The Darkness of Pigeons

Just returned from a trip to the United Kingdom so I missed posting my usually weekly blog. Went to Scotland, Wales and England. One of several trips I’ve made there because I really like the country and people.

Of course I looked for birds between visits to castles and pubs. Nothing to brag about in the way of lifers, I’m afraid. First five days were in chilly Edinburgh. Didn’t see much in the way of birds except for carrion crows, house sparrows, and city pigeons.

Ah, those city pigeons, rock doves, rock pigeons, as they have been variously called, omitting the less appealing appellation of “flying rats.” Strolling downtown in Scotland’s iconic city, I noticed that these pigeons were particularly dark in color, not the usual variety of whites, grays, and blacks. Reminded me of a paper  read a year or two ago about pigeons in London, I believe, and their evolution towards darker coloration. I couldn’t imagine evolutionary pressures selecting for darker colored pigeons. Why? To make them less obvious to predators? To increase their absorption of solar radiation?

Well, it turns out that melanin, the pigment that is responsible for shades of black and brown, and colors the birds’ feathers, also binds to heavy metals such as lead, mercury , arsenic, cadmium, and so on. Growing feathers use melanin as coloration and the melanin-metal combination draws the metals into the feathers. When feathers are molted, the heavy metals are shed as well . So darker birds pass more toxic metals through their bodies more quickly and accumulate less of them and are thus less subject to the effects of heavy metals. So darker birds are healthier and we’d expect their proportions in the population to be greater than that of lighter colored birds. Check this out in the next big city you visit and read the research here. And some interesting research on the same subject that adds a new twist here. For a shorter, somewhat less sciencey explanation, see this article.

What about other birds in the city? I suspect crows and robins and sparrows don’t hang around on the pavement, gutter, power lines, gutters and building ledges the way pigeons do, so they probably don’t pick up as much of the heavy metals. And birds away from big cities probably don’t show any plumage difference due to heavy metal ingestion. But then there has been minimal research on this topic, so stay tuned.

On the other hand, although there is evidence of what i speak, I didn’t do any substantial research while sitting on an Edinburgh park bench. The abundance of dark pigeons just reminded me of that earlier research. That doesn’t preclude park bench sitting from the realm of science- that’s where hypotheses get developed. Although, let’s face it, I was just loafing.