Wherever the Wind May Blow
How did these birds come to have such similar, unusual, life histories? It would be natural to think that the albatrosses (there are between fourteen and twenty species, depending on which ornithologist you agree with) and the frigatebirds (five species) are closely related in evolutionary terms. But they're not. The two groups are members of two well-differentiated evolutionary orders, three major steps up the hierarchy from the species level. But since they're not closely related, what does account for the similarity of life histories? That is a question I've been mulling over for a long time, and one that has been directly guiding my research for several years.
|Albatrosses hold the age record for seabirds, living sixty to seventy years and continuing to reproduce into their fifties.
My work on albatrosses, particularly the wandering albatross (Diomedea exulans), began twenty-five years ago, when I was a graduate student studying breeding behavior on small islands in the subantarctic Southern Ocean [see map below]. At that time, breeding was the only albatross behavior my colleagues and I could observe closely; back then, biologists had no way of learning much more than, say, an observant sailor could discover about the behavior of albatrosses at sea. But in the intervening two and a half decades, technology has come to the rescue. Thanks to generation after generation of ever-smaller electronic tracking devices, biologists have pierced the veil of obscurity and tracked albatrosses on their amazing foraging journeys. Those excursions can cover vast loops more than 9,000 miles long—as if birds nesting in New York City flew to the shores of Italy to forage and then returned to their nests.
As my colleagues and I began to track albatrosses, the question of why their life histories bore such striking resemblances to those of the frigatebirds was never far from my mind. By 2002, when the new tracking technology had revealed many of the secrets of albatrosses, I knew it was time to apply the same methods to frigatebirds. That work is now paying off. The new packages of miniaturized electronic devices are helping biologists understand in detail the many ways frigatebirds are the tropical counterparts of albatrosses. Out of sight of land or ship, the albatrosses and frigatebirds we have fitted with instruments are demonstrating that, despite the birds' genetic distance, the hard facts of soaring and foraging at sea force even the most disparate lives to converge.
My studies of the wandering albatross have repeatedly taken me to two of the most remote islands in the Southern Ocean, Crozet and Kerguelen, where the birds breed and nest. My usual port of departure is Réunion, a French-administered tropical island in the western Indian Ocean. To reach the breeding grounds takes as long as ten days in a supply boat on tossing seas, a voyage I've now made eighteen times. But the bird, a graceful wanderer whose white body is offset by narrow wings that can span eleven feet, has always been worth the trip.
The breeding behavior of wandering albatrosses is much like that of the frigatebirds I have studied, but it is anomalous among birds in general. Usually neither group mates before reaching age ten or twelve—what in other birds would be a ripe old age. Females of both groups lay only one egg, and then take their time raising the chick to the fledging, or independent, stage. The wandering albatross tends its young for nine months, the frigatebird for a full twelve—the longest of any bird. Such protracted parental responsibilities leave both groups with little choice but to take at least a year's "sabbatical" between reproductive efforts.
|The wandering albatross tends its young for nine months, the frigatebird for a full twelve—the longest of any bird. Such protracted parental responsibilities leave both groups with little choice but to take at least a year's sabbatical between reproductive efforts.
Sailors have always known that albatrosses venture far out to sea. Partly because an albatross could appear with strong winds, sailors would respect it as “the bird/That made the breeze to blow,” in the words of Samuel Taylor Coleridge, their best-known bard. (According to legend, albatrosses also carry the souls of mariners lost at sea.)
But where do the avian wanderers go when they are out of sight of land or ships, especially when pressed by the needs of their offspring to make return trips to the nest from the feeding grounds at sea? Where, how, and how often do they encounter their prey—squid, fish, and the remains of dead whales, seals, and penguins—that they partially digest and eventually regurgitate for their chick? Back in the 1980s, when an albatross would pass our ship, we had no way of knowing where it had come from, where it was going, or whether it was a breeding adult, an immature bird, or a bird on sabbatical.
In 1989, however, I happened upon a newspaper article about Japanese scientists who had developed a small satellite transmitter for tracking dolphins. The original versions of such transmitters weighed more than two pounds and were used mainly for following the movements of ships. Biologists had adapted them to track large mammals such as bears or reindeer, but they were still too cumbersome for birds. The Japanese version, though, weighed little more than six ounces, which meant it could be carried by a twenty-six-pound wandering albatross. My colleague Pierre Jouventin, then at the Chizé Center for Biological Studies in Villiers en Bois, France, and I modified the tag, or attachment, of the dolphin transmitter to fit an albatross—and thereby became the first investigators to track seabirds on their foraging flights.
The data transmitted and relayed though satellite to our base amazed us. During a single foraging trip, which typically lasted between ten and fifteen days, the birds flew more than 1,800 miles from their nests and covered as much as 9,300 miles. They traced huge irregular loops, and made smaller-scale zigzagging movements within the loops that added substantially to the total length of the trip. To save energy, they soared on tailwinds or side winds. When the winds died, they alighted and drifted on the sea until the winds picked up again. And those dramatic findings were just the first of many waves of fresh data we collected about the specific activities of each bird along its route.
By the early 1990s, transmitter weights had been whittled down to just a bit more than two ounces. That enabled Rory Wilson, a penguin specialist at the Institute for Marine Sciences in Kiel, Germany, and me to attack another question: Do albatrosses find prey at the end of their foraging route or all along the way? Wilson had the superb insight that when a predator catches a fish or squid from the frigid Southern Ocean, the prey will cool the predator's stomach. So he made a recording thermometer that an albatross could swallow—not a problem for a bird that regularly gulps down six-pound fish whole. The thermometer, combined with the location transmitter, showed that, contrary to our expectations, the birds hunt and eat all along their routes. Generally they find schools of prey species at widely spaced intervals every five to six hours, and each time they do, they swallow a couple of fish or squid.
|The birds hunt and eat all along their routes. Generally they find schools of prey species at widely spaced intervals every five to six hours, and each time they do, they swallow a couple of fish or squid.
We were beginning to build up a picture of how the wandering albatross feeds. Soaring sixty miles an hour (even faster in optimal winds) across huge expanses of open ocean, it searches for rich but isolated schools of prey. It seemed that such long-distance flights would be physically impossible without a highly energy-efficient form of soaring. To find out just how efficient, I worked with Scott A. Shaffer and Dan P. Costa, ecophysiologists at the University of California, Santa Cruz. This time, we deployed three devices in combination: a satellite transmitter weighing only an ounce; a modified heart-rate monitor like the ones runners wear; and an activity recorder attached to the bird's leg, which let us know when the bird was floating on water.
Those instruments confirmed that the soaring flight of the albatross is among the most energy-efficient forms of avian travel known. The heart-rate monitors showed that albatrosses' heart rates during flight are only 10 to 20 percent higher than they are when the birds are at rest. In contrast, the heart rates of other birds in typical flapping flight can rise to as much as 200 percent higher than the baseline level.
As transmitters have continued to shrink (now down to nearly half an ounce), and as Global Positioning System (GPS) monitors have been miniaturized, we have been able to fill in further details about the long sea voyages of the wandering albatross. Low-pressure systems across the Southern Ocean generate a predictable wind pattern, which the birds exploit to the fullest. Flying northward, they typically move in a large, counterclockwise loop; flying southward, they loop clockwise.
Our growing array of electronics has helped me and my collaborators see how the key elements of the albatross's life cycle are interconnected. The patchy distribution of prey requires long-distance foraging. Long-distance foraging means the chicks are fed at long intervals, and so they develop independence slowly. The nine months between hatching and fledging forces the adults to skip a year between breeding attempts. All in all, the bird's slow-paced life probably contributes to its lengthy life span. And perhaps the decade it takes an albatross to reach reproductive maturity is time spent learning how to find the right winds and ride them while keeping a weather eye out for prey.
As my collaborators and I were penetrating the previously hidden lives of albatrosses, the potential value of comparing them with frigatebirds kept percolating in my mind. My interest was stirred in part by the theoretical work of two other ornithologists. The late David Lack, an evolutionary ornithologist at the University of Oxford, had written extensively on how the specific environment of an avian species contributes to its mode of life. N. Philip Ashmole, a seabird specialist at the University of Edinburgh, has further proposed that tropical seabirds are even more constrained than other seabirds by their environment. The thick layer of warm water at the surface of tropical seas restricts the movement of nutrients. And in such nutrient-poor upper layers, prey are even more scattered than they are in colder waters.
As late as 2002, no tracking studies had ever been done on tropical seabirds, and so the time was ripe for testing Ashmole's hypothesis. Olivier Chastel and Christophe Barbraud, ornithological colleagues of mine at the Chizé Center, joined me in April of that year in French Guiana to carry out a pilot study on the foraging ecology of a New World species called the magnificent frigatebird (Fregata magnificens). Having thus gained some experience with frigatebirds, I returned to Réunion in August 2003, but this time, instead of taking the ten-day boat trip to the albatross breeding islands, I took a four-hour flight in a military transport to the islet of Europa in the Mozambique Channel, between Madagascar and the African continent. Europa is just a three-mile-long speck of land, but it serves as a breeding base for a variety of seabirds, including a relative of the magnificent frigatebird, a species known as the great frigatebird (Fregata minor).
All five species of frigatebirds are large black birds with long forked tails and angled wings. As a group they hold the avian record for low "wing loading"—meaning that the ratio of body weight to wing area is lower than that of any other bird. Their physical profile, plus their superb overall flying abilities, makes it possible for them to roam the tropical seas for days on end, coming to land, like the albatrosses, only to breed on such far-flung islands as Europa.
|If a frigatebird lands on water, it's in serious trouble because its feet don't provide enough propulsion for the bird to lift off.
By the time I reached Europa in 2003, I had assembled a formidable tracking arsenal to study frigatebirds. There were location satellite transmitters, GPS data recorders, altimeters, and accelerometers. But my colleague Matthieu Lecorre, an ornithologist at the University of Réunion, and I found working with the frigatebirds to be a challenge nonetheless. We knew that the birds were much more high strung, and thus more difficult to temporarily remove from the nest and “tag,” than albatrosses, which are relatively tame. But on Europa, we also had to work at night. By day, clouds of young and nonbreeding frigatebirds soar over the nesting colonies and are quick to steal twigs from nests in the minute it takes to fit a bird with instruments and return it to its egg or chick. When the tagging was done, we kept watch; within a few days, sometimes even within a few hours, the other member of the frigatebird pair would arrive to relieve its electronically enhanced partner. The partner would then head out to sea to fish for its progeny. We would then be ready to track its course.
The magnificent frigatebirds in our pilot study spent two or three days at sea during incubation, but great frigatebirds stayed away between five and ten days. Once we started to analyze the data from the frigatebirds' flight, we began to see how they could sustain their lengthy foraging trips. Whereas albatrosses have large, webbed feet, which help them “climb” out of the sea and into the air, frigatebirds have minuscule, unwebbed feet and water-permeable plumage. If a frigatebird lands on water, it's in serious trouble because its feet don't provide enough propulsion for the bird to lift off. Our altimeters indicated that frigatebirds remain airborne throughout the foraging trip. Frigatebirds, then, must sleep on the wing. At present, they are the only birds other than swifts known to do so. As it happens, some birds can sleep in one brain hemisphere at a time, and that may be the frigate-birds' strategy.
Our transmitters showed that frigatebirds range hundreds of miles from Europa, some flying to the offshore waters of Mozambique, 360 miles away. Their average speed is only six to eight miles an hour, slow compared to an albatross riding a strong tailwind. But that difference is explained largely by the contrasting styles of flight. Our altimeters showed that during a climb, frigatebirds ride rising warm air masses known as thermals, reaching heights of 9,900 feet, a seabird record. Although the birds sometimes level off, just as a human glider pilot might do, most of their foraging trip is spent climbing and descending.
No doubt because of their inability to take off from the sea surface, frigatebirds rarely get close enough to it to risk a landing. But that raises another, rather obvious question: since they must come down to the sea to feed, how can they do so without getting trapped? The answer is that they often consume flying fish, which leap over the water's surface, and sometimes rob other birds of a meal. But they also feed in conjunction with predators such as tuna and dolphins. During an oceanographic mission that took place while we were on Europa, another colleague studying seabirds, Sébastien Jaquemet of the University of Réunion, observed tuna and dolphins chasing schools of fish and driving them to the surface. As the smaller fish leapt out of the water to avoid their submarine pursuers, low-flying frigatebirds simply snapped them up.
Our studies of albatrosses and frigatebirds have served to sharpen a series of questions I am keen to pursue. How, precisely, do the birds find their prey? How do they navigate? What algorithms do they follow in their search for fish and squid? Do they “memorize” maps of the most promising fishing zones?
|The black frigatebirds, with their sharply angled wings, ride rising thermals, whereas the white albatrosses, with their long narrow wings, catch a lift on a cold gale.
But even as those questions remain, my collaborators and I have been able to draw a picture of the life history of each group that would have been impossible only a decade or two ago. In fact, we now have enough information to answer the question I posed at the beginning of this article: How does it happen that two unrelated groups of birds seem to show such dramatic similarities in life history and in airborne hunting strategies? The black frigatebirds, with their sharply angled wings, ride rising thermals, whereas the white albatrosses, with their long narrow wings, catch a lift on a cold gale. But their foraging strategies converge: soar high, glide long, minimize the expenditure of energy.
Given the pronounced patchiness of their prey, and how albatrosses and frigatebirds have adapted to it, I am convinced that Lack and Ashmole have pointed us in the right direction: the constraints of the environment (in particular, the scarcity of prey in the open oceans) is the primary factor driving the peculiarities and similarities that these remarkable birds display. Evolution has converged, offering a splendid example of how two quite different groups, with two quite different genetic starting materials, can arrive at highly similar life cycles. It is a beautiful evolutionary story, and one that I look forward to documenting in greater detail in the years to come.
Henri Weimerskirch has been studying seabirds of the Southern Ocean for twenty-five years and tropical seabirds for the past five. When Weimerskirch, an ecologist and ornithologist, is not working on remote islands, he is a research director at the Chizé Center for Biological Studies field laboratory, in the middle of a forest in southwestern France (the Chizé Center is part of France's National Center for Scientific Research, widely known by its French acronym CNRS). Weimerskirch is an active member of various conservation groups and is working to develop measures that can reduce the toll taken on seabirds by such practices as long-line fishing. His next seabird project will be a comparative study of masked, brown, and red-footed boobies on Clipperton Island, an uninhabited, French-owned coral atoll in the Pacific Ocean, about 800 miles southwest of Mexico.
Copyright © Natural History Magazine, Inc., 2004