Yesterday afternoon I went for a walk and ran into a mixed feeding flock of chickadees, goldfinches, siskins, and redpolls working their way through a stand of hemlock trees. It was the first time I’d seen redpolls out in the woods rather than at a feeder, so I stopped to watch them for a bit. Eventually, though, it was one of the goldfinches that really caught my eye.
American Goldfinches (Spinus tristis) are sexually dimorphic in the summer, meaning the males and females look different – in this case, the males are bright yellow with a black cap while the females are drab olive. For winter, however, the males molt into olive-drab plumage of their own and are usually almost indistinguishable from their ladies. What I spotted yesterday was a goldfinch that seemed to defy this rule. Here we are in December and he (?) still had a bright yellow face.
I can tell you how and why the males change color – the lemon-yellow color in their feathers comes from pigments in their food called carotenoids. It helps the males attract females, who know that a brightly-colored male must be getting a high-quality diet and might have good genes to pass on to her offspring. In the winter, when they’re not breeding, males presumably switch to a less conspicuous color in order to avoid predators.
What I don’t know is why some males actually keep varying amounts of their yellow color through the winter (and some don’t). In fact, if you pay attention to the goldfinches coming and going from your feeder you can even pick out individuals based on how much yellow they retain and where it on their bodies it is. Does this have something to do with age? Diet? Hormones? Beats me, and I haven’t been able to find any answers online, either.
I’m not even 100% sure these birds are necessarily males. Weird things happen with bird plumage sometimes. In this case, I’m thinking of female Mallards, which can develop male-ish coloration as they get older and their hormones get out of whack.