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<em>Science</em>: For Frigate Birds, Staying Aloft for Months is a Breeze

Frigate birds can stay aloft for months by hitching rides on massive drafts of wind that allow them to preserve energy while flying hundreds of miles a day or more, a study in the 1 July issue of Science shows.

The results provide new insights into flight adaptations that are critical to long-distance migration, but also may inform future flight dynamics of drones and other aircraft.

A male frigate displaying (red throat) with female in flight. | Aurelien Prudor CEBC CNRS

Despite being seabirds, great frigates 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. Some frigates stay aloft for up to two months before finally touching down on land.

"They can do it because their flight costs are extremely low, and they can rest, and sleep while aloft," explained Henri Weimerskirch of the French National Centre for Scientific Research, the lead author of the study.

However, researchers do not exactly understand how frigates expend so little energy while flying. To gain more insights, Weimerskirch and his colleagues outfitted dozens of frigates with solar-powered transmitters, or data-loggers, to measure the birds' heart rate, wing beat frequency, acceleration, altitude, and GPS coordinates.

The data reveal that these birds cover substantial distances, traveling on average 410 kilometers or 255 miles, each day. On average, the birds stayed aloft for 41 days without landing, with one juvenile staying aloft for more than two months.

Most birds flew at an altitude that required very little flapping to stay aloft, somewhere between 30 meters to 2,000 meters, or 98 feet to 6,562 feet. Only when foraging for food, which is far more energy-intensive, did the birds drop to elevations below 30 meters.

While aloft, the birds were found to leverage the circular movements of upward drafts, originating under cumulus clouds, to soar to higher altitudes. After hitching a ride on the drafts, the frigates are able to ascend up to 1,600 meters, or 5,249 feet without flapping their wings, sometimes ascending at rates of four meters to five meters or 13 feet to 16 feet per second. Upon reaching higher altitudes, the birds glide downward for long distances, flying with side winds to achieve the highest ground speeds, before hitching a ride on the next upward draft.

Henri Weimerskirch with a juvenile frigate. | Henri Weimerskirch CEBC CNRS

Weimerskirch said he was surprised that the birds were able to climb up to 4,000 meters in altitude, where they can encounter freezing conditions. "To do this they enter into clouds — which was also completely unexpected."

The study was part of a larger project to understand how juvenile seabirds learn. Remarkably, juveniles were found to embark on even longer journeys than adults, and did so without adult companions, suggesting that their flight patterns are a genetically encoded behavior.

Weimerskirch said the researchers will now study "the learning phase of young frigate birds, how they learn to use these extreme conditions … Also we will test whether living in different oceanic conditions — Galapagos, Western Pacific and Indian Ocean — results in different migratory and dispersal strategies."

[Credit for associated image: Henri Weimerskirch CEBC CNRS]