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.
Bricklin, the Fordham graduate student, is working to tease apart those questions. She monitors bird activity in two research sites—the suburban Mianus River Gorge Preserve and the Bronx Zoo, where her recording equipment is deployed on the World of Birds exhibit building. In the Bronx, birds seeking to refuel during migration are exposed to all the stressful elements that bedevil human city dwellers—from sirens and helicopters to car horns and crowds—plus dogs, cats, bright lights, and more. By capturing and testing birds for blood levels of corticosterone, a hormone released during stressful situations, Bricklin is looking for clues into how birds stopping in city parks respond to stress. The first step, she explains, “is to figure out the differences between which species are stopping at each site and which are flying over. That would be impossible without these new acoustic monitoring methods.”
Elsewhere researchers are studying migrating birds in less urbanized areas. In the North Sea off the coast of Germany, which migrating birds cross as they pass from Scandinavia through the Netherlands, German researchers recorded the calls of Eurasian blackbirds, redwings, song thrushes, and about a hundred other bird species to study what weather conditions prompt migrating birds to congregate at illuminated offshore wind turbines and other structures. Using specialized microphones shielded from gull poop by a stainless steel cover, the biologists ran the data files through software that filtered out the background noises of wind, waves, and rain. The study showed that night-migrating birds called more frequently during nights with high collision rates. In the future, these scientists say, acoustic monitoring systems could serve as a real-time automated collision risk alarm system.
That was one of the goals of an emergency monitoring effort recently completed in Pennsylvania. Last September Audubon Pennsylvania, New York City Audubon, and the American Bird Conservancy learned of a massive public art display planned for Philadelphia that involved 24 robotic searchlights sweeping the night sky for 25 nights during the peak of fall migration. Working frantically in the days before the project’s opening, conservationists convinced the architects to make design changes and to add an emergency alert system in case the lights appeared to confuse migrants. Clark and other researchers assembled an avian radar detection system and three acoustic recorders to monitor bird movements during the light show. Over the course of the project, more than 735,000 migrating birds were tracked through a one-kilometer-wide (about six- tenths of a mile) radar transect over the light show. Thankfully, the monitoring showed no significant problems for the birds, but the studies turned up some intriguing facts. The acoustic monitoring devices recorded six times the number of night flight calls in the vicinity of the light show than at a recording station at the East Reservoir, less than two miles away.
Those findings are right in line with another of Clark’s research projects, a study recording migrating birds’ night flight calls as they passed over North America’s largest metropolis. During just 10 full nights and three partial nights of study in May 2010, he and his colleagues counted more than 300,000 “targets” (some were bats and large insects, but the vast majority were birds) flying over two roughly 550-yard-wide transects of New York City. “And that’s a huge underestimate,” Clark says, because many birds fly higher than the effective range of microphones.
It’s not only career scientists and conservationists using acoustic monitoring to study and assess bird migrations. In fall 2011 and spring 2012, Jessica Fleischman, then an 11th grader in Ossining, New York, set up low-cost microphones on the roof of Ossining High School. She compared her data to sounds recorded on a rooftop in rural Alfred, New York, 263 miles to the west. In 84 nights of recording, she tallied more than 20,500 nocturnal flight calls. With assistance from Clark and Evans, Fleischman showed that wind speed and precipitation were critical factors in calling rates. Her work won her a finalist spot in the American Museum of Natural History’s prestigious Young Naturalist Competition and a Gold Medal placement (top 10 percent of finalists) at the 2013 International Sustainable World Energy, Engineering, and Environment Project Olympiad.