How do frigate birds—seabirds with a 7-foot wingspan—stay aloft for months at a time? Their gliding prowess has been documented for hundreds of years by such varied observers as Christopher Columbus and Walt Whitman. But exactly how the birds accomplished this feat remained largely unknown until now. A new study by Henri Weimerskirch (of the Chizé Centre for Biological Studies in Villiers en Bois, France) and colleagues may help explain how the frigate bird remains airborne for extended periods. This could be of interest to UAV builders, if they can apply lessons from the birds’ flight to their designs.
The team studied frigate birds on Europa Island in the Mozambique Channel between 2011 and 2015, equipping nearly 50 of them with solar-powered transmitters to track their movements. The scientists wanted to determine how the birds completed their long migrations over oceans without landing (their feathers become waterlogged if they settle on the sea’s surface). In particular, they wanted to know if oceanic thermals allowed the birds to remain airborne for long periods.
The study, published in this week’s issue of Science, found that while adult frigates remained aloft for up to 48 days, juveniles spent even longer—more than two months—on the wing. And both groups achieved this feat with low wing-beat frequencies—meaning mainly through soaring—which would conserve energy.
The birds’ flight paths, say the authors, showed that “when soaring, the frigates used circling movements to soar in thermals below cumulus clouds where rising air creates updrafts.” The birds drift with the wind while climbing, but while gliding, they speed up by using side winds. “The resulting movement,” says the team, “is a complex zig-zagging, roller-coaster movement.”
Interestingly, the presence of cumulus clouds could be determined from the birds’ behavior, say the authors. “An ascending flight without or with little flapping of wings can only be made with support of an ascending air current. At sea an ascending air current can only exist in a thermal below a cumulus cloud.... Thus, it can be concluded that there is a cumulus cloud where the bird performs an ascending flight, when birds climb without flapping their wings. Soaring outside but close to the clouds would not be possible because of the downdrafts occurring in the near vicinity of the cloud-edges.”
Both frigate birds and glider pilots use cumulus clouds as indicators of thermals, say Raymond Huey and Curtis Deutsch, in a separate article in Science. But whereas glider pilots typically avoid soaring in clouds (due to Visual Flight Rules), frigate birds “use these clouds to gain altitude and thus to glide extra distance.”
We wonder if this new information might help UAV designers: If a UAV were fitted with sensors that could indicate the presence of a cumulus cloud, perhaps it could use the cloud cover not only to hide, but to gain altitude and coast until finding another thermal.