Anyone who has woken to a cacophony of squawks and chirps knows that birdsong, no matter how melodious, isn't always a welcome sound. Past research suggests that birds aren't keen on human din either. But a new study finds that not all birds think alike: Some species actually appear to seek out noisy environments.
Among birds, noise does more than annoy. It can hinder their ability to communicate. In fact, some scientists suspect that noise pollution is at least partly responsible for the decline of bird populations. Researchers, however, have had a hard time teasing out the impacts of noise from the impacts of other noise-associated factors, such as traffic and development.
To sort out whether noise alone can affect bird nesting and reproduction, community ecologist Clinton Francis of the University of Colorado, Boulder, and his colleagues spent three summers in the pinyon-juniper woodlands of northwestern New Mexico. They located nests belonging to a variety of bird species on 18 wooded plots adjacent to natural gas extraction wells; they then followed those nests throughout the summer to see whether the hatchlings fledged. The study plots were nearly identical except for one key difference: Half of the natural gas wells had compressors so loud the researchers had to shout to be heard. The other half were quiet.
Contrary to the findings of previous studies, which were unable to separate the impact of noise from other confounding variables, the researchers found no difference in bird density: Noisy sites contained as many nests as quiet sites. The team did, however, see differences in species richness. Quiet sites had 32 bird species nesting on them compared with 21 species at noisy sites. Mourning doves and black-headed grosbeaks tended to nest on quiet sites, while black-chinned hummingbirds and house finches seemed to actually prefer noisy sites. Ninety-two percent of hummingbird nests and 94% of house finch nests were located on study plots near compressors. "That was really surprising," says Francis, whose team reports its findings online today in Current Biology. "We haven't found any evidence in the literature showing that species will actually select for these really noisy sites."
Of course, it's also possible that these birds were being forced out of quiet sites, but Francis thinks that's unlikely. His team didn't observe many species that would compete with black-chinned hummingbirds nesting on quiet sites, and house finches succeed almost everywhere.
One possible explanation for the differences is that noise may be hampering communication among some birds but not others. Black-chinned hummingbirds, for instance, communicate in higher frequencies that may still be audible against the low-frequency rumble of the compressor. And house finches can converse in a wide range of frequencies. The two species associated with quiet sites, on the other hand, communicate using lower frequencies that may be drowned out by the compressors.
Ornithologist Wesley Hochachka of the Cornell Lab of Ornithology offers another alternative: Perhaps the species found nesting near compressors simply can't hear the racket. "We really don't know how birds perceive what we would call a noisy environment," he says.
Nest predation may have played a role as well. Francis and his colleagues found that a common nest predator--the western scrub jay--avoided noisy areas. As a result, birds living on those sites had more successful nests. Nearly a third of nests in quiet areas failed to produce any fledglings compared to only 13% of nests on noisy sites.
On the other hand, noise could also make birds more susceptible to predation, notes behavioral ecologist Gail Patricelli of the University of California, Davis. When her graduate student needed to capture sage grouse last year, she took a boom box into the woods and played music by the group Nine Inch Nails. The noise allowed the student to "walk right up to these birds with a spotlight and put a net over their heads," says Patricelli. "They were sort of stupefied."