Albatrosses are amongst the largest of flying birds, and the great albatrosses (genus Diomedea) have the largest wingspans of any extant birds. The albatrosses are usually regarded as falling into four genera, but there is disagreement over the number of species.
Albatrosses are highly efficient in the air, using dynamic soaring and slope soaring to cover great distances with little exertion. They feed on squid, fish and krill by either scavenging, surface seizing or diving. Albatrosses are colonial, nesting for the most part on remote oceanic islands, often with several species nesting together. Pair bonds between males and females form over several years, with the use of 'ritualised dances', and will last for the life of the pair. A breeding season can take over a year from laying to fledging, with a single egg laid in each breeding attempt.
Of the 21 species of albatrosses recognised by the IUCN, 19 are threatened with extinction. Numbers of albatrosses have declined in the past due to harvesting for feathers, but today the albatrosses are threatened by introduced species such as rats and feral cats that attack eggs, chicks and nesting adults; by pollution; by a serious decline in fish stocks in many regions largely due to overfishing; and by long-line fishing. Long-line fisheries pose the greatest threat, as feeding birds are attracted to the bait, become hooked on the lines, and drown.
Morphology and flight
The albatrosses are a group of large to very large birds; they are the largest of the procellariiformes. The bill is large, strong and sharp-edged, the upper mandible terminating in a large hook. This bill is composed of several horny plates, and along the sides are the two "tubes", long nostrils that give the order its former name. The tubes of all albatrosses are along the sides of the bill, unlike the rest of the Procellariiformes where the tubes run along the top of the bill. These tubes allow the albatrosses to have an acute sense of smell, an unusual ability for birds. Like other Procellariiformes they use this olfactory ability while foraging in order to locate potential food sources. The feet have no hind toe and the three anterior toes are completely webbed. The legs are strong for Procellariiformes, in fact, almost uniquely amongst the order in that they and the giant petrels are able to be models.
The adult plumage of most of the albatrosses is usually some variation of dark upper-wing and back, white undersides, often compared to that of a gull. Of these, the species range from the Southern Royal Albatross which is almost completely white except for the ends and trailing edges of the wings in fully mature males, to the Amsterdam Albatross which has an almost juvenile-like breeding plumage with a great deal of brown, particularly a strong brown band around the chest. Several species of mollymawks and North Pacific albatrosses have face markings like eye patches or have grey or yellow on the head and nape. Three albatross species, the Black-footed Albatross and the two sooty albatrosses, vary completely from the usual patterns and are almost entirely dark brown (or dark grey in places in the case of the Light-mantled Sooty Albatross). Albatrosses take several years to get their full adult breeding plumage.
The wingspans of the largest great albatrosses (genus Diomedea) are the largest of any bird, exceeding 340 cm (over 11 feet), although the other species' wingspans are considerably smaller. The wings are stiff and cambered, with thickened streamlined leading edges. Albatrosses travel huge distances with two techniques used by many long-winged seabirds, dynamic soaring and slope soaring. Dynamic soaring enables them to minimise the effort needed by gliding across wave fronts gaining energy from the vertical wind gradient. Slope soaring is more straightforward: the albatross turns to the wind, gaining height, from where it can then glide back down to the sea. Albatross have high glide ratios, around 22:1 to 23:1, meaning that for every metre they drop, they can travel forward 22 metres. They are aided in soaring by a shoulder-lock, a sheet of tendon that locks the wing when fully extended, allowing the wing to be kept outstretched without any muscle expenditure, a morphological adaptation they share with the giant petrels.
Albatrosses combine these soaring techniques with the use of predictable weather systems; albatrosses in the southern hemisphere flying north from their colonies will take a clockwise route, and those flying south will fly counterclockwise. Albatrosses are so well adapted to this lifestyle that their heart rates while flying are close to their basal heart rate when resting. This efficiency is such that the most energetically demanding aspect of a foraging trip is not the distance covered, but the landings, take-offs and hunting they undertake having found a food source.[13] This efficient long-distance travelling underlies the albatross's success as a long-distance forager, covering great distances and expending little energy looking for patchily distributed food sources. Their adaptation to gliding flight makes them dependent on wind and waves, however, as their long wings are ill-suited to powered flight and most species lack the muscles and energy to undertake sustained flapping flight. Albatrosses in calm seas are forced to rest on the ocean's surface until the wind picks up again. The North Pacific albatrosses can use a flight style known as flap-gliding, where the bird progresses by bursts of flapping followed by gliding. When taking off, albatrosses need to take a run up to allow enough air to move under the wing to provide lift.
Distribution and range at sea
Most albatrosses range in the southern hemisphere from Antarctica to Australia, South Africa and South America. The exceptions to this are the four North Pacific albatrosses, of which three occur exclusively in the North Pacific, from Hawaii to Japan, California and Alaska; and one, the Waved Albatross, breeds in the Galapagos Islands and feeds off the coast of South America. The need for wind in order to glide is the reason albatrosses are for the most part confined to higher latitudes; being unsuited to sustained flapping flight makes crossing the doldrums extremely difficult. The exception, the Waved Albatross, is able to live in the equatorial waters around the Galapagos Islands because of the cool waters of the Humboldt Current and the resulting winds.
It is not known for certain why the albatrosses became extinct in the North Atlantic, although rising sea levels due to an interglacial warming period are thought to have submerged the site of a Short-tailed Albatross colony that has been excavated in Bermuda.[9] Some southern species have occasionally turned up as vagrants in the North Atlantic and can become exiled, remaining there for decades. One of these exiles, a Black-browed Albatross, returned to gannet colonies in Scotland for many years in a lonely attempt to breed.
The use of satellite tracking is teaching scientists a great deal about the way albatrosses forage across the ocean in order to find food. They undertake no annual migration, but disperse widely after breeding, in the case of southern hemisphere species, often undertaking circumpolar trips. There is also evidence that there is separation of the ranges of different species at sea. A comparison of the foraging niches of two related species that breed on Campbell Island, the Campbell Albatross and the Grey-headed Albatross, showed the Campbell Albatross primarily fed over the Campbell Plateau whereas the Grey-Headed Albatross fed in more pelagic, oceanic waters. Wandering Albatrosses also react strongly to bathymetry, feeding only in waters deeper than 1000 m (3281 ft); so rigidly did the satellite plots match this contour that one scientist remarked, "It almost appears as if the birds notice and obey a 'No Entry' sign where the water shallows to less than 1000 m".There is also evidence of different ranges for the two sexes of the same species; a study of Tristan Albatrosses breeding on Gough Island showed that males foraged to the west of Gough and females to the east.
Diet
The albatross diet is predominantly cephalopods, fish and crustaceans, although they will also scavenge carrion and feed on other zooplankton. It should be noted that for most species, a comprehensive understanding of diet is only known for the breeding season, when the albatrosses regularly return to land and study is possible. The importance of each of these food sources varies from species to species, and even from population to population; some concentrate on squid alone, others take more krill or fish. Of the two albatross species found in Hawaii, one, the Black-footed Albatross, takes mostly fish while the Laysan feeds on squid.
The use of dataloggers at sea that record ingestion of water against time (providing a likely time of feeding) suggest that albatross predominantly feed during the day. Analysis of the squid beaks regurgitated by albatrosses has shown that many of the squid eaten are too large to have been caught alive, and include mid-water species likely to be beyond the reach of albatross, suggesting that, for some species (like the Wandering Albatross), scavenged squid may be an important part of the diet. The source of these dead squid is a matter of debate; some certainly comes from squid fisheries, but in nature it primarily comes from the die-off that occurs after squid spawning and the vomit of squid-eating whales (sperm whales, pilot whales and Southern Bottlenose Whales). The diet of other species, like the Black-browed Albatross or the Grey-headed Albatross, is rich with smaller species of squid that tend to sink after death, and scavenging is not assumed to play a large role in their diet.
Until recently it was thought that albatross were predominantly surface feeders, swimming at the surface and snapping up squid and fish pushed to the surface by currents, predators or death. The deployment of capillary depth recorders, which record the maximum dive depth undertaken by a bird (between attaching it to a bird and recovering it when it returns to land), has shown that while some species, like the Wandering Albatross, do not dive deeper than a metre, some species, like the Light-mantled Sooty Albatross, have a mean diving depth of almost 5 m and can dive as deep as 12.5 m. In addition to surface feeding and diving, they have now also been observed plunge diving from the air to snatch prey.
|
