I was just mowing the lawn and discovered a large feather in my path.
I stopped to retrieve the molted red-tailed hawk primary before it became just a bit more mulch in the yard. It was one of the outer primaries, one that creates the “fingers” on the tips of the wings of a soaring bird.
I could tell it was from a 1-year-old bird because the feather lacked the barring typical of the outer primaries of the adult. It was mostly white except for the last four inches, which were nearly black. There’s a story in those last four inches of feather.
Feathers are absolutely incredible, far more complex than the hair of mammals. Put one under a microscope or powerful magnifying glass, and the intricate structure it reveals is amazing. There are barbs that branch from the shaft. Then there are the barbules that branch again, each with Velcro-like hooks and rolled edges that bind adjacent barbs together. This creates a light weight sheet that is not only strong and tough, suitable for aerodynamic feats, but can also insulate and help a duck float.
There are many secrets buried in the growth and structure of feathers. Most will have to wait for another time. It’s the color in the tips of wing feathers I wish to explore here.
Feathers must be able to resist the abuses they incur on a daily basis. Flapping to escape predators or migrating great distances places stresses on feathers. Brushing against stems, leaves and branches also takes a toll. Sunlight itself subjects feathers to degradation. Many feathers, including flight feathers, are replaced only once a year, so they must endure.
Feathers are mostly made up of keratin. It is the same substance that comprises hair, fingernails, claws and horns. It is a tough substance by itself, but there is more. Melanin is a common chemical that produces brown and black colors in many organisms. The pigment in dark hair and tanned skin is due to melanin. Most blacks and browns in feathers are produced by the same chemical. Melanin has other attributes. Not only is it a pigment, it also provides strength and toughness.
This is why the feathers that experience the greatest level of wear and damage are usually dark in color especially at the tips. Flight feathers, especially the primaries at the ends of the wings, are subject to great wear and damage and are usually heavily pigmented. Even mostly white birds tend to have black primaries. The only feathers on the white pelican that aren’t white are the flight feathers. The same is true for snow geese and whooping cranes. Most gulls are very pale and yet they, too, tend to have black wingtips.
There are exceptions. This is biology after all. Egrets are some of the few white birds without black wingtips. However, if you consider their lifestyle, they fly slowly and infrequently and occupy open pond habitat where potential abrasion is low. The pure white wings of swans are more difficult to explain.
Well, I’ve stalled long enough. I better get back to mowing the lawn.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.