At nightfall in spring and summer, a male Common Nighthawk puts on a fabulous show. He soars and swoops and makes a nasal peent call. Then, he nose-dives and flexes his white-striped wings downward. As the wind rushes through the largest feathers at the bird’s wingtips, it makes a roaring noise, like a vehicle speeding past on a highway. “They call it a boom, but that’s not what it sounds like. It sounds more like a whoosh,” says Rebecca Suomala, leader of Project Nighthawk at New Hampshire Audubon, which is unaffiliated with the National Audubon Society. “It is an amazing display.”

But for all their splendor in the sky, nesting nighthawks on the ground keep a low profile. A female’s main defense is camouflage—she’ll lay her eggs right on the dirt and park her body on top of them, rarely moving when a predator comes by.

While their cryptic coloring may help deter foxes and raccoons, it also makes things very difficult for scientists trying to find their nests. But now, new research could make that job considerably easier and help strengthen efforts to conserve nighthawks.

The findings, published in Ornithology, confirm what some nighthawk researchers had long suspected, but not formally tested (an idea the paper’s title calls “The Big Boom Theory”): The sound is a territorial signal the birds use to mark their nest sites. By tracking 21 male Common Nighthawks in northeastern Alberta, researchers showed that the areas where each bird performed the wing-boom had barely any overlap. What’s more, the birds were significantly more likely to do this display closer to their nests.

For scientists studying nighthawks, this new insight dramatically narrows the area they have to search for a mother and chicks, which could make field work during dark, sweaty, buggy summer nights much easier. “That wing-boom is restricted to a pretty small territory,” says Elly Knight, a postdoctoral researcher at the Smithsonian Migratory Bird Center and lead author on the paper. While nighthawks hunt for flying insects across a range that can be larger than 40 square miles, they boom in much smaller areas of about 25 acres, according to the study. By observing a nighthawk, researchers can record where it wing-booms the most. “And you can target your searching to that area,” Knight says.

More importantly, confirming the link between wing-booms and nest sites points the way toward better understanding and protecting this still-mysterious species. Despite their wide range and familiarity, Common Nighthawks haven’t been thoroughly studied, in part because it’s so hard to find their nests to gather basic information, such as determining breeding success. Research has shown that nightjars, the family that includes Common Nighthawks, choose their nest location based on how well the surroundings match their own individual plumage. “It’s a big deal for them,” Knight says. “You could be two feet away from the nest and not know it.”

The new findings could help prioritize areas for conservation by identifying habitats that are most useful to nesting nighthawks, says Elora Grahame, a Ph.D. candidate at the University of Guelph who was not involved with this paper. “If you have booming going on at a site, theres a pretty good chance that there is breeding happening,” she says. “That would mean that it would be a prime place to put in some conservation efforts.”

Nighthawks are federally listed as threatened in Canada, and they’re rapidly disappearing from several northeastern states. Birds that eat flying insects are among the fastest-declining groups in North America, and Common Nighthawk numbers have plunged by around 60 percent over the past half-century. Suomala, who was not affiliated with the study, and Knight say that habitat loss, pesticide use, and climate change might all play a role.

With this new understanding of how the wing-boom functions, scientists can gain a better idea of population trends in this declining species. In songbirds, the song is a territorial signal that scientists use to estimate population trends or density. Though it’s not a vocal sound, the wing-boom functions the same way: It represents an exclusive territory, telling researchers how many nighthawks might be breeding in a region. “Part of why I published this paper was because I intended to use it as a tool for these other applications,” such as estimating population size, Knight says. The wing-boom is “actually pretty fundamental to how we research nighthawks.”

The study may prove useful for other species, too. Along with nightjars, birds such as woodpeckers, hummingbirds, snipes, and grouse make non-vocal sounds that might also be associated with territory, or with finding a mate. The nighthawk findings could bolster efforts to ensure that all of these birds—and their booming, drumming, and humming—fill our skies well into the future.

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