How frigatebirds stay aloft for weeks and weeks

Great frigatebirds (Fregata minor) are marvels of nature, for they’re known to stay aloft—without landing—for weeks and even months, though they have to descend to the water’s surface to fish. And it’s a good thing, too, for, unlike nearly every other seabird, their feathers aren’t waterproof and their legs are too small to land on the water. That could be tricky, as they make their living by catching fish from the ocean. (Of course, these features could have evolved after their aerial habits or have coevolved with them.) Frigatebirds have the largest wing area/body mass ratio of any living bird, and their only rivals in staying aloft are swifts, which can also remain in the air for months without resting or sleeping.

Here’s a great frigatebird on the wing, and those wings are huge:

West of Christmas Island, Kiribati, November 21, 2012 © Donald H. Gudehus. Source.

The males have red gular sacs that they inflate to attract females during the breeding season (photo from Wikipedia).

Here’s their range:

How do these birds manage to stay aloft for so long? Do they sleep on the wing? A team of researchers headed by Henri Weimerskirch give some answers in a paper published in the July 1 issue of Science (free download, reference and link below).

The answer is that the birds have a very low expenditure of energy, flapping their wings as rarely as possible. They accomplish that by taking advantage of wind and weather. To track these birds and their energy expenditure, the team put solar-powered transmitters on 49 birds (24 adults, 25 juveniles) caught on the island of Europa between Madagascar and mainland Africa. An additional 11 adult females were fitted with “loggers” measuring their GPS location, acceleration, and heart rate. The birds were tracked when they left Europa after the breeding season ended and moved northward to fish.

During this period, birds were continuously aloft except for occasional resting periods—landing on islets for only 8-48 hours. On average, the birds’ maximum time aloft was 41.2 days each, with the record being 2.1 months in the air without a rest. The researchers knew this because they could monitor the altitude and position of the birds. On average, a bird traveled between 420 and 450 km per day (260-280 miles, greater than the distance between New York City and Washington D.C.), depending on whether they were circling around the doldrums (see below) or traveling in a directional way.

So how do they stay in the air so long without exhausting themselves? The researchers found several things:

• During the period these birds were aloft, there are areas of no wind (“doldrums”) between Madagascar and India. The transmitters showed that the birds flew around the edges of these doldrums, for those edges have updrafts the birds can use to gain altitude without expending energy. Here’s a track of a single adult male. The colors, as indicated on the scale, give wind speeds in meters per second (it’s zero in the doldrums). All figure captions are from the paper:

Two successive clockwise movements from Aldabra Island (Seychelles) of an adult male great frigate bird (no. 138502) in relation to wind strength (in meters per second, color scale) and direction (arrows). (A) In May 2015, a 24-day foraging trip around the doldrums (shown by the absence of wind, in white), with 1 day of rest in Chagos. (B) In June 2015, a 28-day foraging trip, with a 36-hour rest in Chagos.

• The birds gain altitude for soaring (which then allows them to glide downward without expending energy) by moving with the wind, using thermal updrafts to lift them. They then glide down with the wind at their side. Here’s a diagram of that movement, which the authors describe as “a complex zig-zagging roller-coaster movement”. They go up about 600-700 meters in each rise before a gliding descent:

Fig. 3 Three-dimensional movement of a frigate bird at three scales. (A) Section of a track of a frigate bird traveling with side winds. (B) Enlargement showing the movement alternating gliding and soaring, resulting in a zig-zag and roller-coaster movement. (C) Detailed schematic representation of a single cycle of soaring and gliding, illustrating the climb by circling, with a resulting drift due to wind, followed by the descent.

 

• Perhaps the most surprising finding is this: the birds deliberately fly up into cumulus clouds, which have strong updrafts. Inside those clouds the birds can ascend at 5 meters/second, and they can go as high as 4000 meters, where it’s below freezing! Then they glide down, and after such a big rise can then glide about 60 kilometers without flapping. Juvenile birds do this, too, suggesting that this behavior isn’t learned but genetically hard-wired.

• Finally, do they sleep? The authors don’t know, and neither does anyone else. Here’s what they say:

“Long periods in continuous flight are interrupted by very short periods of rest on land, suggesting that frigate birds might sleep while airborne. Periods of low activity (no flapping) occur mainly during soaring episodes and may allow sleep. However, periods of completely motionless (no flapping at all) flight, potentially corresponding to periods of sleep, are relatively short, (~2 min, never exceeding 12 min). Animals such as frigate birds may have evolved the ability to dispense with sleep when ecological demands favor wakefulness such as during extended flights, but studies are needed to determine how they sleep during much longer-lasting flights.”

I’m sure there are many readers who would appreciate an evolved ability to dispense with sleep! It is a big mystery why these birds can do it (if they do it), while other animals absolutely require sleep if they’re to live. At any rate, we have yet another study that, using modern technology, is able to uncover how much more wonderful animals are than we ever suspected.

Here’s a video of these birds, produced to illustrate the findings of Werimerskirch et al. Sadly, it uses a robotic voice.

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Weimerskirch, H., C. Bishop, T. Jeanniard-du-Dot, A. Prudor, and G. Sachs. 2016. Frigate birds track atmospheric conditions over months-long transoceanic flights. Science 353:74-78.