How frigate birds stay aloft for months

When some of the longest-distance flyers in the seabird world embark on months-long migrations, they get a leg-up from passing clouds, a new study has found.

Frigate birds fly for up to 48 consecutive days, according to the research, during which time they take advantage of hugely variable atmospheres over deserts, mountains and oceans.

“Great frigate birds are the only birds other than swifts to be able to stay aloft for months,” explain the researchers in their paper, published this week in the journal Science.

“Long periods in continuous flight are interrupted by very short periods of rest on land, suggesting that frigate birds might sleep while airborne.”

To find out the secrets behind this extreme endurance, an international team of researchers led by Henri Weimerskirch at the University of La Rochelle in France strapped solar-powered trackers to dozens of these seabirds to monitor heart-rate, flapping frequency, angles of ascension and speed over a four-year period.

Frigate birds (Fregata minor) are an unusual species of seabird in that their feathers are not waterproof. The findings suggest these birds expertly capitalise on wind currents to reduce the need for flapping, and to minimise ocean rest-stops, because their feathers would absorb too much water.

Adult birds were observed hitching a ride on the upward drafts beneath cumulus clouds, ascending up to 1,600 metres without a single flap of their wings. Once inside the clouds, their ascents are quick, soaring at rates of up to five metres per second.

The birds reach altitudes higher than 2,300 metres before gliding steadily down until it’s time to catch another upward draft.

“The birds repeat these soar-glides multiple times per day, rising more than 15 kilometres on average,” write Raymond B Huey and Curtis Deutsch, two researchers at the University of Washington, commenting on the findings.

“They mainly use flapping flight when pursuing prey near the sea surface.”

The birds descend to the sea when it’s time to forage. But feeding intervals are infrequent, given the low energy-cost of their flying method. Using this efficient method of migration, the birds can average 420 kilometres per day.

The findings also revealed juvenile birds have even higher capabilities, taking on extreme flight patterns independently of their adult counterparts.

One youngster on record was aloft for over two months, and another flew farther than the Earth’s total circumference in 185 days, stopping for rests of no more than four days at a time.

Alongside amazing statistics, the researchers also warn that climate change could impact the conditions that allow frigate birds their epic migrations.