I regularly receive reports of birds with unusual coloration. Often it is just partial albinism in which patches of white show up on the bird in various places, or incomplete albinism in which the eumelanin (dark pigment) is diluted instead of absent, so a dark bird might appear gray instead of black or tan instead of brown. I once had a Yellow-billed Magpie frequenting my back yard which was gray and white instead of black and white. That’s genetic or developmental. There are other cases in which the environment is changing the birds’ color. We know, for example, that the male House Finch seeks out red berries which will turn his plumage red in order to attract females.
Originally from Asia, the highly invasive Morrow’s Honeysuckle has become common in the suburbs of the northeastern U.S. Cedar Waxwings, common in these landscapes, typically exhibit yellow-tipped tail feathers. After about 1950 these birds were being found with orange-colored feather tips, a result of the birds ingesting the honeysuckle berries. In central Ohio the brightness of the male Northern Cardinals’ red plumage decreased with the increase in urbanization and more abundant exotic honeysuckle plants as the berries contain more orange than red pigment. The brighter red males from rural or forested areas which had little or no honeysuckle bred earlier and raised more offspring than their dull city dweller cousins. In Europe, the favorite food of the Great Tit is caterpillars, which contain carotenoids that the larvae obtained from eating leaves. Carotenoids are important as precursor vitamins, antioxidants, and pigments in birds, in this case the yellow of the chest and abdomen of the Great Tit. Urban-dwelling caterpillars contain lesser amounts of carotenoids, so urban parent Great Tits have to make more foraging trips to feed their young than do forest-inhabiting nesters. If the urban area is polluted, the carotenoid levels of the insects are so low that the young birds leave the nest with a dull yellow chest rather than a bright one. Follow up studies are needed to determine the long-term effect.
Researchers in Paris discovered that darker city (feral) pigeons are healthier, partly because darker birds seem to be able to rid their bodies of heavy metals. Wild birds kept in cages for a year had their flight feathers tested for heavy metals such as zinc, cadmium, copper, and lead. Eumelanin, the pigment responsible for the black-brown dark-colored feathers, binds to heavy metals. At the next molt the new feathers were again tested for heavy metals; the levels were down by 75 percent. In addition, darker birds ridded their bodies of more toxins than lighter colored birds. There is some evidence that darker pigeons are increasing as a proportion of the population, perhaps because their survival rates are higher and reproduction more successful. Over the past few years in my travels to different cities, I have taken the time to sit on a park bench and look at the pigeons. I have no data, but it certainly does appear that they are getting darker.