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In the US, urban development is causing monogamous birds to divorce

Breeds of birds which generally tend towards long-term monogamous relationships feel the strain of the city just as much as anyone else. A recently-published study showed that urban development in North America is encouraging more monogamous duos to “divorce” one another, as they’re forced to flee encroaching urban environments.

The conclusions were based on research undertaken by scientists at Washington University, Seattle, alongside others. It was published in the journal PLOS ONE under the title “Breeding Dispersal by Birds in a Dynamic Urban Ecosystem”.

The scientists differentiated between “sensitive forest” species and those “tolerant of suburban lands”. They termed the first group, which consisted of the Pacific wren and Swainson’s thrush, “avoiders”; the latter, which consisted of four other species (song sparrow, spotted towhee, dark-eyed junco and Bewick’s wren), the labelled “adapters”.

All six species studied were considered “socially monogamous”. But new urban developments appear to require some species of birds to abandon their old homes and find new mates, when they otherwise would not have done.

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“What we saw with those two avoiders, in the areas that were being developed, where trees were literally being cut down and roads put in during our study, we had birds that were forced to move and divorce by that activity. And those birds suffered a reproductive cost,” explains John Marzluff, a professor of wildlife science at Washington University.

This was particularly problematic for “avoiders” who “divorced” at a steady rate, the study found.

“You see an actual bump in success in the adapter group in the sparrows and the flexible species. But you don’t see that in the avoider species,” one coauthor, David Oleyar, a senior scientist at HawkWatch International, told Smithsonian Magazine.


Between 2002 and 2010, the scientists mapped 6,363 territories of a set size, assessing the locations of individual adults and adult birds’ locations over two consecutive years. They studied hundreds of birds to form their conclusions.

“By far the hardest piece of bird biology to get a hand on is movement. How do they move between successive breeding attempts,” Marzluff explains. “The reason we were able to do it was that we marked thousands of individual birds at the sites we studied and then monitored them over a 12 year period.”

Defining “divorce” among birds has itself had a difficult history. “I remember publishing a study when I was a graduate student back in the 1980s,” he said. “Boy, we got a lot of static for trying to use that term in that paper. So we took it out”

But it’s since been used on a regular basis, he adds. “It’s simply when an individual gets a new mate at the same time as former mate is still alive and in the area.” Mazluff theorised that, while the sample size wasn’t quite big enough to examine gender differences, if could be that males try to attract females on territory which the female deems unsuitable. “It’s the males that stay longer,” he said.

Urbanisation isn’t always bad news for birds, however, and can even lead to an increase in the diversity of bird species. “On the fringes, where we mix things up a lot, have a lot of different land cover – diversity is high there,” he said, adding there had been an increase in bird species in inner London over the last 100 years.

While the study revealed a previously unnoticed impact of urbanisation on certain species of birds in North America, the impact could be different among different species and in different cities. There have been no such studies conducted in the UK to date, but there is significant potential for the research to be expanded.

Professor Richard Gregory, from the RSPB Centre for Conservation Science, says that the study “casts new light” on the potential impact of urbanisation “and why there might be winners and losers” among birds.

“The costs of breeding dispersal, with lower reproductive success and survival, mean that some more sensitive bird species will decline in the face of urbanization. Yet less sensitive birds might thrive,” he says.

“We’d very much encourage similar studies in the UK because our understanding of the urban environment is surprisingly poor.”

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