City Birds are Tougher Than Country Birds

tags: researchblogging.org, conservation, birds, ornithology, ecology, environment, endangered species

I suppose this seems obvious to you city dwellers, but recently published research demonstrates that urban birds are better overall survivors than their country-dwelling cousins. Apparently, this is because city-dwelling birds are less specialized in their requirements, and thus, are more broadly adaptable to a variety of microenvironments, such as those found in large cities. While this might seem to be a mere conversational topic to many of you, this observation is important when trying to design conservation programs to preserve threatened and endangered species, particularly when these species' needs come into direct conflict with the ever-expanding human population.

"The urban habitat is usually more severe than the habitats these birds historically occupied," said study team member John Wingfield of the University of Washington, who was also my dissertation advisor. "Urban habitats aren't easy, so the birds have to have developed coping mechanisms."

The team, led by my grad school friend and colleague, Frances Bonier, who now is at Virginia Tech, predicted that environmental generalists would be tougher -- have a greater environmental tolerance -- than those species that have more specialized needs because the conditions of human-disturbed habitats are more likely to fall within those tolerated by generalist species (figure 1);

It is difficult to precisely define the relative "toughness" of any particular species, but for the purposes of this study, the team defined generalist species as those with a wide geographic distribution. Because a broad latitudinal and elevational distribution correlates strongly with a wide variety of ecological conditions tolerated by both animals and plants, this is a reasonable measure for a given species' environmental tolerance.

To identify which species of birds do best in city environments, the researchers sent questionnaires to ornithologists, biologists and birdwatchers all over the world asking them to list the 10 most common native breeding birds found in their cities.

To test this hypothesis, Bonier and her colleagues compared and analyzed 217 urban birds found in 73 of the world's largest cities and compared them to 247 rural native bird species from 14 taxonomic orders. To be included in the study, each rural species' breeding distribution had to geographically overlap with at least one of the study cities for which an urban species within the same genus ("congener") was reported as a common native breeder.

According to Bonier, some well known city-dwellers included starlings, parrots, crows, sparrows, pigeons and doves.

The team found that urban birds had significantly broader elevational and latitudinal distributions than did their rural congeners (figure 2). Surprisingly, this pattern remained unchanged even when the team's analyses were restricted to only one avian genus or one city within the United States.

This is the first phylogenetically controlled comparison between urban and rural avian species. It suggests that urban birds demonstrate a much wider variety of behavioral, physiological and ecological flexibility than demonstrated by rural birds, which makes urban birds more likely to survive and do well in a disturbed habitat, such as that occupied by large numbers of people. Further, other studies reveal that there are similar differences between individuals within the same species -- a factor that impacts birds that are kept and bred in captivity.

This study supports other studies and predictions, those birds that are specialists with small ranges are more likely to be at a greater risk for extinction than those species that have larger ranges and that exhibit a greater flexibility in their requirements.

"In the face of global climate change and human disturbances, such as increased urbanization and deforestation, we may be able to identify species that can cope with such changes," Wingfield observed. "Then we may be able to identify the species that cannot cope with these changes, or might even go extinct in the face of increased disruption."

"Our findings aren't surprising," Bonier pointed out in email. "[B]ut [they] are still exciting (at least to me), because our study was very broad and showed a really strong pattern, suggesting that this is something that matters a lot for determining which birds fare well with urbanization, and which don't."

Sources

Bonier, F., Martin, P.R., Wingfield, J.C. (2007). Urban birds have broader environmental tolerance. Biology Letters, 3(6), 670-673. DOI: 10.1098/rsbl.2007.0349 [PDF]

LiveScience (quotes).