Late summer is a time of transition for birds and a time of tremendous variety for birders. You can see adult birds in breeding plumage (looking worn after a long breeding season), fresh nonbreeding plumage, and any stage in between. And in the same flock, you might find newly fledged birds still in juvenal plumage, and others that fledged earlier in the summer and are already in fresh first-winter plumage.
Considering the dramatic changes in feathers, and the variety of appearances, it’s not surprising that most birders miss the changes in bill color that happen at the same time. A bird’s bill is not a lifeless bony growth, like a horn or an antler. Bones form the foundation, but over the bones is a layer of living tissue, and covering that is a thin layer of hard but translucent keratin. The living cells just below the keratin layer can change color, and because the keratin is translucent, the colors are visible.
In bills just as in feathers, all bright yellow and red colors are produced by carotenoid compounds. Because carotenoids must come from the diet and are also important in immune-system functions, a bright yellow or red bill can be a signal that a bird is healthy — that is, finding a good diet and having no immune-system stress.
American Goldfinch, American Robin, European Starling, and other songbirds send this signal by developing yellow or orange bills in the breeding season. As the season winds down, adults need to signal less, and their bills fade to a grayish color. Juveniles grow up with drab dusky bills and won’t develop the brighter color until the following spring, when breeding season begins.