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For frigate birds, staying aloft for months is a breeze

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Credit: Aurelien Prudor CEBC CNRS

Frigate birds, which can stay aloft for months at a time, capitalize on atmospheric conditions in order to spend very little energy while flying over hundreds of miles a day, a new study shows. Despite being seabirds, great frigates (Fregata minor) do not have water-repellent feathers, meaning that they often avoid landing in water and fly for long distances and periods of time to find food. Yet, how F. minor accomplishes these long flights remains largely unknown. Here, Henri Weimerskirch and colleagues outfitted dozens of frigates with solar-powered transmitters, or data-loggers, to measure heart rate, wing beat frequency, acceleration, altitude, and GPS coordinates. The data reveal that these birds cover substantial distances, traveling on average 410 kilometers (255 miles) each day. Remarkably, juveniles were found to embark on even longer journeys than adults, and to do so without adult companions, suggesting that their flight patterns reflect a genetically encoded behavior. Most birds stayed at an altitude that required very little flapping to stay aloft, somewhere between 30 to 2000 meters (98 and 6562 feet). Only when foraging for food, which is much more energy-intensive, did they drop down to elevations of 0 to 30 m. While aloft, the birds were found to use the circular movements of upward drafts, originating under cumulus clouds, to soar to higher altitudes. After hitching a ride on these drafts, the frigates are able to ascend up to 1,600 meters (5,249 feet) without flapping their wings, sometimes ascending at rates of 4 to 5 meters (13 to 16 feet) per second. Upon reaching higher altitudes, the birds could then glide downwards for long distances, flying with side winds to achieve the highest ground speeds, before hitching a ride on the next upward draft. A Perspective by Raymond Huey and Curtis Deutsch discusses these findings in greater detail.

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