Urban woodlands provide vital ‘refuelling’ stops for migrating birds

Even the smallest forest areas are valuable, say researchers
May 2010: Even the smallest patches of woods in urban areas seem able to provide food and protection for migrating birds, according to an American study. The findings are important and encouraging, as with the expansion of cities worldwide, migrating landbirds must regular pass through vast urban areas offering only very limited forest habitats.

TAGGING: A researcher fits a radio transmitter to a Swainson's thrush. They were attached with a special glue and fell off within weeks. Picture: Ken Chamberlain, Ohio State University

‘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 the university.

The researchers published two related studies: one will appear in an upcoming issue of the journal Landscape Ecology and the other is in a recent issue of The Condor. Both studies involved a secretive relative of the American Robin called Swainson's thrush.

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 91 of the migrating birds at a woodlot on the Ohio State campus and fitted them with tiny radio transmitters before releasing them at one of seven woodland sites in the area. These ranged from less than one hectare (1.7 acres) to about 38 hectares (93.9 acres) in size.

They then tracked how long the thrushes would stay in the woodlots - leaving quickly after release would suggest the site 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 per cent of the birds moved to other sites in the Columbus region.

Smallest sites met their stopover requirements
‘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.

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.'

Low-weight birds would stay to bulk up
Other factors seemed also to play a role. Later arriving birds appeared to be in a greater rush to leave so they would reach breeding grounds in time, and good flying conditions would also tempt the thrushes to leave promptly. However, birds with lower body mass tended to stay longer, 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. Furthermore, birds preferred to be away from the edges of woodlots, so those staying on smaller sites moved around less, indicating they were more restricted in the area where they could forage for food.

Researchers cautioned that this study was of just one species, so it was impossible to confirm similar results would be achieved with different birds, but it certainly seems to be encouraging news for anyone trying to save green lungs in urban areas.

‘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.'