Nearly 30 years ago, a simple question sent University of Washington researchers into the Puget Sound lowlands to look for birds. With Seattle’s population swelling and housing developments pushing into once-forested suburbs, researchers wanted to know how land conversion was impacting birds, and whether the type of development mattered.
The researchers returned to various development sites three to four times a year to log which birds they saw and heard. They found that when developments preserved native vegetation and tree cover between homes, a wider variety of bird species remained. The difference was most significant to sensitive species that tend to avoid humans – like the Wilson’s warbler and Swainson’s thrush – which fared better in conservation-minded developments.
This study highlights a dynamic that is often overlooked under the assumption that all development is bad for habitat.
“This work shows that birds care how land is developed, which gives developers a choice during the planning process. There are tradeoffs between developments designed for humans and those designed with the ecosystem in mind, but balancing the two can benefit both,” said senior author John Marzluff, UW professor emeritus of environmental and forest sciences.
The results were published May 31 in Scientific Reports.
For the study, researchers organized birds into three groups based on their adaptability. Avoiders included native forest birds such as chickadees, wrens and woodpeckers; adapters included sparrows and robins; and crows, pigeons and hummingbirds were among those classified as exploiters because they can benefit from development by visiting bird feeders and foraging on scraps.
The researchers returned to lowland sites several times a year during the spring and summer breeding season to look and listen for birds. Pairing visual and acoustic signals gave them a more complete bird census.
During the initial years of observation, when the habitat was changing but before construction began, species diversity increased at all development sites, but prolonged disturbance sent many native species into decline. This effect was more pronounced when housing density increased.
“The greatest species diversity was right in the middle years of development, likely due to an increase in habitat heterogeneity,” said lead author Jack DeLap, who completed this work as a doctoral student of environmental and forest sciences at the UW and is now an associate professor at Seattle University. “Our Pacific Northwest forests aren’t terribly diverse as forests go, but when you start to mix that up by clearing trees, it opens the canopy to another suite of species.”
As construction progressed, avoider and adapter populations declined. Avoiders showed the most uniform response, but adapters were more split – some declined while others persisted. Exploiters were present in both development types, contrary to the researchers’ hypothesis that they would fare better in denser developments.
These results come at a pivotal moment for Washington’s avian residents. Earlier this year, Birds Connect Seattle issued a report describing declines in bird abundance and diversity throughout the city. Average bird counts are down 21% since the 1990s and species richness has dropped 18%. At the same time, developers are buying up tracts of timberland to build homes.
Just last week, bidding closed on the auction of 4,833 acres of forest in the Snoqualmie Valley, which could allow construction of up to 30 new homes.
“It’s becoming less and less profitable to keep land for timber than it is to sell it for home development,” DeLap said.
Even if development compromises habitat quality, knowing which factors support biodiversity could help developers balance profitability and ecosystem health.
These results underscore the need to consider sensitive species when planning projects, and caution surveyors against assuming that all birds within a guild will behave the same way. Changing the land will change the ecosystem, bringing birds together that might not have shared habitat otherwise.
“Given that a lot of forest land will likely be converted in the coming years, I hope this will help folks recognize what they stand to gain and lose through development,” DeLap said. “There might be other configurations worth evaluating, but now at least we know what happens on the landscape in these two scenarios.”
Additional co-authors include Sarah Converse, a UW professor of aquatic and fishery sciences and leader of the USGS Washington Cooperative Fish and Wildlife Research Unit.
This study was funded by the U.S. National Science Foundation and the University of Washington Rachel Wood’s Endowed Graduate Program and James W. Ridgeway Professorship.
