A tail of plumage colouration: disentangling geographic, seasonal, and dietary effects on plumage colour in a migratory songbird

Plumage ornamentation in birds serves critical inter- and intra-sexual
signaling functions. While carotenoid-based plumage colouration is often
viewed as a classic condition dependent sexually selected trait, plumage
colouration can be influenced by a wide array of both intrinsic and
extrinsic factors. Understanding the mechanisms underlying variation in
colouration is especially important for species where the signaling
function of ornamental traits is complex or the literature conflicting.
Here, we examined variation in the yellow/orange tail feathers of American
redstarts (Setophaga ruticilla) passing through two migratory stopover
sites in eastern North America during both spring and fall migration to
assess the role of geographic variation and seasonality in influencing
feather colouration. In addition, we investigated whether diet during
moult (inferred via stable isotope analysis of feather δ15N and δ13C)
influenced plumage colouration. Our findings indicate that geographic
variation, season, and diet all influence individual differences in
American redstart colouration, represented by both traditional and
tetrahedral colour variables. The extent to which these factors influence
colour expression however is largely dependent on the colour metric under
study, likely because different colour metrics reflect different
attributes of the feather (e.g., structural components vs. pigment
deposition). The effects of diet (δ15N) and season were pronounced for
brightness, suggesting a strong effect of diet and feather
wear/degradation on feather structure. Though hue, a metric that should
strongly reflect pigment deposition, also changed from spring to fall,
that effect was dependent on age, with only adults experiencing a
reduction in ornamentation. Taken together, our results highlight the
numerous sources of variation behind plumage coloration and underscores
the difficulty of unraveling complex visual signaling systems, such as
those in American redstarts.