Polluted air pushes pigeons to fly faster

By now you’ve probably seen those viral photos of a hazy, smoky, pollution-riddled Beijing. Skyscrapers just 1,000 feet away are barely visible behind a thick curtain of grey and brown, trees and rivers are occluded by air heavy with more than 20 times the contaminant level deemed safe by the World Health Organization. Pollution is bad news, and has been linked to everything from heart disease to lung cancer in humans. But few researchers have investigated the effects of pollution on animals. Here’s a surprise from one group of researchers who have: pigeons actually navigate better in polluted air than they do in clean air.

You might expect, as did researchers Zhongqui Li from Nanjing University, Franck Courchamp from the University of Paris, and Daniel Blumstein from UCLA, that birds would be sensitive to air pollution; the scant research available on the question suggests that their health suffers just like ours. And you’d probably expect that their homing abilities, among the best-understood animal navigation systems on our planet, would be similarly negatively affected by pollution. The most obvious reason is that heavy haze makes it harder to see. While pigeons do use solar and magnetic cues to orient themselves, they rely on olfactory and visual ones to locate themselves in space. The ability to orient in space without the ability to figure out where they are simply wouldn’t be enough to get home.

Given that, the researchers expected that on more heavily polluted days, homing pigeons would fly more slowly and would be less successful in finding their way back home. To verify that, they extracted data from 415 pigeon races on the North China Plain organized by the Chinese Racing Pigeon Association. The air there is particularly dirty: in the study, air quality scored as high as 482, on a scale where 500 is the worst possible score. All the racing data they needed was easily accessible – if you can read Chinese – on the association’s website.

It was more than a little surprising when they found the opposite of their predictions. “Pigeons homed significantly faster when flying through more polluted conditions,” they wrote last week in the journal Scientific Reports, even after accounting for other factors like wind speed or the amount of visible sunlight. When flying the median distance (186.4 miles), a pigeon would get home 22.7% faster on average under conditions of heavy pollution. Put another way, they traveled more than 8 miles per hour faster under those conditions.

The researchers aren’t certain just why pigeons home faster in filthy air, but Blumstein says he is “certain the pattern is there.” They do have two ideas, though.

One is that despite pollutions’ negative impacts on pigeon health, it could actually enhance their navigation ability. Not in the visual modality, of course, but in the olfactory one. The pollution, which is mostly produced by the anthropogenic burning of coal and biomass, might create a salient odor map that the birds can use, relying upon olfactory landmarks rather than visual ones.

The other is that pollution could increase the birds’ motivation to get home, resulting in an increase in flying speed. Air pollution might act as a harbinger of avian illness, and the birds might therefore be driven to limit their exposure—a kind of escape response. Or perhaps reduced visibility interferes with the pigeons’ ability to detect predators. That, too, could ramp up their motivation to home more rapidly. Either way, the idea would be to limit their exposure to a dangerous environment.

These intriguing results conjure up more questions than they offer satisfying answers. But what they do provide is an important reminder that nature doesn’t always behave the way we think it will. And also, a 23% increase in pigeon homing efficiency probably isn’t a good reason to keep dumping pollutants into our skies. – Jason G. Goldman | 13 January 2016

Source: Li, Z., Courchamp, F., and D.T. Blumstein. 2016. Pigeons home faster through polluted air. Nature Scientific Reports 5, 18989. DOI: 10.1038/srep18989.

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