Berta grigia

Identificazione

Sooty shearwaters are 40–51 cm (16–20 in) in length with a 94–110 cm (37–43 in) wingspan. It has the typical "shearing" flight of the genus, dipping from side to side on stiff wings with few wing beats, the wingtips almost touching the water. Its flight is powerful and direct, with wings held stiff and straight, giving the impression of a very small albatross. This shearwater is identifiable by its dark plumage, which is responsible for its name. In poor viewing conditions, it looks all black, but in good light, it shows as dark chocolate-brown with a silvery strip along the center of the underwing.<p>Usually loud, sooty shearwaters coo and croak while on the breeding grounds.<p>In the Atlantic, it is the only such bird, whereas in the Pacific part of its range, other all-dark large shearwaters are found. The short-tailed shearwater in particular is almost impossible to tell apart from the present species at a distance.

Tassonomia

The sooty shearwater was formally described in 1789 by the German naturalist Johann Friedrich Gmelin under the binomial name Procellaria grisea. The shearwater had been briefly described in 1777 by James Cook in the account of his second voyage to the Pacific, and in 1785 the English ornithologist John Latham had described a museum specimen. The sooty shearwater is now placed in the genus Ardenna, that was introduced in 1853 by Ludwig Reichenbach. The genus name Ardenna was used to refer to a seabird by Italian naturalist Ulisse Aldrovandi in 1603, and the specific epithet grisea is medieval Latin for "gray". The species is considered to be monotypic; no subspecies are recognized.<p>It appears to be particularly closely related to the great shearwater (A. gravis) and the short-tailed shearwater, all blunt-tailed, black-billed species, but its precise relationships are obscure. In any case, these three species are among the larger species of shearwaters that have been moved into a separate genus Ardenna based on a phylogenetic analysis of mitochondrial DNA.

Nome scientifico

Ardenna grisea

Autorità

(Gmelin, JF, 1789)

Ordine

Procellariformi

Famiglia

Procellaridi

Genere

Ardenna

Areale e habitat

Sooty shearwaters breed on small islands in the south Pacific and south Atlantic Oceans, mainly around New Zealand, the Falkland Islands, Tierra del Fuego, and in the Auckland Islands and Phillip Island off Norfolk Island. They start breeding in October, and incubate their young for about 54 days. Once the chick hatches, the parents raise their chick for 86 to 109 days.<p>They are spectacular long-distance migrants, following a circular route, traveling north up the western side of the Pacific and Atlantic Oceans at the end of the nesting season in March–May, reaching subarctic waters in June–July, where they cross from west to east, then return south down the eastern side of the oceans in September–October, reaching to the breeding colonies in November. They do not migrate as a flock, but rather as individuals, associating only opportunistically; in June 1906, for example, two were shot near Guadalupe Island off Baja California, Mexico, several weeks before the bulk of the population would pass by. Likewise, the identity of numerous large, dark shearwaters observed in October 2004 off Kwajalein in the Marshall Islands remain enigmatic; they might have been either sooty or short-tailed shearwaters, but neither species is generally held to pass through this region at that time.<p>In the Atlantic Ocean, they cover distances in excess of 14,000 km (8,700 mi) from their breeding colony on the Falkland Islands (52°S 60°W) north to 60 to 70°N in the North Atlantic Ocean off north Norway; distances covered in the Pacific are similar or larger; although the Pacific Ocean colonies are not quite so far south, at 35 to 50°S off New Zealand, and moving north to the Aleutian Islands, the longitudinal width of the ocean makes longer migrations necessary. Recent tagging experiments have shown that birds breeding in New Zealand may travel 74,000 km in a year, reaching Japan, Alaska, and California, averaging more than 500 km per day.<p>In Great Britain, they move south in late August and September; with strong north and north-west winds, they may occasionally become "trapped" in the shallow, largely enclosed North Sea, and heavy passages[clarification needed] may be seen flying back north up the British east coast as they retrace their path back to the Atlantic over northern Scotland.

Areale riproduttivo

Worldwide : s temperate and subantarctic islands from s Chile and Falklands to s Australia and Tasmania, Macquarie (far se of Australia) and outlying islands of New Zealand

Areale non riproduttivo

n to North Atlantic to Greenland and nw Europe, and n Pacific to Gulf of Alaska and w USA

Migrazione

Complete migrants

Comportamento

The sooty shearwater feeds on fish and squid. They can dive up to 68 m deep for food, but more commonly take surface food, in particular often following whales to catch fish disturbed by them. They also follow fishing boats to take fish scraps thrown overboard. Isotopic analyses revealed significant niche overlap between sooty shearwaters and great shearwaters.<p>They breed in huge colonies and the female lays one white egg. These shearwaters nest in burrows lined with plant material, which are visited only at night to avoid predation by large gulls. The architecture of sooty shearwater burrows can vary within and between breeding colonies, and is influenced by competition for breeding space and habitat type, with soil under dense tussac grass being easier to excavate than other substrates.<p>In New Zealand, about 250,000 muttonbirds are harvested for oils and food each year by the indigenous Māori population. Young birds just about to fledge are collected from the burrows, plucked, and often preserved in salt. In 2022, climate change was thought to be impacting this cultural harvest by Ngāi Tahu.<p>Its numbers have been declining in recent decades, and it is presently classified as near threatened by the IUCN. In 2009, the harvest reported record-low catches, on average a trapping cage yielded nearly 500 birds; in 2009, the number was estimated to be closer to 40 per cage.

Conservazione

Tendenza della popolazione

declining