Election Section

Thrush? Modest Coat Belies Brilliant Skills

By JOE EATON Special to the Planet, Photo by: Peter LaTourrette
Tuesday May 18, 2004

Here’s a suggestion: Take an early morning or late afternoon walk in Tilden Park, along the trail that starts at the Lone Oak picnic area and follows Wildcat Creek. This time of year you’ll be surrounded by birdsong—black-headed grosbeaks, warbling vireo s, Wilson’s warblers—but one voice in particular will stand out. The performance may start with a soft “whit,” likened by some listeners to the drip of water into a bucket. Then the Swainson’s thrush, newly returned from its Mexican and Central American w intering grounds, will get serious. From somewhere in the oaks and bay laurel will come what Alexander Skutch, who has heard these birds warming up in Costa Rica, called “slender liquid spirals of song.” The smooth notes flow in an ascending scale, with a reedy effect as the pitch rises. If you’re lucky, you’ll hear several males with adjacent territories matching voices, the song-duels echoing off the cliffs that rise above the creek. 

For my money, these modestly plumaged olive-brown birds are the bes t singers on the West Coast. Some rank the hermit thrush higher, but we seldom get to hear those outside the Sierra forests and patches of coastal conifers. The Swainson’s thrushes check in late—April 15 is an exceptionally early date for Berkeley—and I’v e always thought of them as the last act in the spring chorus. Although they prefer riparian growth, they can turn up in unexpected places. A couple of years ago one chose a territory in my neighborhood, and I could hear it most mornings as I walked to th e BART station. That voice made the grim daily commuting ritual a lot more bearable. 

The thrushes may have spent the winter anywhere from the West Mexican states of Nayarit and Tamaulipas down to the mountains of Bocas del Toro in Panama. Some frequent t he fruiting trees that shade coffee plantations (which, properly managed, can be havens for neotropical migrant birds). Others choose moist evergreen or deciduous forests. They eat a lot of berries and fruit in winter, augmented by the insects flushed from the forest floor by swarms of army ants. “The thrush hovers about the outskirts of the swarm,” wrote Skutch, “and I have not seen it dash into the midst of the fray to seize a fugitive, in the manner of tropical birds more adept at this kind of hunting.” 

How have they found their way back from these tropical settings to the familiar woods of Tilden Park? A study recently reported in Science sheds new light on how Swainson’s thrushes—and by extension, other migratory birds—orient themselves in flight. M ig ration researchers have speculated for years that birds use a complex set of cues to set their courses; steering by the stars or the moon, or using rivers, mountains, and other geographic features as route markers. But it’s become clear since the 1970s, w hen pioneering work was done on European robins, that magnetism is the single most important source of guidance for the vast numbers of birds that migrate by night. 

Birds, it turns out, may be able to “see” the Earth’s magnetic field; that’s been verifie d for Savannah sparrows and Australian silvereyes, at least. (A bird’s-eye view is different from ours in significant ways; they’re adapted to see into the ultraviolet range.) In bobolinks, the ophthalmic nerve, which feeds visual input to the brain, is s ensitive to magnetic stimulation and contains traces of iron oxide. What birds perceive is not the magnetic field itself, but the plane of polarized light caused by the setting sun. They seem to use this to calibrate their internal compasses. Workin g with captive Savannah sparrows, biologist Frank Moore found the birds became spatially disoriented if he placed mirrors around their cage to shift the apparent position of sunset. 

Until recently, though, no one had taken the study of magnetic orientation outside the lab. William Cochran, who had done preliminary work with the Illinois Natural History Survey, teamed up with Martin Wikelski of Princeton and Henrik Mouritsen of the University of Oldenburg in Germany to study the Swainson’s thrush and its close relative, the gray-cheeked thrush. They trapped northbound migrant thrushes in Illinois and glued lightweight radio transmitters to their feathers. At dusk, the biologists used portable equipment to expose the captives to false magnetic fields, rotated 80 degrees to the east of true magnetic north. Then they set them free. 

The thrushes flew through the night, exchanging their flight calls: “a single, mellow or plaintive and far-carrying whistled queep,” by one account. In a 1982 Oldsmobile crammed with tracking devices, the scientists followed. Several times the battered vehicle with its roof-mounted antenna was pulled over by suspicious Midwestern cops. 

What Cochran and his colleagues found was that on the first night after their release, the thrushes flew skewed courses, veering to the west. But on the following night, they resumed their normal northward flight heading. One view of the true magnetic field allowed them to recalibrate. 

Although the Swainson’s thrushes that nest in California don’t cros s the equator, the Cochran study may help explain how trans-equatorial migrants, which can’t tell magnetic north from magnetic south, avoid getting turned around. Using only a magnetic compass would risk confusion, but the sunset calibration could help ke ep the birds on track. They may also have an onboard clock that corrects for changes in latitude and season. 

At some level, knowing all this—how these tiny-brained birds navigate the immensity of the night—is just as satisfying to me as hearing t heir songs on the breeding ground. Letting the mystery be is all well and good, but for some of us there’s an almost visceral pleasure in learning how another piece of the wonderfully complex natural world really works.