The hidden cost of development: some birds divorce, pack up, move out — with years of breeding productivity lost.

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The suburbs are no love nest, at least not for two of the region’s most beloved songsters, the Pacific wren and Swainson’s thrush.

Instead, when their forest homes are cleared for development, these birds are forced to divorce, flee and search for a new home and mate.

The disruption is so lasting, these birds can lose as much as half their breeding years as they struggle to start over, reported John Marzluff, professor of wildlife science at the University of Washington’s School of Environmental and Forest Sciences, in a new paper published Dec. 28 in the journal PLosONE.

Marzluff dubs Pacific wren and Swainson’s thrush “avoiders” because they are the species most affected by suburban development. They need ground cover, brush, fallen trees, root balls, shrubs and ferns for their nests — not the bald, tidy yards of the average suburban tract home.

To keep these birds in our midst as the region grows, it will be necessary to retain our nearby reserves of forests where the displaced avoiders may make a new home, and those already there may sustain viable populations, Marzluff wrote.

But other birds, which Marzluff called “adapters” or “exploiters,” will tolerate and even thrive around human development. Song sparrows, spotted towhees, dark-eyed juncos and Bewick’s wrens all can live quite successfully in the edges of backyards, small greenbelts and even bird boxes, Marzluff and his team discovered.

To maintain a habitat for more than 70 bird species, John Marzluff, professor of wildlife science at the University of Washington, preserved an acre of native forest in his suburban backyard off Highway 9. (Steve Ringman / The Seattle Times)

The findings were the fruit of more than 10 years in which Marzluff and many of his graduate students monitored six common ground and shrub-nesting bird species found in Seattle-area suburbs. They observed the birds in three types of landscapes: forest preserves, already developed suburban neighborhoods, and new developments built where there used to be forest.

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Starting in 2000, the researchers eventually banded hundreds of individual birds and mapped the birds’ travels, to determine their fate in the landscape around them, and document how urbanization influenced bird breeding and dispersal.

Researchers were able to tell over time if the birds relocated, broke from their mates, or stayed put from year to year.

The varied thrush is one of the many winter birds that can be attracted with a backyard feeder. This thrush is feasting on seed in Maltby, Snohomish County, during a winter snow shower. (Steve Ringman/The Seattle Times)
The varied thrush is one of the many winter birds that can be attracted with a backyard feeder. This thrush is feasting on seed in Maltby, Snohomish County, during a winter snow shower. (Steve Ringman/The Seattle Times)

“Development forces the birds to move, forcing them to abandon the places they selected and go elsewhere, which often entails finding new mates, when they wouldn’t have otherwise,” Marzluff said in a statement.

That could mean lower populations of avoiders over the long term, as pair bonds are broken by lost habitat and development refugees lose years of reproductive success.

“Conservationists interested in sustaining sensitive species in urbanizing regions have a difficult task ahead of them,” Marzluff said. “Our results emphasize the importance of nearby reserves where displaced avoiders might find a place to breed (and possibly adapt to a more urban life) and existing sensitive species might be able to continue to reproduce at sustainable levels.”

Marzluff has long been fascinated with birds living their extraordinary lives in and amid one of the country’s fastest-growing regions.

In his most recent book, Welcome to Subirdia (Yale, 2014) and in the video he created to go with it, Marzluff explains homes for people and for wildlife don’t have to be in conflict.

John Marzluff, a professor of wildlife science with the University of Washington’s School of Environmental and Forest Sciences, shows off a suburban environment where birds like to live. (Steve Ringman/The Seattle Times)
John Marzluff, a professor of wildlife science with the University of Washington’s School of Environmental and Forest Sciences, shows off a suburban environment where birds like to live. (Steve Ringman/The Seattle Times)

At his own home, just a stone’s throw from busy Highway 9, as traffic hums in the near distance, there are other sounds, too: the quiet conversation of an Anna’s hummingbird, in a berry-studded shrub. Meanwhile a dark-eyed junco was busily at the seed Marzluff had put out in his backyard. A red squirrel sat in the feeder, chewing with gusto.

A multitude of other birds feasted too, during a December snow shower: varied thrush, fox sparrow and more.

Suburbia can teem with busy animal lives as long as food, cover and safety are all present for the birds, amphibians and mammals that will use it, Marzluff said, embarking on a stroll through the scant 2 acres of woods behind his home.

Even a few acres of forest, such as Marzluff has at his suburban lot, can help. “Phst! Phst! Phst!” said Marzluff to the Pacific wren scuttling about in the salal near his driveway.

The dark-eyed junco is one of the Seattle-area bird species that has adapted well to suburban development.Otherspecies, such as theSwainson’sthrush and Pacific wren, lose years of breeding productivity when forced from theirforest home by clearing and development. (Steve Ringman/The Seattle Times)
The dark-eyed junco is one of the Seattle-area bird species that has adapted well to suburban development.Otherspecies, such as theSwainson’sthrush and Pacific wren, lose years of breeding productivity when forced from theirforest home by clearing and development. (Steve Ringman/The Seattle Times)

While it is no wilderness redoubt, Marzluff has seen coyotes, salamanders, frogs and a multitude of birds on and near his property just beyond the whizzing traffic.

Keeping the trees, and even topping one to create a snag, was his way of sheltering as many birds as possible. The blank industrial lawns cultivated by neighbors are nearly devoid of birds, he noted.

“This is the kind of thing to leave,” Marzluff noted, walking past the brush he has piled shoulder high over the years, moldering and composting in the woods, providing the thickety-thick tiny lives need.

“Most people chip and haul this away or burn it, but Pacific wren will hide in here.”

The birds are seeking insects that also can shelter in the brushy material, as well as the understory. Salmonberry is prized by Swainson’s thrush and other birds relying on the cover and food it provides.

Salal is a good plant for birds, as is sword fern — which should be allowed to pile up its cinnamon-colored fronds year by year, creating a snug, secure nesting spot.

“It doesn’t take much,” Marzluff said. “Some salal, sword fern and salmonberry can go a long way.”

His is a more than professional interest: Marzluff enjoys the daily drama of all the lives around him, in the mix of habitats a suburban setting can provide.

“You get the most diversity, with the adapters and exploiters able to utilize changing landscapes.

“This is my backyard, but it’s the birds’ backyard, really.”

The fox sparrow is a wintermigrant to the Puget Sound region that enjoys a birdseed feast. (Steve Ringman/The Seattle Times)
The fox sparrow is a wintermigrant to the Puget Sound region that enjoys a birdseed feast. (Steve Ringman/The Seattle Times)