The topic of this column and the next was inspired by an email from Lisa Jones.  She sent me a video of an albino American Robin in her backyard in Clinton.  You can see the video in another post on this website.

The robin that Lisa found is a pure albino.  All of the feathers are white and eyes are red.

Many birds, of course, have feathers of strikingly different colors.  Rose-breasted Grosbeak males are stunning birds with black, white and red feathers.  Ducks, especially drakes, often have many different hues in their feathers.  A Green-winged Teal is nice example.

How are these feathers colored?  We need to realize that a fully formed feather has no living tissue associated with it.  During the formation of the feather, tissues in the feather-forming structures called follicles form the intricate shape of the feather to be.  The cells in the follicle tissues secrete keratin, the same basic material in your fingernails.  Once the complex feather is formed from the secreted keratin, the follicle cells withdraw, leaving the non-living keratin behind.  A feather is produced!

Often, follicle cells lay down pigments in the developing feather.  The most common type of pigment is melanin.  Melanin gives a feather dark coloration (black, browns, grays).

A bird is able to manufacture the melanin it needs to color it feathers.  In the process of digestion, proteins are split into their basic units of amino acids.  A protein is nothing more than a long chain of amino acids.  One of these amino acids is tyrosine.   Birds use tyrosine to manufacture melanin.

The follicle cells intersperse melanin granules in the keratin secreted to make a feather.  A feather with no melanin or other pigments appears white because all wavelengths of light are reflected back to a viewer.  Dark melanins absorb all the wavelengths of visible light and hence are black to our eyes.

In addition to providing color, melanins also appear to strengthen the feathers.  In a flying bird, the primary feathers at the tips of the wings undergo pronounced deformation during the downward stroke of powered flight.  The tips of larger birds are often richly endowed with melanin.  Snow Geese, American White Pelicans, Northern Gannets, Wood Storks, American Bitterns, and most species of gulls provide only a partial list.

The glorious yellow of an American Goldfinch, the brilliant orange of a Baltimore Oriole and the vivid red of a male Northern Cardinal are produced by a different type of feather pigment called carotenoids.  Carotenoids are plant pigments used in the process of photosynthesis.  Birds cannot manufacture carotenoids so must recycle the carotenoids from their food.  The pathway of carotenoid acquisition can be complicated.  For instance, flamingoes are pink because they sequester carotenoids from their small shrimp prey, which in turn get the carotenoids from the unicellular algae growing in saline lakes.

Have you ever seen a House Finch male that was dull orange rather than red?  Such a washed-out bird is not acquiring enough carotenoids in its diet to make deep red breast and head feathers.  Animal behaviorists call the intensity of the coloration an honest signal.  A Scarlet Tanager with stunning red colors is advertising its ability to forage effectively.  That red coloration can’t be faked.  On the other hand, a Scarlet Tanager with a washed out appearance is advertising its poorer foraging abilities.  Which male do you think a female Scarlet Tanager would choose for a mate?

Scarlet Tanagers use their carotenoid resources wisely.  Their contour feathers only have red carotenoids at the tips.  Since the contour feathers overlap like shingles, only the outer tip of each feather is seen.  There is no benefit in coloring parts of feathers that will never be seen. Female Scarlet Tanagers have an olive-green color.  This color represents an interaction of melanin and carotenoid pigments in the same feathers.

[Originally published on Jul 20, 2014]