of plumage phenotypes.

Melanins are the most common pigments in animals and are known to experience bleaching (molecular degradation) under UV and visible radiations. However, melanin photobleaching effects on the appearance of animals under natural sunlight conditions are unclear. Here, we collected body feathers from developing Spanish imperial eagles Aquila adalberti, mainly pigmented by the orange pheomelanin, and monitored their reflectance properties during a 15-week sunlight exposure regime. Feather brightness significantly increased with exposure time following a power function, resulting in a 1.87-times increase in paleness and an obvious loss of feather integrity. Photobleaching thus explains the gradual increase in plumage paleness exhibited by juvenile imperial eagles, changing from dark orange to yellowish during the first months of age without the course of feather molt. Bleached plumage characterizes eagle immature phenotypes until reaching a contrasting blackish phenotype by progressive molt after 5-6 years, a period during which the feather degradation that accompanies bleaching may limit flight performance. Given the pheomelanin-pigmented plumages commonly observed in juvenile raptors, and in other groups of birds in which color disappears independently of molt (e.g. wheatears, genus Oenanthe), photobleaching arises as a source of phenotypic expression that may also drive life-history strategies such as crypsis and migration.