1. Melanins are the most common pigments in vertebrates and, as such, fufill multiple adaptive functions, including honest signaling. This is the case of male pied flycatchers Ficedula hypoleuca, whose dorsal plumage, composed of black and grey feathers pigmented by eumelanin, is sexually selected by females regarding the proportion of black.

2. However, the basis of such mating preferences and other associations with life-history traits are unknown. 

3. Here I take advantage of recent advances in Raman spectroscopy analysis to investigate the monomeric composition of eumelanin, constituted by 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunits, in male pied flycatchers. 

4. I found that plumage blackness (percentage of dorsal black feathers) increases with the DHICA:DHI ratio of the constituent black freathers, but not with that of grey feathers. The repeatability of DHICA:DHI measurements in black feathers is two times that in grey feathers. 

5. Eumelanin chemistry may thus constitute the basis of signal honesty in pied flycatchers, as females preferentially mate with males with higher relative DHICA feather contents and this may be related to the lower pro-oxidant effect of DHICA. Given the the ubiquitous nature of melanin-based pigmentation phenotypes, the monomeric composition of eumelanin should therefore be considered, instead of eumelanin as a whole, for a better understanding of how and why phenotypes are linked to life-history traits in animals.