These Lovely Birds Do More Than Sing—They Sleep Around
At dusk in North American forests, wood thrushes (Hylocichla mustelina) fill forests with a rising and falling “ee-oh-lay” song with a strange reverb. Like Tuvan throat singers, these pot-bellied brown birds have the ability to sing their own harmony and descant.
Alas, wood thrush populations have dropped 55 percent since 1966, and they’ve been identified as a species in danger of extinction. In order to conserve a bird, you need to know where it lives. That’s a tough question to answer for wood thrushes—they migrate to Central America and back each year, traveling over 4,000 km. In both their northern and southern ranges, loss of forest cover is a major contributor to declining numbers.
Some basic questions about wood thrush biology are still unanswered: Do they roost communally at night? Where do they sleep? What kinds of habitat do they sleep in? New research from The College of William and Mary details 2 years of tracking wood thrushes both night and day through coastal Virginia. Braving gunfire, the CIA, and swamps, the researchers’ nocturnal adventuring revealed at least one bit of information about the sneaky species: Male wood thrushes were sleeping around.
What’s the Radio Telemetry Frequency Kenneth?
It’s not enough to know where birds nest and what they eat. If a scientist wanted to develop a conservation plan for humans, but only studied what we did while the sun was up, a significant portion of our behavior would be missed. But how do you track a bunch of little brown birds in a murky forest? “First,” says Vitek Jirinec, lead author on the study, “catch the bird.”
The researchers caught 37 male and 9 female birds with a combination of guile, audio trickery, and really big nets. Then they attached a radio transmitter “backpack” on the birds with jewelry supplies. Radio transmitter technology has been used for decades in wildlife tracking, and is cheap and easy to acquire. (GPS trackers cost up to $465 per bird, in comparison.)
Forty-six birds might not sound like a lot, but the labor involved in radio tracking is intense. Researchers stumbled around in the forest schlepping a big directional antenna, a receiver about the size of a coffee maker, headphones, batteries, notebooks, binoculars, and assorted bird nerd gear. Each bird backpack has a unique frequency; that’s dialed into the receiver to try to find an individual bird.
Remember, half of this project was done in the dark, because the researchers wanted to know where the birds were sleeping. “I was walking for hours and hours. You would not only run into trees, but you would fall into swamps and creeks,” says Jirinec. He was assisted by 14 undergraduate students, learning the hard way that ecology field research is not for wimps.
One unexpected complication: Many of the largest bits of intact forest in coastal Virginia are federal lands. Camp Peary—the CIA training facility known as The Farm, was an unexpected destination. And a dead end. They had better luck with the Naval Weapons Station Yorktown where they were able to get security clearance to chase birds after an extensive background check. “When [we were] doing the daytime tracking, basically the back drop would be machine gun fire,” says Jirinec.
Far ranging birds were tracked down with a plane, piloted by 74-year-old Veteran pilot Caton Alexander “Captain Fuzzzo” Shermer (three Zs, with the middle Z silent, according to Jirinec). Fuzzzo flies aerial surveys for a bald eagle census yearly, so swooping low over trees in search of birds was nothing new for him. The researchers did run into a problem, though—there wasn’t an obvious way to mount their radio antennas on his tiny plane.
As is traditional in scientific endeavors, duct tape solved the problem. Of course, the radio antenna wire still had to come back into the cabin of the plane—so flights were made with the windows open. As you can see from the photos, it was a very small plane with one bantam-sized pilot and two large researchers (plus gear) stuffed inside. It cannot have been comfortable.
Ruling the Roost
Some results were expected: Female thrushes with active nests slept at home, on her nest. Paired males with nests, however, slept all over, sometimes hundreds of meters away from their nest. Mated pairs without nests slept together. The researchers hypothesized the male was sticking close to prevent extra-pair matings (a polite term for bird adultery). Males who had a nest were perhaps roaming around looking for their own bit of bird tail with their nocturnal rambling.
Using LiDAR data, a remote detection system that uses lasers and reflected light, the researchers determined that thrushes that ventured out of their home range were roosting in areas with 7 percent more vegetation. That was a significant difference statistically, and makes sense for this bird of deep forests. But is a 7 percent density difference in a forest important for conserving this species?
Julie Craves, Director of Avian Research for the Rouge River Bird Observatory, told me this was an interesting and well-done paper. But “at the risk of sounding like an alarmist, determining if wood thrush roost sites are 7 percent denser than their diurnal home ranges and managing habitat for this is like re-arranging the deck chairs on the Titanic,” she says. “The thing we have to prioritize is overall habitat preservation. Otherwise, we are losing sight of the forest for the trees, rather literally.”
In other words, these birds can’t live without forests. Worrying about a small difference in forest density is of less urgency than saving what remnants of forest we still have left in North America.
Your Cuppa Can Save Birds
Most of us can’t afford to purchase remnant forest lands, but we can make a positive daily choice to help wood thrushes and other migratory song birds: drinking “bird-friendly coffee.” The Smithsonian Migratory Bird Center certifies coffee plantations with foliage cover, tree height, and diversity suitable for birds that winter down south. Coffee grown in the shade has a richer taste, as it ripens more slowly. Here is their list of bird-friendly coffee roasters and importers.
Your choice of benign, sustainable agricultural practices can be helpful for both birds and people—and keep the beautiful songsters of summers returning home to North America for decades to come.
If you need 10 minutes of Nature Zen at your desk, here’s a longer recording of this beautiful bird.
Mismatch between diurnal home ranges and roosting areas in the Wood Thrush (Hylocichla mustelina): Possible role of habitat and breeding stage. http://www.aoucospubs.org/toc/tauk/133/1.
Stanley, et al. 2014 Connectivity of wood thrush breeding, wintering, and migration sites based on range-wide tracking. Conservation Biology, 29, 164–174. DOI: 10.1111/cobi.12352