Spring is a time of change in many ways. Over the past couple of weeks, I’ve watched the goldfinches (males, mostly) transform from the drab tan and gray winter plumage into their bright, golden, summer one.
The process is rather gradual — growing feathers is an expensive and time-consuming process — and consequently, male Goldfinches in the spring exhibit variegated colors.
Like many songbirds, Goldfinches undergo two feather molts annually: a complete body, wing, and tail molt in the fall, and a partial molt of just body feathers in the spring before breeding.
The yellow color comes from dietary carotenoid pigments, which must be consumed while the feathers are coming in. (Read more about pigment coloration in birds here.) Millet seed provides much higher concentrations of carotenoids than sunflower and thistle seed (which are quite low), so perhaps Goldfinches change their diet slightly in the spring to increase their carotenoid uptake.
Studies have shown that the male Goldfinches that are brightest in color are the most successful in attracting a mate and have the highest levels of carotenoids circulating in their blood, which might be attributed to their greater success in seeking out the appropriate food sources.