Data from: Eumelanin levels in rufous feathers explains plasma testosterone levels and survival in swallows

Pigment-based plumage colouration and its physiological properties have
attracted many researchers to explain the evolution of such ornamental
traits. These studies, however, assume the functional importance of the
predominant pigment while ignoring that of other minor pigments, and few
studies have focused on the composition of these pigments. Using the
pheomelanin based plumage in two swallow species, we studied the
allocation of two pigments (the predominant pigment, pheomelanin, and the
minor pigment, eumelanin) in relation to physiological properties and
viability in populations under a natural and sexual selection. This is
indispensable for studying the evolution of pheomelanin-based plumage
colouration. Pheomelanin and eumelanin share the same pathway only during
their initial stages of development, which can be a key to unravel the
functional importance of pigment allocation and thus of plumage
colouration. Using the barn swallow, Hirundo rustica, a migratory species,
we found that plasma testosterone levels increased with increasing the
proportion of eumelanin pigments compared with pheomelanin pigments, but
not with the amount of pheomelanin pigments, during the mating period. In
the Pacific swallow H. tahitica, a non-migratory congener, we found that,
during severe winter weathers, survivors had a proportionally smaller
amount of eumelanin pigments compared with pheomelanin pigments than that
in non-survivors, but no detectable difference was found in the
pheomelanin pigmentation itself. These results indicated that a minor
pigment, eumelanin, matters at least in some physiological measures and
viability. Because the major pigment, pheomelanin, has its own
physiological properties, a combination of major and minor pigments
provide multiple information to the signal receivers, potentially
enhancing the signalling function of pheomelanic colouration and its
diversification across habitats.