None of us is so foolish as to think that winter is over.  However, the spring bird migration is about to begin.  It is likely that Red-winged Blackbirds will begin to appear in Maine before the month is over.  March will bring Common Grackles, American Woodcock, Turkey Vultures, Eastern Phoebes and Osprey.

As spring approaches, many songbirds will change from a drab winter plumage to a more gaudy breeding plumage.  Males are much more likely to “get dressed up” for the breeding season.  This change in costume is accomplished by molting of feathers.

Like our fingernails, bird feathers are dead structures once they are formed.  In fact, feathers are mostly composed of keratin, the same material that makes up our fingernails.  As feathers are formed in special structures called follicles, various pigments are deposited as small grains in the feather that gives a feather its distinctive coloration.

By far the most common type of pigment in bird feathers are the melanins.  Melanins of different types impart black, brown and other earthy tones to a feather.  Humans produce melanins too.  The black skin of a person of African heritage is caused by the high concentration of melanin in the skin cells.  Lighter-skinned humans produce melanins when exposed to strong sunlight.  That’s how we get a suntan.  Melanins are produced by the cellular machinery of birds by recombining amino acids from the breakdown of proteins.

Besides providing color to feathers, melanin has another important function in birds.  The presence of melanin grains strengthens the feathers.  You can probably think of a number of large birds that have black wing tips.  Northern Gannets, Snow Geese, Tundra Swans, White Ibis, Wood Storks, American White Pelicans, and Swallow-tailed Kites are good examples.  For a flying bird, the outer feathers of the wing experience the most stress during flight.  The birds above are not close relatives.  All of them have evolved black-tipped wings to reduce the wear on their outer wing feathers.  The presence of similar structures in unrelated species is called convergence; black wing tips are an excellent example of convergence.

Lots of our local birds have brilliant red, yellow and orange feathers in their breeding plumage.  In most birds, these colors are imparted by pigments called carotenoids.  Unlike melanins that a bird can manufacture, carotenoids cannot be made by a bird’s cells.  Rather, the carotenoids are acquired in the diet either by consuming plant material or indirectly by eating animals like caterpillars that consume plants.  Carotenoids are made by plants.  The brilliant yellows, reds and oranges of fall trees come from carotenoids in the leaves that are unmasked only after the green chlorophyll pigment is resorbed in the fall.  Birds retain carotenoids from their plant food for their own decoration.

The amount of carotenoids in the diet of a bird can determine how colorful that bird’s breeding plumage is.  For instance, a male House Finch typically has deep red feathers on its head, breast and rump.  Males that are good at finding food rich in carotenoids have deep red feathers.  Males on deficient diets are yellowish rather than red.  Research has shown that female House Finches choose their mates based on the color of the plumage.  Red males are preferred.  Females seem to know that if a male is good at finding food for itself, his food-finding abilities will come in handy when it is time to feed hungry nestlings.  Yellow male House Finches will normally not be picked as mates by females.

A third type of pigments in bird feathers are called porphyrins.  These compounds are similar to the hemoglobin molecules in our blood.  Porphyrins generally impart reddish and brownish tones.  These pigments are found in a number of owls.  One characteristic of porphyrin pigments is that they fluoresce under ultraviolet light.  Birds can see into the ultraviolet portion of the spectrum so porphyrins are likely more vivid for birds than they are for us.

Eastern Bluebirds, Blue Jays and some other local birds have what appear to be blue feathers.  However, there are no blue pigments in bird feathers.  The appearance of blue is brought about by the particular structure of the feathers.   That structure allows all the different colors of visible light (all the colors of the rainbow) to enter the feather but only blue colors are reflected back out.  All the other colors are absorbed by the feather and can’t be seen.   So, a Blue Jay appears to be blue not because of blue pigments but by selective reflection of light.  As I tell my students, blue color in birds is a pigment of your imagination.

[Originally published on February 21, 2010]