Ohio State University EurekAlert 18 May 10;
COLUMBUS, Ohio – Even tiny patches of woods in urban areas seem to provide adequate food and protection for some species of migrating birds as they fly between wintering and breeding grounds, new research has found.
The results are important because, with the expansion of cities worldwide, migrating landbirds increasingly must pass through vast urban areas which offer very little of the forest habitats on which many species rely.
"The good news is that the birds in our study seemed to be finding enough food in even the smaller urban habitats to refuel and continue their journey," said Stephen Matthews, co-author of the study and a post-doctoral researcher in the School of Environment and Natural Resources at Ohio State University.
Matthews conducted the study with Paul Rodewald, an assistant professor of environment and natural resources at Ohio State.
The researchers published two related studies: one will appear in an upcoming issue of the journal Landscape Ecology and the other appeared in a recent issue of The Condor.
Both studies involved a secretive relative of the American Robin called Swainson's Thrush (Catharus ustulatus). Swainson's Thrushes winters mainly in Central and South America, and travel through the eastern United States to their breeding grounds in the boreal forests of Canada.
The researchers captured up to 91 Swainson's Thrushes at a woodlot on the Ohio State campus while they were migrating through Columbus in May or early June, 2004 to 2007. They then fitted them with tiny radio transmitters and released them at one of seven wooded sites in the Columbus area. (The radio transmitters were glued to back feathers and naturally fell off within a few weeks.)
The sites had forest sizes that ranged from less than one hectare (1.7 acres) to about 38 hectares (93.9 acres) in size.
Using the radio transmitters, the researchers tracked how long the thrushes would stay in the woodlots where they were placed. If they left soon after release, that would suggest that the sites did not provide the food and habitat that they required.
Results showed that at the five largest release sites, all the birds stayed until they left to continue to their migration north. At the two smallest sites (0.7 and 4.5 hectares), 28 percent of the birds moved to other sites in the Columbus region.
"The fact that a majority of the birds stayed at even our smallest sites suggests that the Swainson's Thrushes were somewhat flexible in habitat needs and were able to meet their stopover requirements within urban forest patches," Rodewald said.
The study revealed that the birds stayed at each site from one to 12 days, with the average being about four days. There was no difference in how long the thrushes stayed across the seven sites.
"If our study sites differed strongly in habitat quality, we should have seen differences in how long the birds stayed," Matthews said. "The fact that the stopover duration was similar suggests that all the sites were meeting the needs of the thrushes as they prepared for the next leg of migration."
The study did find that the later the calendar date, the shorter the thrushes stayed at the sites. That may be because the later-arriving birds would be in more of a rush to reach their breeding grounds, Matthews said.
Weather was also a factor: birds tended to leave the sites when winds were light, following a drop in barometric pressure.
Birds also tended to stay longer if they had lower body mass, suggesting they needed to bulk up more to continue their journey.
While nearly all sizes of woods appeared adequate for the thrushes, they still seemed to prefer larger forested areas, the study revealed.
In one of the studies, the researchers found that in the larger urban woodlots, the thrushes would stay farther in the interior and not get as close to the forest edge. The birds also moved less during a three-day period in the smaller sites, indicating they were more restricted in the area where they could forage for food.
The researchers cautioned that this study was done with just one species, so it is impossible to say whether the results will apply to other species. But the Swainson's Thrush is one of the more forest-sensitive species, so the fact that it could make do with even small, fragmented woodlots is encouraging.
"These findings suggest that remnant forests within urban areas have conservation value for Swainson's Thrushes and, potentially, other migrant landbirds," Rodewald said.
"Obviously, larger forest patches are better, but even smaller ones are worth saving."
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The study was funded by the Ohio Division of Wildlife.
Urban trees 'help migrating birds'
Mark Kinver BBC News 21 May 10;
Even a small urban forest can help migrating birds, a study has said.
US researchers found that birds used the patches of greenery to rest and refuel in the middle of their journey between winter and breeding sites.
The scientists gathered the data by fitting tiny tags to thrushes, which recorded the birds' movements.
Writing in the journal Landscape Ecology, they added the findings were important because the world was becoming increasingly urbanised.
"With the expansion of urban areas worldwide, migrating songbirds increasingly encounter fragmented landscapes where habitat patches are embedded in an urban matrix," wrote co-authors Stephen Matthews and Paul Rodewald, landscape ecologists at Ohio State University in the US.
"Yet, how migrating birds respond to urbanisation is poorly understood."
Unique data
In order to learn more, the pair attached small tags to more than 100 Swainson's thrushes (Catharus ustulatus) and managed to gather data on the movements of 91 birds during the four-year study.
"The tags provided us with a unique data-set on migration during a stopover," Dr Matthews told BBC News.
"We were able to record how long the birds were staying, and we found that it was not necessarily the forest size that was influencing the birds.
"Instead, they were responding to internal factors, such as [the amount of] fat reserves they had."
He added that the importance of the urban tree cover was highlighted when they examined the area the birds were using to forage for food.
"We started to see the importance of these smaller forest patches.
"The birds ability to search around was limited by the extent of these forested areas."
The study examined seven urban forests, the smallest of which was an arboretum that covered less than one hectare.
"It seemed that the birds were able to utilise these small forest patches during their stopover," Dr Matthews observed.
"In five of the sites, the birds never left the forest patch... and were not exploring in folks' backyards."
The team collected the data by using radio-transmitter tags.
"The tags were very small, weighing an average of 0.66g, and we glued them to the back feathers of the birds using a very mild adhesive," he explained.
"The battery life only lasted a couple of weeks, so the transmitter just fell off after a short while."
Dr Matthews said the tiny tags emitted a signal that they could pick up via a hand-held receiver and record the birds' locations.
"We recorded the GPS location, and then we would go back and do the analysis [of the birds' movements] later. It would have been nice if it was all automated, but there was a little bit of fieldwork involved.
The researchers said that migration had been identified as a critical period in the annual cycle of migratory birds.
"Within migration, land birds spend up to 90% of their time resting and regaining energy at stopover sites, making habitat a key component," they explained.
"This information is necessary to evaluate the conservation importance of urban habitats for stopover migrants."
"These findings suggest that remnant forests within urban areas have conservation value for Swainson's thrushes and, potentially, other migrant land birds," Professor Rodewald said.
"Obviously, larger forest patches are better, but even smaller ones are worth saving."
