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Extreme Flight Tracked by Electronics on Frigate Birds

Evolution News


Named for fast-moving French warships of the 18th century, the frigate bird is built for gliding. With one of the largest wing surface ratios of any bird, they soar from the ocean surface up to the high clouds, staying aloft for weeks at a time. Recently, scientists traveled to Europa Island, a speck of land in the Mozambique Channel between Madagascar and the African mainland, to study their navigation patterns. What they found pushes the limits of flight physics. Here is a bird that flies over the open ocean for up to two months, yet is unable to land on water.

You may have seen these eye-catching birds in photos of Galápagos wildlife. Both sexes are outfitted in black feathers, having long beaks hooked downward at the tip. Females have white heads, and males have a large red pouch on their necks that they inflate for mating displays. When aloft, they are easy to distinguish by their huge wingspans and forked tails. News from CNRS, the French national center for scientific research, tells more about them:

A sea bird that is difficult to observe, which makes it somewhat of an enigma, the frigate bird (Fregata minor) is extremely light and equipped with very long and wide wings that give it an extraordinary ability to glide and climb in updrafts without beating its wings. With the longest period of parental care of all birds, and its inability to land on water (even while depending entirely on flying fish that it must capture in flight), the frigate bird is a subject of choice for the Early life research project focusing on the behavior of young marine predators. A team of researchers thus conducted a major program to tag frigate birds on Europa Island in the Mozambique Channel, a breeding ground for the species. [Emphasis added.]

The team outfitted 24 adult and 25 juvenile birds with geolocators somewhat similar to those placed on Arctic terns described in Illustra’s film Flight: The Genius of Birds. Frigate birds, being larger than the terns, could manage larger units with transmitters. This enabled scientists to monitor information in real time about altitude, heart rate, wingbeat, and speed of flight. A four-minute video describing the project — part of an award-winning documentary — contains stunning footage of the birds on land and in the air.

Frigate birds are “completely atypical” of other marine birds, the narrator says. Though they have webbed feet, they lack the waterproofing on their feathers to land on the water. All their feeding, therefore, has to be done on the wing. They follow dolphins who force their prey, flying fish, up to the surface. The frigate bird swoops down to catch them without ever touching the water. The transmitters allowed the scientists, for the first time, to follow their migration. How do they maintain the energy required to stay aloft? When do they sleep?

On a large scale, the recordings show that frigate birds make their transoceanic flights between Africa and Indonesia by following the edge of the tropical cyclone formation area near the Equator, often called the doldrums by sailors. They use favorable winds (trade winds) to effortlessly make immense circles in the Indian Ocean. The juveniles in particular, who leave their birthplace for the first time, travel thousands of kilometers and, even more surprisingly, can remain airborne for over two months without touching ground.

On a smaller scale, within these circular trajectories, the studies reveal that frigate birds follow a roller-coaster flight pattern. Using the convection under cumulus clouds, they gain altitude by gliding without beating their wings, and with very little energy expenditure. The recordings then show short periods of total inactivity, suggesting that frigate birds potentially sleep for a few minutes during this ascent phase.

New Scientist adds that one of the birds traveled 55,000 km (34,000 miles) in 185 days with only 4 days of rest on islands. That’s a long time to hold the wings outstretched. Imagine holding your arms out for over two months; even with lift, it would be tiring. The birds must be built for comfort in that position, even though on land they can fold their wings like other birds. Only some swifts are known to stay aloft for longer times.

The paper mentions these other amazing facts about frigate birds:

  • Young frigate birds left their birthplace at the same time as adults, but independently of their parents.

  • Juvenile individuals are able to master the flight strategy of adults as soon as they become independent. When they leave their birthplace, they all head north to reach the equator and circle the entire Indian Ocean. This stereotyped movement suggests a genetically encoded behavior that brings young individuals directly to a predictable, favorable, and large-scale atmospheric feature located thousands of kilometers from their birthplace.

  • Probably as a consequence of these extreme attributes, frigate birds have the longest period of parental care in birds, suggesting a long period of learning to acquire flight and foraging abilities in early life.

  • Whereas active foraging is very costly for frigate birds, requiring high dynamic body acceleration and heart rates, traveling periods have a remarkably low energy expenditure, with few wingbeats…, suggesting that overall field metabolic rate during months at sea is likely to be exceptionally low.

  • 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).

  • To deal with the local scarcity of clouds and gain longer gliding distances, birds regularly soar inside cumulus clouds to use their strong updraft, and they can reach altitudes of 4000 meters [13,000 feet], where freezing conditions occur.

It gets very cold up there, but these birds lack down feathers to keep warm. Somehow they endure the harsh conditions. The height gives them enough of a boost to glide for up to sixty kilometers.

Why study these particular birds? “Biologists have long been attracted to locomotor extremes,” the biologists explain in the paper, “because they provide clear examples from which information about structure-function relationships can be drawn.” Unfortunately, they never do so in the paper or in the news item except for a couple of passing references to evolution, e.g.:

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.”

Did they “evolve the ability” on purpose? Not according to Darwin’s theory. Perhaps the ones that fell asleep dropped like bombs into the water, preventing them from passing on their genes. One would think, though, that if evolution could keep the birds awake for months at a time, it could endow them with waterproof feathers just as easily, so that they could rest on the water surface like other oceanic birds. More likely, the inability to land on the water represents a loss of function in the modern species compared with their ancestors, since it is apparently not needed. It’s easier to explain loss of function by evolutionary theory.

Design science, of course, is compatible with variation over time. The fossil record of frigate birds, compared with the five species alive today, shows differences in wingspan, leg length and body proportions. Those, however, are minor adjustments to existing structures. Claiming birds evolved powered flight from land-based ancestors is another matter. There are too many independent physical requirements that need to operate simultaneously, as Paul Nelson explains in the Illustra documentary:

You look at the anatomy of a bird, its behavior, its metabolism, the structure of its feathers, the structure of its muscles and so forth — these are multiple independent points in a complex space, out of which flight emerges. And I think from a biological standpoint, to fly at all requires a cause that’s able to visualize a distant functional endpoint, and bring together everything necessary to achieve that endpoint. Uniquely, and universally in our experience, only intelligence is capable of that kind of causal process.

Ask Bryan Allen, who pedaled the Gossamer Condor across the English Channel in 1979. Ask Burt Rutan, whose Voyager flew around the world without stopping or refueling in 1986. Ask Bertrand Piccard, who just crossed the Atlantic in a solar-powered plane. Flight does not “emerge” unless stringent requirements are met. None of the artificial flyers stayed aloft for months at a time. Nor could their pilots swoop down in flight and catch a flying fish.

Photo credit: Benjamint444 (Own work) [GFDL 1.2], via Wikimedia Commons.