Listening to Migrating Birds at Night May Help Ensure Their Safety
On autumn and spring evenings, hundreds of thousands, sometimes millions of birds migrate across North America. Cutting-edge recording devices are capturing the tiny chips and chirps these birds make while in flight, helping conservationists plot a protected course.
Recent years, however, have brought about “an explosion of professional and amateur research in acoustic monitoring and the study of nocturnal migration,” says Jeff Wells, senior scientist for the Boreal Songbird Initiative. For decades, scientists have used radar monitoring to track large-scale movements of migrating birds. Radar has limitations, however. On a finer scale, it can’t differentiate between “targets,” such as birds, bats, and large insects. It provides no information on species, and is expensive and complicated to use in the field. By contrast, microphones calibrated specifically for recording nocturnal migrants—some of the best can capture the high-frequency note of a warbler flying 1,000 feet overhead or the hoarse buzz of a grosbeak at twice that distance—have become so affordable that avid birders are adding them to their wish lists alongside high-quality binoculars. Analyzing hours of recordings remains cumbersome, but automated sound-recognition software and sound-analysis software is now available for free or for a reasonable price. “These developments,” says Wells, “have placed this field at a new threshold.”
Most birds migrate at night. The stars and the moon aid night-flying birds’ navigation. Free of daytime thermals, the atmosphere is more stable, making it easier to maintain a steady course, especially for smaller birds such as warblers that might fly as slowly as 15 miles per hour. Cooler nighttime temperatures also help keep hard-working birds from overheating. And for birds that frequently wind up on the menu of hawks, cats, and other daytime predators, flying under cover of darkness can be a lifesaver.
While scientists aren’t certain about the reasons that migrating birds call when flying at night, there are some widely accepted theories. Research has shown that birds do it more frequently in rough weather and when navigating headwinds and crosswinds, so the calls likely help birds stick with their flocks. Communicating at night might also prevent fatal crashes. High call rates coincide with incidences of mass collisions with tall, lit structures such as high-rises and offshore energy structures. It may be that night flight signals serve as a warning to other birds about such perils.
These sounds are nothing like the beloved springtime breeding choruses. “We’re talking about cryptic little buzzes and whistles,” explains Bill Evans, director of a nonprofit called Old Bird, which facilitates acoustic monitoring. Evans was first intrigued with night-calling birds while delivering pizzas during college. In the 1990s he worked as a technician with Cornell University’s Laboratory of Natural Sounds, the world’s largest archive of wildlife audio and video recordings. Evans spent more than a decade recording and analyzing the night flight calls of eastern migrants. In 2002 he and naturalist Michael O’Brien published a CD with the recorded calls and spectrograms of 211 North American terrestrial bird species, an event that helped kick-start today’s revolutionary research. “Few of these calls are longer than a half-second,” he says, “and many are much shorter than that.”
Consider the hooded warbler. Its night flight call is perhaps one-twentieth of a second long. To the human ear, it sounds like a buzzy cricket-like chirp. “Analyze the spectrogram,” Evans says, “and it’s a completely different language. There’s a lot going on inside the sound.” The spectrogram reveals a cup-shaped call in the 6-to-7.5-kilohertz range, with a ragged modulation—“a smiley face with teeth,” Evans laughs. No other bird calls with that particular signature; it’s like an auditory fingerprint that reveals the warbler’s unseen presence above.
With enough practice, many night flight calls can be identified by ear. The dickcissel’s buzzy note is distinguishable from the buzzy one made by the painted bunting and the blue gros- beak. There’s the chestnut-sided warbler’s low, buzzy dzzew call. The swamp sparrow’s buzzy zee is distinct from the short whistle of the hermit thrush. Researchers have described this variety in an array of onomatopoeic notes: wok (black-crowned night heron), skwonk (great blue heron), creeeenk (least sandpiper), and vheeu (veery). Other birds are not yet identifiable by species but are grouped into “complexes” of calls—the buzzy zeet group of the cerulean, Connecticut, and magnolia warblers, for example, and the sibilant, rising tseet group of the Tennessee, Nashville, and black-throated green warblers.