SUMMER2008/VOLUME 22, NUMBER 3
Life in the City
How does urban stress affect birds?
Traffic got you jumpy? Fed up with crowds? Jolted awake by sirens in the night? Welcome to life in the city. If you’re tired of all the commotion, imagine what birds have to cope with.
In recent years, scientists have begun to test urban birds, from European Blackbirds and European Starlings to White-crowned Sparrows and Florida Scrub-Jays, for physiological signs of stress. They’re finding that urban birds indeed often live more stressful lives than their rural relatives.
One direct way to gauge a bird’s stress level is to take a tiny blood sample and analyze it for substances such as corticosterone, an adrenaline-like hormone. For birds caught in researchers’ nets, corticosterone levels spike as quickly as three minutes after capture. Over the short term, this is good: the hormone helps move energy-rich sugars into the bloodstream and readies the bird’s brain for quick action.
But if you’ve ever lived near an airport or next to the train tracks, you know it’s possible, even desirable, to acclimate to situations that seem unbearable at first. City birds seem to do the same thing. In a 2007 study published in General and Comparative Endocrinology, suburban Florida Scrub-Jays living near cats, dogs, and bird feeders actually had lower stress-hormone levels than those living in the tranquility of wild scrub.
This habituation to urban life may even be passed from parent to offspring. European Blackbirds from the heart of Munich, Germany, had lower responses to abrupt disturbances (as measured by corticosterone) than those from a forest 25 miles out of town, according to a 2006 Ecology article. That pattern held even though the blackbirds had been transported as nestlings to the same laboratory room and hand raised, side by side.
Remaining unflappable amid city bustle may be an important way birds reduce the harmful effects of long-term stress. In another 2007 General and Comparative Endocrinology article, repeated stresses (such as loud radios) at the nests of European Starlings caused birds to raise fewer young than at unbothered nests. A 2007 study of White-crowned Sparrows in California and Washington, published in Behavioral Ecology, found that females with higher stress-hormone levels were also less successful at raising young.
These starlings and sparrows have run up against what scientists call a “sublethal effect” of urbanization. If city living doesn’t kill birds outright, it may contribute to lower reproductive success. And fewer offspring can be just as deleterious to a population’s prospects.
For example, a British study found that the harm cats do to bird numbers simply by creating an unsafe atmosphere could exceed the toll from hunting. (The research appeared in Animal Conservation in 2007.) Cautious birds spend more time watching from a safe perch and less time gathering food, resulting in fewer fledglings each summer. The researchers suggested that this effect might partially explain the House Sparrow’s alarming 60 percent decline across parts of England and Europe.
So if you’re out birding around town, take a moment to admire not just the sweet songs and bright colors, but the remarkable adjustments—both psychological and physical—birds have made to survive in what is for them an alien and often hostile environment.
For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: firstname.lastname@example.org