Frigatebirds

 

Frigatebirds are amazing!

They have the largest ratio of wing area to body weight of any bird. This lets them fly very long distances while only rarely flapping their wings. They often stay in the air for weeks at time. And one being tracked by satellite in the Indian Ocean stayed aloft for two months.

Surprisingly for sea birds, they don’t go into the water. Their feathers aren’t waterproof. They are true creatures of the air. They snatch fish from the ocean surface using their long, hooked bills—and they often eat flying fish! They clean themselves in flight by flying low and wetting themselves at the water’s surface before preening themselves.

They live a long time: often over 35 years.

But here’s the cool new discovery:

Since the frigatebird spends most of its life at sea, its habits outside of when it breeds on land aren’t well-known—until researchers started tracking them around the Indian Ocean. What the researchers discovered is that the birds’ flying ability almost defies belief.

Ornithologist Henri Weimerskirch put satellite tags on a couple of dozen frigatebirds, as well as instruments that measured body functions such as heart rate. When the data started to come in, he could hardly believe how high the birds flew.

“First, we found, ‘Whoa, 1,500 meters. Wow. Excellent, fantastique,’ ” says Weimerskirch, who is with the National Center for Scientific Research in Paris. “And after 2,000, after 3,000, after 4,000 meters — OK, at this altitude they are in freezing conditions, especially surprising for a tropical bird.”

Four thousand meters is more than 12,000 feet, or as high as parts of the Rocky Mountains. “There is no other bird flying so high relative to the sea surface,” he says.

Weimerskirch says that kind of flying should take a huge amount of energy. But the instruments monitoring the birds’ heartbeats showed that the birds weren’t even working up a sweat. (They wouldn’t, actually, since birds don’t sweat, but their heart rate wasn’t going up.)

How did they do it? By flying into a cloud.

“It’s the only bird that is known to intentionally enter into a cloud,” Weimerskirch says. And not just any cloud—a fluffy, white cumulus cloud. Over the ocean, these clouds tend to form in places where warm air rises from the sea surface. The birds hitch a ride on the updraft, all the way up to the top of the cloud.

[…]

“Absolutely incredible,” says Curtis Deutsch, an oceanographer at the University of Washington. “They’re doing it right through these cumulus clouds. You know, if you’ve ever been on an airplane, flying through turbulence, you know it can be a little bit nerve-wracking.”

One of the tagged birds soared 40 miles without a wing-flap. Several covered more than 300 miles a day on average, and flew continuously for weeks.

• Christopher Joyce, Nonstop flight: how the frigatebird can soar for weeks without stopping, All Things Considered, National Public Radio, 30 June 2016.

Frigatebirds aren’t admirable in every way. They’re kleptoparasites—now there’s a word you don’t hear every day! That’s a name for animals that steal food:

Frigatebirds will rob other seabirds such as boobies, particularly the red-footed booby, tropicbirds, shearwaters, petrels, terns, gulls and even ospreys of their catch, using their speed and maneuverability to outrun and harass their victims until they regurgitate their stomach contents. They may either assail their targets after they have caught their food or circle high over seabird colonies waiting for parent birds to return laden with food.

Frigatebird, Wikipedia.

8 Responses to Frigatebirds

  1. Albert says:

    Fascinating animals, I love birds! Thanks for sharing!

  2. Anon says:

    Cool! Thanks for posting.

    Albatrosses appear to use a different mechanism to extract energy from the air. Using it, they can also fly with little expenditure of metabolic energy.

    See
    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3434196/

    Bob

    • John Baez says:

      Nice article!

      […] it would be difficult to explain albatrosses’ foraging strategies without considering proper adaptations necessary to reduce energy expenditures. The first of these adaptations is anatomical. Albatrosses have an elbow-lock system operating which keeps their wings open without any muscle activity, e.g. no energy expenditure. The second adaptation concerns a flight mode – dynamic soaring. It enables albatrosses to gain the energy required for flying from the constant strong winds common in the marine environment. What physical rules in terms of special flight technique or interaction between bird’s motion and wind govern such outstanding travelling performance in regard to the energy cost of flying?

      […]

      This paper presents the first experimental results on the small-scale movements constituting dynamics soaring which enables albatrosses to fly at no mechanical energy cost. We use in-flight-measurement data to show how birds gain the energy required for flying without flapping their wings from the moving air in the shear wind above the sea surface. Based on these high-precision data achieved with a new, in-house developed GPS-signal tracking method, it was possible to analyse the dynamic soaring flight manoeuvre in detail.

    • Gerald Wedekind says:

      Q for the many human readers more physically astute than I (or for pretty much any albatross, but please don’t tell me “we just wing it”)-

      I think the albatross article describes how the bird can gain energy (and ultimately glide long distances) using winds whose speed is low at low altitude and high at higher altitudes, and whose direction is horizontal and constant over space and time.

      Is that right? If so, is there any constraint on the bird’s large scale travel direction relative to the wind direction? (E.g. could the semicircle in diagram (a) have been completed to a circle, without any variability in wind direction)?

      • John Baez says:

        I’m no expert in bird dynamics, but presumably if you’re an albatross, once you’ve gained a lot of height, and thus potential energy, you can spend it by coasting down in any direction you want… though of course you’ll go further with the wind than against it.

        I imagine people who fly hang gliders know quite a bit about this kind of issue.

        • Gerald Wedekind says:

          Thanks John. And looking more closely at diagram (c) I see that during ascent, the bird’s ground displacement is almost exactly crosswind(!). So in that scenario at least, it would seem that a left-of-windward glide or dive/glide from point 2 could indeed have yielded a net windward ground displacement after one soaring cycle, with nary a flapped wing.

  3. domenico says:

    I had thought that the trajectories of the frigatebird could be used by autonomous little gliders to take low cost weather measures (temperature, pressure, humidity and others) for long times without fuel (using solar panels for the batteries used for transmissions and commands); if there are not metallic parts, then there should be no problem with glider strike against an aircraft engine.
    If the cost of the measures is low, then the cost of the weather knowledge may be lower.

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