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One of the most astounding phenomenon in nature is occurring right now in your backyard. American Goldfinches are molting their body feathers and transforming from oft-ignored drab, olive birds into the most spectacular of brilliant sun-yellow beauties. The transformation is no less dramatic than the opening of a flower bud.

Nearly every songbird replaces all of its feathers in autumn, shortly after the heavy burden of the breeding season ceases.

Both male and female goldfinches acquire a tan plumage and are not easily recognizable. Come spring, certain species molt in new feathers for the breeding season – not the sturdy wing and tail feathers used for flight– but the small colorful feathers covering the head and body. Just in time to impress females, male goldfinches trade their tan for brilliant yellow feathers on their bodies, and a striking black cap on the head. Their blackish beaks become glowing gold. Because their wing feathers are not replaced, the whitish edges have worn off by May so their wings become strikingly black for the breeding season as well.

But why do they go to all of this trouble? Replacing thousands of feathers is expensive, especially when one considers that all of that brilliant yellow pigment must be acquired from plants by eating their carotenoid-rich seeds.

Like so much else in nature, it comes down to sex. Female goldfinches are more likely to mate with males that have bright yellow back feathers and orange bills. Molting in dull yellow feathers is like having a big “kick me” sign taped to your back.

Is this preference of females for brighter males just an arbitrary fashion decision? No, evolution has led to this system of female preference because brighter colors in male goldfinches actually indicate to females which potential mates have better health and genes. Having offspring with a brighter father means he is less likely to be infected by parasitic flukes, and has a strong immune system to pass on to the kids.

The same carotenoid pigments that males transport into their feathers in spring are also useful to the immune system for fighting disease. A male that can spare a lot of yellow for his feathers is likely to have genes that make for a strong immune system. The beak of a male who is stressed will get duller within a few hours, revealing his plight to all females.

If you don’t have a feeder up with the goldfinch’s favorite food, nyjer (a.k.a. “thistle) seed, run right out and get some this week. It is not too late to see this brilliant product of evolution turn from bird into precious gem.

Cristol teaches in the Biology Department at the College of William and Mary and can be contacted at dacris@wm.edu. To discover local birding opportunities visit http://williamsburgbirdclub.org/