October 15, 2019

A Framework of Feathers

Posted by: Rock Delliquanti

A Framework of Feathers

Feathers are an iconic feature to birds. They are fundamental to a bird’s ability to fly, to defend territories, and to breed. But what exactly are feathers? On a molecular level, they’re not that different than our hair or fingernails--they’re made of the same protein called keratin. Each type of feather serves an important role in the birds life: some are for flight, some for warmth, some for sensing, but all of them are made from these same building blocks.


So why, then, are some birds colorful, while others are drab? By default feathers are white, such as those on the Snowy Egret pictured below. These feathers have no added pigments or structures which modify color and are therefore perceived as white. In order for a bird to have non-white colors it has to employ the use of added pigments or altered feather structure.


Photo: John Sutton/Macaulay Library at the Cornell Lab of Ornithology (ML25935961)


Birds that have brown or black feathers owe this to the pigment called melanin--the same pigment that gives our skin its color. Birds can produce melanin themselves and varying amounts of it will result in different shades of browns and blacks. Melanin can be combined with other pigments as well resulting in varying shades of other colors we will talk about. Additionally, melanin strengthens feathers making them more resistant to being damaged; this effect is often utilized on high-value feathers such as those fundamental for flight. Wood Storks are an excellent example of a bird with non-pigmented, white feathers on their body and upper wing, but have strengthened, melanin-rich, black flight feathers to aid in their ability to efficiently soar.


Photo provided by Phil Murdaco (on Instagram as @phillipmurdaco)


But birds are more than black, white, and brown. In order for birds to produce feathers of warmer colors, birds need help from their diet. Pigments called carotenoids are responsible for creating the reds, oranges, and yellows present in feathers. Carotenoids, however, are only produced through photosynthesis which means birds cannot directly make them like they do with melanin. To work around this hurdle, birds will consume the plants, fungi or bacteria that produce carotenoids; if they consume high enough quantities of them, the birds will have access to these warm pigments. Some birds, such as the Roseate Spoonbill, do not eat the carotenoid-laden algae themselves, but instead they eat the shrimp and crustaceans that consume these carotenoid producers allowing them to acquire the colors secondhand. These pinks and reds are indicators for fitness as well--if a bird is eating well, these diet-based colors will be stronger and more vibrant; if you see a particularly dingy Northern Cardinal it may be a sign that he could not find adequate food.


Photo provided by Phil Murdaco (on Instagram as @phillipmurdaco)


Lastly, in order to achieve cooler colors, birds have to  take advantage of the structure of their feathers. By altering the structure of the proteins, air pockets are introduced within the keratin which allows certain wavelengths of light to reflect while others to pass through. This allows birds to create blues, purples, and greens. To really see this in action, if you find a blue feather--for example from an Eastern Bluebird--you can shine a light on it from above to see it reflect blue; however if you shine a light from below the “prism” of the feather isn’t aligned properly and you only see the natural brown and black melanin within the feather and no blue coloration.


Photo provided by Phil Murdaco (on Instagram as @phillipmurdaco)


As you can see from these examples, many birds do not rely on a single color for their feathers. In fact, many birds use some combination of them. An example of a bird found here on LSSI that uses all of these coloration tricks is the female Belted Kingfisher. She has a white belly and collar due to a lack of pigments or structural colors; she has strengthened, black wingtips resulting from the introduction of melanin into the feathers; she has structurally adjusted and melanin enriched blue-grey feathers on her head, back, and wings ; and lastly she sports a beautiful chestnut colored belly band due to the introduction of carotenoids.

Photo: Ilya Povalyaev/Macaulay Library at the Cornell Lab of Ornithology (ML39304341)


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