Fat a question of feathers for shearwaters

By Mark Brazil | Aug 16, 2000

The fact that young animals and birds not only start off small, but remain smaller than their parents for a long time, seems to be a dominant rule of life. Think of fox or badger cubs, think of young sparrows or bulbuls — from birth, or hatching, and for some time after they remain smaller than their parents. The cygnets of the whooper swans that migrate to Japan each winter, though capable of the arduous migration, are still noticeably smaller than their parents six months after hatching, an age by which most small birds will be indistinguishable from their parents both in size and in plumage.

There is one group, though, that not only bends this rule, it shatters it. The long-winged gliders of the ocean, the seabirds known to ornithologists as Procellariformes and to naturalists as albatrosses, shearwaters and petrels, definitely do not play by the accepted rules. Their offspring routinely reach weights that may be 60 percent heavier than their parents; they accumulate fat at an astonishingly fast rate — and then lose most of it before leaving the nest.

This strange weight change has for a long time been assumed to be an insurance strategy. Albatross, shearwater and petrel parents feed at sea, where foraging conditions and food availability are notoriously variable. On foraging trips they may be away for days at a time. Some albatrosses take that to extremes and circumnavigate Antarctica on their foraging trips away from their colony.

During the prolonged absences of their parents, the fluff-ball chicks must wait out both hunger and whatever the fickle climate throws at them. Many of the albatrosses, shearwaters and petrels nest on remote islands in the southern hemisphere where summer storms, even snow, are likely. The chicks may not eat for days and may experience chilling winds, rain or snow.

Their fat layer, then, is their insurance policy against the weather and the delayed return of their parents — a reserve for them to live off.

That, at least, was the perceived wisdom, and it seemed to make good sense. Recent research has shown, however, that it is only part of the story.

Once young albatrosses, shearwaters and petrels leave the nest they are on their own. No more free hand-outs, no more family support: They have to go it alone from the day they depart for life at sea.

The young bird will have spent weeks cooped up in a burrow, its wings an unknown quantity until its instincts eventually drive it to begin stretching them at the entrance of the burrow. Until this stage all food has been delivered to it by its parents. A richly concentrated, partly predigested, seafood chowder has been delivered to its door with a regularity that has allowed it to lay down an excess layer of fat.

Then one day neither of its parents returns. Suddenly on its own, it must metamorphose from a parasite totally dependent on the efficiencies of its parents’ foraging, into a lean flying machine capable of finding its way across oceans and of finding its own food on the way.

Isn’t this actually the point in their lives when the chicks are at the greatest risk? Shouldn’t this be the period they carry the fat for, to see them through their first inexperienced days of learning to fly and to recognize and catch food? Yet, generation after generation, these birds put on weight massively while in the nest, reach a stage where they dwarf their parents, then slim right down orey leave the nest.

There is clearly something missing from the story.

The remote island of South Georgia, in the South Atlantic between the Falkland Islands and the Antarctic Peninsula, has long been the research territory of the British Antarctic Survey (BAS). Thorough research on the species that breed there has continued for decades and fascinating discoveries keep coming to light.

Recent research there on gray-headed albatrosses, for example, has revealed two distinct phases of growth as the chicks grow up. During the first phase the internal organs develop, including the organs for processing and using that rich food supply that their parents keep delivering. Then, during the second phase, when the chicks are between 60 and 80 days old, they develop the muscles and feathers that they require for flight.

If the insurance-policy theory were correct, then the chicks would be at risk right from the beginning and so they should accumulate fat right from the beginning. The researchers found, however, that they only begin to lay down fat during the second phase of growth, that is, after they are 60 days old.

The BAS researchers have found that this later phase of growth has stricter limitations than the first. If there is a food shortage during the younger phase of a chick’s life, its inner organs will just grow more slowly for a while; as soon as food is delivered again its inner growth can catch up.

In the later growth phase, though, it is the crucial flight feathers that are being grown. Any interruption in their growth would weaken them, and ultimately make the bird a poorer flier.

Growing feathers requires energy, and growing a full set from scratch requires an enormous amount of energy. Adult seabirds replace their flight feathers, their wing and tail feathers, slowly. They drop symmetrical pairs, one from each wing, or one from each side of the tail, and the replacements grow in slowly. They spread the need for energy for growing over many months.

The chicks don’t have this luxury. They have 70 days, at most, to grow a full set of 60 wing feathers and 14 tail feathers. It is not a temporary set just to see them through, either; these feathers will last them for several years, so they have to produce quality as well as quantity, and they have to do it in a very limited time. Food shortage during this growth phase could be fatal. The fat layer that the chicks have accumulated helps power this growth and insures against food shortages that might hamper it.

The Procellariformes are master gliders, shearing low over the waves, swooping up on the updraft. Long wings, strong feathers, a light frame and a low weight-to-wing-surface ratio are crucial, so the final phase for the chicks is to burn off excess fat. Any fat left after the feathers are fully grown will be extra baggage.

Once they have achieved the ultimate — self-powered flight — and once they have learned to navigate and feed, the young petrels, shearwaters and albatrosses set off on their journeys. The shearwaters that breed on the cliff tops of islands around New Zealand and Tasmania avoid the cold winters of the southern oceans by making their way in a huge loop all the way around the North Pacific. They pass through Japanese waters on their way and many of them even reach the Bering Sea.