The wandering albatross has a huge home range consisting of the 77 million square kilometres of the southern oceans. It can circle the world from the Tropics to Antarctica! Although an albatross often has difficulty in taking off, especially if there is no wind to help it, once airbourne it can fly for long periods. The bird glides down towards the sea on folded wings and when just above the water, it swings sharply into the wind and is blown back up to its original height by the rising air.
In this way, the bird progresses in a series of zig-zags. Albatrosses secure much of their food using an extraordinary technique which involves them spinning around on the ocean surface furiously, often for hours on end, a leading animal expert has discovered.
Although it may sound like an exhausting and bizarre way to get a meal there is method to this apparent madness because the spinning of the bird — wingspan around 14 feet — sets in motion a chain of events resulting in a belly full of squid.
The movement causes all the bioluminescent — light emitting — animals in the area to put on their brightest show, attracting the squid which, like moths, are attracted to the light. Introduced predators such as rodents and feral cats—the islands have no native land mammals—pose a danger, especially to defenseless chicks, which are left alone for long periods while their parents shuttle back and forth from distant feeding grounds.
In one of the most extreme examples of seabird predation, mice on Gough Island, in the South Atlantic, are decimating the populations of petrels and albatrosses that breed there, killing an estimated 1, Tristan albatross chicks a year. Natural disasters also cause heavy losses.
In , storm surges washed over two royal albatross breeding islands in the Chathams, killing chicks and, even more problematic, removing much of the islands' scant soil and vegetation. With the albatrosses lacking nesting material in subsequent years, the breeding success rate dropped from 50 percent to 3 percent: the birds laid their eggs on bare rock, and most eggs were broken during incubation.
Yet the most pernicious threats to albatrosses today are not to chicks but to adult birds. Along with other seabirds, they are locked in a competitive battle with humankind for the food resources of the sea—and the birds are losing. This is not just because of the efficiency of modern fishing practices but because fishing equipment—hooks, nets and trawl wires—inflict a heavy toll of injury and death. John Croxall, a seabird scientist with the British Antarctic Survey, has described the decrease in numbers in some albatross species as "catastrophic.
Over the past two decades, high-tech tracking devices such as the GPS loggers used by Scofield on the Pyramid have begun to fill in gaps in our knowledge about where albatrosses roam and where they are coming into lethal contact with fishing operations. Previously, when an albatross flew away from its breeding island, it virtually disappeared, its activities and whereabouts unknown.
But now the lives of these birds are being revealed in all their unimagined complexity, stunning accomplishment and tragic vulnerability. GPS loggers can give a bird's position to within a few yards. Some loggers also have temperature sensors. By attaching them to the legs of their study birds, scientists can tell when the birds are flying and when they are resting or feeding on the sea, because the water is generally cooler than the air.
As nifty as GPS loggers are, there is a snag: you have to get them back—an outcome by no means guaranteed. Among the larger albatrosses, chick-feeding forays can last ten days or more and encompass thousands of square miles of ocean. Lots of things can go wrong on these outings, particularly in and around commercial fishing grounds, where birds die by the thousands, done in by hooks, nets and the lines that haul them.
And because albatrosses have to struggle to take flight in the absence of a breeze, birds may be becalmed on the sea. The Chatham albatrosses' feeding forays tend to be relatively short—only a few days—and there was little chance of his birds becoming becalmed in the windy latitudes they inhabit, meridians known to mariners as the Roaring Forties, Furious Fifties and Screaming Sixties.
More worrisome to Scofield was the knowledge that the area adjacent to the Chatham Islands—known as the Chatham Rise—is one of New Zealand's richest commercial fishing grounds, replete with orange roughy and several other deep water species. Albatrosses, too, know where fish are found, and the birds sample the most productive fishing areas much as human shoppers make the rounds of favorite stores. And what expeditions these birds make!
From mollymawks, as the smaller species are known, to the great albatrosses, these super-soarers cover tens of thousands of miles in their oceanic forays.
Individuals of some species circumnavigate the globe, covering miles a day at sustained speeds of 50 miles per hour. And then they somehow find their way home—even when home is an outpost in the ocean like the Pyramid, not much bigger than an aircraft carrier. At the start of their breeding season, albatrosses have been tracked making almost ruler-straight trips from distant foraging areas to their nests. Because the birds maintain their course day and night, in cloudy weather and clear, scientists believe they use some kind of magnetic reckoning to fix their position relative to the earth's magnetic field.
The birds also seem able to predict the weather. Southern Buller's albatrosses were found to fly northwest if a low-pressure system, which produces westerly winds, was imminent, and northeast if an easterly wind-producing high-pressure system prevailed. The birds typically chose their direction 24 hours prior to the arrival of the system, suggesting they can respond to barometric cues. In his autopsy room in Wellington, ornithologist Christopher Robertson slit open a plastic bag containing a white-capped albatross.
This does not mean, however, that the bird spends years in the air flying without stopping. In a statement emailed to Reuters, Charles Eldermire, Bird Cams Project Leader of the Cornell Lab of Ornithology here confirmed that albatrosses indeed land on water to feed, though they may spend days aloft.
Andrea Angel, Albatross Task Force Manager for nonprofit BirdLife South Africa here explained to Reuters via email that albatrosses feed solely from the ocean and therefore need to land on water to do so. As they cannot dive to great depths, albatrosses scavenge for garbage off fishing boats, or capture fish or squid close to the surface of the ocean.
0コメント