Why Dark-Colored Pigeons Are More Common in Cities

Melanin seems to help the birds get rid of potentially toxic compounds from the environment

Photo: Randy von Liski

The feathers of feral pigeons can range in color from pure white to a dark, dark pigment—although you wouldn't necessarily know that, if your experience with pigeons is limited to city birds, like the ones poking around Central Park. In cities, pigeons tend to be darker, and, according to a new study, that's probably not a coincidence. In cities, dark pigeons' melanin might give them an edge, performing double-duty by helping the birds rid themselves of chemical toxins, reports ScienceNOW

Melanin is responsible for pigmentation, and the darker a bird (or person, or any other animal), the more melanin they have. Melanin also conveniently binds to metals, such as lead, ScienceNOW explains. In theory, melanin would bind to heavy metals that make their way into the birds' bloodstream through food or water, and then remove those metals as the melanin forms and grows into new feathers.  

To test whether or not this hypothetical explanation is plausible, researchers caught around 100 pigeons living in Paris. (Parisian pigeons are certainly carrying their weight as scientific subjects: dark pigeons from the city have previously been shown to have superior immune systems and better defenses against parasites.) The scientists kept the birds in cages for a year and took feather samples when the animals were first captured and at the end of their time in captivity.

They measured the feathers for zinc levels, ScienceNOW reports, and found that those levels dropped by nearly 30 percent. They also found that the darker pigeons had significantly higher levels of zinc in their feathers than the lighter birds. "That suggests that when fed the same diet and housed under the same conditions, darker birds remove more zinc—and possibly other noxious heavy metals—from their bloodstream than light-colored birds do," ScienceNOW writes. 

To really solidify this hypothesis, however, the team will have to perform tests that take into account heavy metal levels in the birds' blood as well as in their feathers, ScienceNOW adds. 

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