Falling Leaves, Falling Feathers: How Birds Create Colors

The color of our own skin and hair is produced by pigments called melanins. As you may be able to surmise, these pigments are able to produce a limited range of colors, mostly brown, black, gray, tan, and buff. These pigments are also present in the feathers, beaks, and skins of birds. Melanins are created within specialized cells called melanocytes, which allow birds’ bodies to have minute control over where melanins are deposited in the growing skin and feather cells. Because of this, patterns produced by melanins can be complex, such as the variety of facial patterns on sparrows, and the striped “pants” worn by goshawks. Beyond mere aesthetics, feathers containing melanins are stronger than feathers without. All pigments strengthen feathers a little bit, with pigmentless white feathers being the weakest and most prone to damage, but melanins offer great strength to the areas where they are found. This is why many birds, even those with white bodies like seagulls, will have black wingtips. These feathers will resist the wear and tear of flight, maintaining their shape and allowing the bird to slice through the air with ease.

While the colors produced by melanins can be lovely, they are limited. Other colors require other pigments, and many of the warm colors are produced by a class of pigments called carotenoids. Reading that word, you may be put in mind of bright orange carrots, and that association isn’t incorrect. Carrots produce a high density of carotenoids, and in fact, the entire class of pigments are produced only within the tissues of plants. So how do they find their way into feathers such as the bright orange face and chest of a blackburnian warbler? Well, they get there by following the food chain upwards. This means that the pigments that create the vibrant colors of autumn leaves create the same colors in feathers. Not merely the same class of pigments, but the exact same pigments themselves. Carotenoids dissolve readily in fat (in fact, they’re responsible for the sunny yellow of an egg yolk), and when birds eat plants containing the pigments, or other animals who have done so, the pigments enter their bloodstream and flow through their bodies, being collected and deposited in the appropriate places. Due to this harvesting and delivering mechanism, colors produced by carotenoids tend to be applied in broader strokes than the precise pattern-creating melanins. These large swatches of carotenoid color often manifest as reds, oranges, yellows, and can contribute to other colors - though we will discuss that mechanism later.

The third classification of pigments are called porphyrins, and they are not present in all birds. Rather, they have been discovered in around 13 orders, and are produced by modifying amino acids within the bird's body. While each porphyrin is chemically different, they are identifiable due to a unique property; they fluoresce when placed under black light and glow a bright red. Under normal lighting conditions, porphyrins can appear as red, brown, or even green. Turacos - birds native to sub-saharan Africa - produce two unique porphyrins, turacin and turacoverdin. Turacoverdin is a copper-based pigment, and is the only green pigment found in birds.

‘But wait,’ you may say. ‘I’ve seen tons of green birds, from parrots to hummingbirds. Heck, even mallards have green heads!’ Well, you are correct that you see green when looking at those birds, however, there is actually no “green” present in those feathers. Rather, there are microscopic structures that refract incoming light, splitting it into its constituent colors, and bouncing some of them back toward our eyes. These “structural colors” can be identified by grinding or crushing a feather. If the feather’s color is derived from pigments, the pigment-based color will still be present in the dust, but structural color will be lost. Take shades of blue, for example, such as those present in the feathers of blue jays. Blue is exclusively produced structurally, and the pigments in the feather are a dull brown. If you shine a light on a blue feather, it will appear as blue. But if you take that same light and shine it through a feather, it will appear brown, since the refracted light is bouncing off the feather in the opposite direction. Some feathers have such complex stacked structures that the feathers will split the light into a rainbow, allowing different colors to be seen depending on the viewing angle. Hummingbird bibs, peacock tails, and the oil-like sheen on a starling are all created by structural iridescence. By combining pigments and structural colors, birds are able to create even more varied and specialized hues. Budgies, for example, appear green due to stacking yellow carotenoids and structural blue in the same feathers.

By combining different pigments and structural colors across their bodies, birds are able to make rich hues in every color of the rainbow, and even beyond! (We didn’t even touch on the colors that birds can express that we can’t see, due to their eyes having sensitivity to ultraviolet light.) So though cold weather is coming, and the birds have already begun their southward migrations, hopefully you will be able to look at those birds that are passing through, and those that stay through the winter with a greater appreciation for the colors, patterns, and the microscopic interactions that bring a level of complexity to even the most drab of winter plumages.