Goatsuckers Get Some Respect
Recent studies paint North America's little-known family of nocturnal birds in new light
Mark Brigham is a rara avis in the flock of North American ornithologists. A good clue is the Saskatchewan scientist's business card. Under the usual stuff like "University of Regina, Department of Biology," there's this line: "Nocturnal Flying Critters." In summer, most of Brigham's peers are up at dawn investigating the usual suspects, like migratory songbirds--a currently hot research subject. At dusk, when those researchers are sitting down to supper, Brigham is likely to be out in the grassy hills of western Canada looking at bats and bullbats, properly known as common nighthawks.
Bats, of course, are mammals albeit flying ones and are odd critters for a birdman to be studying. Nighthawks, on the other hand, are birds albeit curious ones. They owe their colloquial name to the erratic, rather batlike way they fly and the supposedly bullish noise they make during late-evening feeding forays.
As Brigham has found, nighthawks and their three wide-ranging North American relatives--the whip-poor-will, chuck-will's-widow and common poorwill--definitely are not hot research subjects. "I gave a talk at a meeting of 600 ornithologists and maybe a dozen people showed up," he says. "My bird talks at bat meetings generate more interest."
All four species have largely been ignored by ornithologists since John James Audubon painted dramatic portraits of these gape-mouthed bug-catchers in the early 1800s. Maybe that's because of the hours the robin-sized birds keep and their secretive nature: They're so cryptically colored in intricate patterns of browns and grays that a nighthawk nesting on the ground in broad daylight is virtually invisible. Or perhaps, as Brigham suggests, most bird students think they're too weird.
The neglect ended when Brigham, after studying the roosting behavior of big brown bats in the mid-1980s for his master's thesis, started comparing the ways insect-eating night birds and insect-eating bats make a living and cope with food shortages. "What I found," he says, "is that the similarities extend far beyond what they eat and when."
Nighthawks and their kind are the stuff of myth. They belong to a worldwide family with a scientific name, Caprimulgidae, that means "goatsucker." Superstitious goat-herds in ancient Greece saw night birds fluttering open-mouthed around their livestock and believed the birds came out at dark to drink milk from the mammals' teats. Not true! Ancient Hopi Indians called the common poorwill Hôlchoko, which means "the sleeping one," in the belief that the birds tucked themselves away in rock crevices and hibernated like bats when the desert country turned cold. True!
Most of the 70 or so goatsucker species dwell in tropical climes, and their number includes several birds in which the muted males undergo astonishing changes at courtship time. For example, the outer primaries (feathers) of the pennant-winged nightjar in southern Africa grow into fluttering streamers more than two feet long, which break off once the bird dazzles a mate.
The daylight aerial displays of the pennant-winged nightjar, which is said to be voiceless, take the place of the nocturnal arias of other goatsuckers. Indeed, the whip-poor-will, chuck-will's-widow and poorwill are named for their strident territorial calls. On bright moonlit nights, a male whip-poor-will might cry its name 16,000 times between sunset and sunrise.
Whip-poor-wills, like most goatsuckers, usually are heard but not seen: They're as silent on the wing as owls, their closest relatives on the avian evolutionary tree. Night- hawks are North America's best-known goatsuckers, as they often nest on flat, graveled roofs in towns and cities; forage in loose groups over downtown streets and woodsy neighborhoods; and rend the dusk with their loud, nasal calls. Then there is the booming sound produced by air rushing through a male nighthawk's primary feathers as he dive-bombs a roosting female, a rival for her affections or a human intruder.
Tropical goatsuckers tend to lead sedentary lives, given the year-round abundance of food in their insect-ridden haunts. But species that breed in temperate parts of the world have no such luxury. Common nighthawks, which nest as far north as the Yukon and Labrador, winter as far south as Argentina. That's one of the longest migrations by any North American bird.
Southbound nighthawk movements can be spectacular events. "There's nothing like a big nighthawk night," says Tom Gagnon, a Massachusetts bird-watcher who has monitored the birds' passage down the Connecticut River Valley for the past 20 years and once counted 3,673 nighthawks plundering a "huge bug hatch" on a single evening in 1991. In contrast, whip-poor-wills from eastern deciduous forests move a shorter distance to the Gulf Coast and Central America, as do chuck-will's-widows from pine-oak woodlands in the Southeast.
The common poorwill, a denizen of arid western lands, is an enigma. Little is known about the species' seasonal movements, but studies dating to the 1940s suggest that poorwills in the southwestern desert do not migrate. Rather, the poorwill is alleged to be the only bird in the world that hibernates during extended cold and foodless periods.
The poorwill is one of a very few North American birds with the proven ability to reduce its metabolism and fall into a state of lethargy, or torpor, for short periods. Brigham and his Regina students found that on cold spring nights the body temperature of poorwills dropped to around 40 degrees Fahr- enheit. The normal temperature of active birds is 104 degrees.
"This is the lowest naturally recorded body temperature for any species of bird," says Brigham, "and very similar to the temperature of torpid bats." The oxygen consumption of a torpid poorwill is reduced by 90 percent, and its energy use is one-twentieth that of an active bird, he explains. "A poorwill should have no problem surviving several days of bad weather without feeding."
But do poorwills really hibernate? A year ago, Brigham's answer was "probably not." Last winter, however, one of his students monitored two torpid poorwills in the mountains near Tucson, Arizona, for a month before they woke up due to unseasonably warm weather. "Chris will get a larger sample next winter," says Brigham, "but the bottom line is this: They do hibernate!"
Poorwills' use of torpor may even influence their choice of prey. Nighthawks eat mostly small insects--especially flying ants, 2,175 of which once were found in the stomach of one bird. The chuck-will's-widow, which has an immense mouth that gapes a full two inches, also is an opportunistic feeder and even will nab a late-flying hummingbird or warbler. But Brigham's work in British Columbia's Okanagan Valley showed that the diets of both poorwills and Townsend's long-eared bats are dominated by large beetles--a rich source of the unsaturated fat that is easily metabolized by torpid animals--and large moths.
Poorwills and Townsend's bats are sit-and-wait predators. Bats are not blind; they have good vision but its function is a mystery since they rely on high-frequency sound called biosonar--or echo-location--to pinpoint prey and avoid obstacles. The large eyes of goatsuckers, on the other hand, are adapted for enhanced night vision, and the birds apparently can discriminate between different kinds of insects silhouetted against twilight or a moonlit sky. "Poorwills forage like kingbirds," says Brigham. "They make four or five sallies a minute from the ground to a height of no more than 10 feet, and the flights never last longer than five seconds."
Whip-poor-wills also sally, and like poorwills they are active at night when the moon is up. Whips, however, go one step farther: They synchronize their nesting cycle so their eggs hatch during the young waxing moon. Thus the adults have the greatest amount of moonlight for foraging during the critical first two weeks of their nestlings' lives. A month later, when the young whips gain their independence, the moon again will be on its way to full.
Lunar light plays no role in the lives of bats or common nighthawks. With rare exceptions, nighthawks are active only at twilight--the hour or so between sunset and full night and between full night and sunrise. "'Crepuscular hawk' might be a better name," Brigham jokes. Nighthawks forage in daylight during stormy weather or in fog, he notes. "You'll also find parties of nighthawks feeding at night in the bright light from stadiums like Boston's Fenway Park." With only a small window of time in which to find food, nighthawks forage continuously on the wing like swifts, swallows and the big brown bats that share the Okanagan's ample insect resources.
Brigham clocked the foraging times of both nighthawks and bats and found that the birds hunted an average of 70 minutes nightly, making 18 attacks a minute on caddis flies and flying ants. Big brown bats also averaged 18 attacks a minute, mainly on caddis flies, but fed from 85 to 160 minutes a night. Contrary to long-standing belief, nighthawks do not fly open-mouthed through swarms of insects, sucking them up like some feathered vacuum cleaner. "They identify and attack individual insects," says Brigham. "You can hear their bills snap."
Goatsuckers have been a fertile field of research for Brigham and his students. Still, he admits that he's bothered by the apathy of his colleagues toward these uncommon birds. And the indifference has a serious downside. The common nighthawk as well as the whip-poor-will and chuck-will's-widow appear to be declining species, and no one knows why or to what extent. But goatsuckers do have their votaries. Like Tom Gagnon and Glenn d'Entremonte, another Massachusetts birder who censuses whip-poor-wills every June in Myles Standish State Forest to satisfy his own curiosity--and wonders why he gets 30 calling males today where there were 50 or more 15 years ago.
Night bird people know that summer evenings without the memorable sounds of whips, chucks and bullbats would be empty indeed.
Nighthawks no longer haunt the evening skies over the small western Michigan town where Field Editor Les Line watched them in awe as a boy a half century ago.