IN THE FIELD: Migrating songbirds, calls in the dark

Sheffield — This time of year, if you go outside at night and listen, you will hear tiny, soft “peeps” in the sky above you. These are the flight calls of migrating songbirds, en route from northern breeding grounds to the neotropics of Central and South America. They migrate at night, needing daylight hours for feeding. Mixed-species flocks of songbirds — warblers, vireos, flycatchers, thrushes, and the like — pass overhead in late summer and fall, undetected except for these sprinklings of their calls in the dark.

But these seemingly nondescript flight calls are actually species-specific — expert birders could distinguish the split-second, monosyllabic flight call of an American Redstart from that of a Yellow Warbler. Moreover, acoustic monitoring stations can now decipher these flight calls and reveal the species composition of a migrating flock.

Technology can indeed help us unravel the mysteries of bird migration. Cornell Lab’s monitoring program called BirdCast uses radar, weather, and citizen science data to forecast bird migration. Migrating birds can actually be picked up by weather radar. On the Web site of BirdCast (birdcast.info), one can watch a Doppler of birds spreading across the map of North America.

Regional migration analyses list species increasing and decreasing in numbers. Charts for individual species track their timing and numbers in a given region. A national forecast map depicts wind directions and precipitation events. Northerly winds, for example, facilitate fall migration, and storms are likely to ground birds.

A bay-breasted warbler. Photo: Maha Katnani

The most recent regional summary of migration in the Northeast tells us that in the past week, numbers have spiked due to northerly winds and cooler temperatures. The East can expect “moderate to heavy flights featuring Ruby-throated Hummingbird, Acadian Flycatcher, Blue-headed Vireo, Philadelphia Vireo, Gray Catbird, Brown Thrasher,Tennessee Warbler, Bay-breasted Warbler, American Redstart, Summer Tanager, and Baltimore Oriole.”\

The site posted videos taken at the Tribute in Light memorial in Manhattan, showing these surging numbers of migrating birds swarming in the light beams like insects. The Tribute occasionally turns the upward-pointing lights off when the birds get caught in them too long. Someone estimates there are “conservatively 5,000 birds in the beam at a given time.”

In the morning, migrants drop into the trees — a “fallout,” if there are so many birds you don’t know where to look. I’ve never actually used BirdCast to plan birding excursions, but it is tempting. Even without optimizing your timing, however, migration season is an exciting time to look for birds. While we may be able to predict which species, how many, when, observing birds in the wild is still at heart about the sheer pleasure of their unpredictability.

Hiking recently in Beartown State Forest, I came to a clearing in the woods — a swampy beaver pond. The forest had been quiet and still, but there in the open, birds were astir. Cedar Waxwings and woodpeckers flew back and forth between the dead trees. A flash of yellow caught my eye in the distance, some kind of warbler — the tiny, colorful songbird that is typically the highlight of migration for birders.

However, warblers like to be high in treetops. They never stop moving. Identifying one is often a matter of piecing together incomplete, split-second glimpses of their dulled fall plumages that are trickier to distinguish in the first place. From where I sat on a log at the edge of the beaver pond, I could tell that several of the birds suddenly quivering in the trees were Black-throated Green Warblers. One sang a half-hearted version of its bright spring song, as if the songs, like the colors, were toned down to a muted, carefree version. Another looked mostly grayish; another enticed me as a possible Bay-breasted Warbler, a species only seen during migration, as it breeds far to the north of us.

I lay back on my log with my binoculars, relieving the neck strain usually required for bouts of warbler-watching. The light was terrible, overcast skies silhouetting the birds. Often all I could see was the undersides of their tails. Conveniently, the undersides of warblers’ tails can be distinctive enough to provide important clues for ID. Some are tan and yellow. Some are white outlined in black. Actually — how did I never notice this? — most are white outlined in black. But the exact ratio and shape of the white and black varies. A little.

A week later I lay reading on the grassy summit of Jug End Mountain, quiet but for the occasional raven. Suddenly, the chatter of chickadees. Often this is a sign that warblers are present, too, as birds move through their feeding areas in mixed-species flocks. And right on cue, they appeared. A Black-throated Green Warbler, a Black-and-White, like a parade to which numerous species had sent representatives. Two Magnolia (sporting a truly distinctive under-tail, by the way: half black and half white). They zipped around me for a while, barely noticing me there on the ground, then moved on, and the mountain resumed its quiet.