Study: suburban development forcing some songbirds to divorce, flee

A new study finds that urban sprawl is kicking one group of songbirds, called "avoiders", out of their territory, forcing divorce and stunting their ability to find new mates.

The findings, published in the journal PLOS ONE, were the results of research for about 10 years by John Marzluff, a professor of wildlife science in the University of Washington's School of Environmental and Forest Sciences, and a number of graduate students.

By identifying and monitoring hundreds of individually marked songbirds from six common species found in suburbs around Seattle, the largest city in Washington state, the researchers tracked bird activity in three types of landscapes: forested preserves, already developed suburban neighborhoods and neighborhoods transitioning from forest to subdivision.

They were able to tell when a bird relocated, broke up from its mate or stayed put year to year, as bands were placed around the birds' legs and sightings of mated pairs and nest locations were mapped.

Avoider birds are species that are known to decline in response to urbanization, for example when forested areas are removed for developments. In the U.S. Pacific Northwest, two avoiders are the Pacific wren and Swainson's thrush, birds that are generally shy of humans and rely on groundcover and brush such as fallen trees, root balls, shrubs and ferns for breeding.

Monogamous birds will "divorce" their mate and move to a new territory if they have a reason to. Maybe they miss a season to reproduce because of a poor partner, and moving is beneficial because ultimately it increases their reproductive success.

But for sensitive species, the opposite is true when movement is forced. The manicured yards of many suburban neighborhoods often do away with native habitat, and these species must flee to be able to nest and mate.

"The hidden cost of suburban development for these birds is that we force them to do things that natural selection wouldn't have them do otherwise," Marzluff, lead author on the study, was quoted as saying in a news release from University of Washington on Tuesday. "Because development requires that these birds move, we force them to abandon the places they selected and go elsewhere, which often entails finding new mates when they wouldn't have otherwise."

When forced to move, the avoider birds largely failed to reproduce again for at least one year after relocating. The whole transition to a new home and often a new partner might cause a bird to lose half of its breeding years.

"These birds don't like to move once they have established a territory," Marzluff said. "But when it comes to having enough food and safety for a nest, and being able to attract a mate, that's when things get tough. That's probably when they decide to move." Endit