Despite the colder temperatures and snow of late across most of Colorado, the birds have definitely been signaling that spring is on its way. In fact, astronomically, the spring equinox happens next month (this year it’s March 20th). When you think of spring, what comes to mind? For some people, me included, one of the most common indicators of spring is the reappearance of the American robin in your backyard.
The American robin (Turdus migratorius) is one of the most ubiquitous song birds in North America. While it’s commonly believed that they migrate south in the fall (and some do, particularly those in the upper reaches of Canada and the U.S.), a lot of robins actually spend their winters where they spend their summers. They just might not be as visible because they’re clustered together in trees eating berries instead of on the ground foraging for worms.
A few weeks ago, I noticed that the robins had returned. Spurred on by hormones, their winter flocks were breaking up and they were starting to lay claim to spring breeding grounds. One day, I was watching a couple of robins on the roof of my neighbor’s gazebo when I did a double take; one of the robins had a partially white head! What was going on?
Color in animals is dictated by the amount of pigments in the cells of their skin, hair/fur, scales, or feathers. One of those pigments is called “melanin.” The darker the animal’s features are, the more melanin is being produced. Albinism is the total absence of melanin and results in an all-over white or pale pink coloring (the pink is the blood vessels showing through) and, often, red eyes. Last summer, I visited the Colorado Gators Reptile Park in Mosca, Colorado, and saw several albino alligators, but this robin clearly wasn’t like them; Aside from his head, the robin was a normally colored bird. What else could explain his unusual look?
Similar to albinism, leucism also involves a reduction in color in the skin, hair/fur, etc. However, scientists believe that leucism differs in that pigments (melanin as well as other pigments) are produced, but not deposited in particular areas. This can result in an animal that’s completely white with normal-colored eyes, one that is a muted shade of its usual coloring, or even one where only patches of the animal are white (called piebald). The latter is probably the category my robin fits into. Piebald is a common color pattern in domestic animals such as horses, dogs, cats, and cows.
Leucism, like albinism, can make an animal more susceptible to predators by reducing its natural camouflage. In addition, because melanin provides structural support to feathers (hence why the wing tips on even white birds are typically black), an albino or leucistic bird where the leucism affects the wings, may experience more feather wear. This, in turn, could cause problems during migration. It’s also been theorized that albinism and leucism could cause problems in animals, such as some birds, that rely on color for mating displays, or that it may cause problems trapping heat in cold climates because paler colors reflect more solar radiation than darker colors.
So far there doesn’t seem to be much of an impact on my leucistic robin neighbor, but the real test will be this upcoming breeding season.