Berta maggiore

Identificazione

This shearwater is 43–51 cm (16.9–20.1 in) in length with a 105–122 cm (3.5–4.0 ft) wingspan. It is identifiable by its size, dark upper parts, and white under parts, with the exception of a brown belly patch and dark shoulder markings. It has a black cap, black bill, and a white "horseshoe" on the base of the tail. The stiff flight, like a large Manx shearwater, is also distinctive. The only other large shearwaters in its range are the Cory's Shearwater and the all-dark Sooty Shearwater.

Tassonomia

The great shearwater was formally described in 1818 by the Irish naturalist Bernard O'Reilly and given the binomial name Procellaria gravis. The great 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. The specific epithet gravis is Latin meaning "heavy" or "weighty". The species is monotypic: no subspecies are recognised.

Nome scientifico

Ardenna gravis

Autorità

(O'Reilly, 1818)

Ordine

Procellariformi

Famiglia

Procellaridi

Genere

Ardenna

Areale e habitat

The great shearwater, like the sooty shearwater, follows a circular migration route, moving north up the eastern seaboard of first South and then North America, before crossing the Atlantic in August. It can be quite common off the southwestern coasts of Great Britain and Ireland before heading back south again, this time down the eastern littoral of the Atlantic.

Areale riproduttivo

AO : Kidney I. (Falkland Is.), Nightingale, Inaccessible and Gough (Tristan da Cunha group; sc Atlantic Ocean)

Areale non riproduttivo

to s Atlantic off coasts of s South America and s Africa, and n to temperate and Arctic North Pacific Ocean to s coasts of Greenland and nw Europe

Migrazione

Complete migrants

Comportamento

This bird has the typically "shearing" flight of the genus, dipping from side to side on stiff wings with few wingbeats, the wingtips almost touching the water. Its flight is powerful and direct, with wings held stiff and straight.<p>This species breeds on Nightingale Island, Inaccessible Island, Tristan da Cunha, and Gough Island. It is one of only a few bird species to migrate from breeding grounds in the Southern Hemisphere to the Northern Hemisphere, the normal pattern being the other way around. This shearwater nests in large colonies, laying one white egg in a small burrow or in the open grass. These nests are visited only at night to avoid predation by large gulls.<p>The great shearwater feeds on fish and squid, which it catches from the surface or by plunge-diving. It occasionally feeds on crustaceans, fish entrails and other refuse discarded by fishing vessels. It readily follows fishing boats, where it indulges in noisy squabbles. This is a gregarious species, which can be seen in large numbers from ships or appropriate headlands. They have a piercing "eeyah" cry usually given when resting in groups on the water.<p>Great shearwaters are among the seabird species with the highest incidence of plastic ingestion.

Conservazione

Tendenza della popolazione

stable