We’re never too long between migrations here. Most of the northbound birds have reached their destinations by now, even the chronic laggards like the yellow-billed cuckoo. But this is just a pause in the action between the first wave of shorebirds come through on their way south, starting next month; then the rufous hummingbirds tracing the Sierra southward, and the early-migrant passerines, and the raptors and the waterfowl.
So June is not such an odd time for a migration column, inspired by recent research at Ohio State University on the use of stopover habitat by the Swainson’s thrush. These birds occur in California as well, although ours are a different subspecies. They’re furtive creatures; in their breeding season, you’ll hear a dozen for every one you see. (Gifted singers, too; one of the most extraordinary voices in the chorus.) The Swainson’s thrush looks a lot like its bolder relative, the hermit thrush, but it’s more uniform in coloration and sleeker in shape. The two rarely overlap; by now our wintering hermits are back in the mountains where they nest.
The Ohio State study was an attempt to define this bird’s minimum habitat requirements when it touches down for a breather during migration. It’s a forest species: nests in northern boreal or western riparian forest, winters in tropical rainforest, seeks out forest or forest fragments as stopovers. It’s been a long time since Ohio had great unbroken swathes of deciduous woods. The thrushes now have to settle for something less, including remnant forest patches in urban areas. The question driving the research: how small can those patches be without losing value to the birds?
It look a while for scientists to appreciate the value of stopover habitat. The tendency was to focus more on breeding grounds and wintering grounds. For a handful of birds, like the bar-tailed godwits that migrate directly from Alaska to New Zealand, the issue is moot: they just don’t stop. That kind of extreme migration. requiring special physiological adaptations, is beyond most birds, though. Typical migrants need to rest and refuel somewhere along the way.
Birds are attracted to some stopover spots by seasonally abundant resources. The phalaropes and grebes that cover Mono Lake in the summer, gorging on brine shrimp, are on their way south; the phalaropes will end up in the southern oceans, consorting with whales. Northbound surf scoters time their journey to match the spawning runs of herring; the ducks are after the nutritious roe, not the fish themselves. Another kind of spawning event draws migrating red knots and other shorebirds to Delaware Bay to feed on horseshoe crab eggs. Overexploitation of these ancient arthropods—not true crabs, they’re more closely related to spiders—has put a major knot population at serious risk.
Migrating Swainson’s thrushes are looking for food too, insects in their case, as well as shelter from predators. In the Ohio State study, professor Paul Rodewald and postdoc Stephen Matthews, netted 91 thrushes at a woodlot on campus in May and early June from 2004 through 2007. The birds were equipped with radio transmitters, each weighing two-thirds of a gram, which were glued to their back feathers. Each bird was then released in one of seven forest fragments in the Columbus area, ranging from just under two to 94 acres in size.
After that kind of trauma, you’d think the birds’ first priority would be getting the hell out of Columbus. They didn’t, though. About a quarter of the thrushes released at the two smallest sites moved to larger forested areas. Most of those at the five largest sites stayed put until they headed north again, an average of four days (extremes of one and twelve.) Except for the birds that traded up to a larger woodlot, length of stay didn’t correlate with size of site.
Other findings: thrushes with lower body mass when captured tended to stay longest, bulking up before resuming their flight. Those caught late in the season had shorter stays, as if they somehow knew that time was running out: got to get back to Manitoba before all the good territories are taken.
“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 Mathews, in a press release. Small sites can still be valuable: “"If our study sites differed strongly in habitat quality, we should have seen differences in how long the birds stayed. 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."
It was also welcome news that Swainson’s thrushes are more flexible in their stopover habitat requirements than had been thought. Flexibility is a good thing. Let’s hope we can be flexible enough to preserve those patches of urban forest.