Uria beccogrosso

Identificazione

Since the extinction of the great auk in the mid-19th century, the murres are the largest living members of the Alcidae. The thick-billed murre and the closely related common guillemot (or common murre, U. aalge) are similarly sized, but the thick-billed still bests the other species in both average and maximum size. The thick-billed murre measures 40–48 cm (16–19 in) in total length, spans 64–81 cm (25–32 in) across the wings and weighs 736–1,481 g (26.0–52.2 oz). The Pacific race (U. l. arra) is larger than the Atlantic race, especially in bill dimensions.<p>Adult birds are black on the head, neck, back and wings with white underparts. The bill is long and pointed. They have a small rounded black tail. The lower face becomes white in winter. This species produces a variety of harsh cackling calls at the breeding colonies, but is silent at sea.<p>They differ from the common murre in their thicker, shorter bill with white gape stripe and their darker head and back; the "bridled" morph is unknown in U. lomvia – a murre has either a white eye-stripe, or a white bill-stripe, or neither, but never both; it may be that this is character displacement, enabling individual birds to recognize conspecifics at a distance in the densely packed breeding colonies as the bridled morph is most common by far in North Atlantic colonies where both species of guillemots breed. In winter, there is less white on the thick-billed murre's face. They look shorter than the common murre in flight. First year birds have smaller bills than adults and the white line on the bill is often obscure, making the bill an unreliable way to identify them at this age. The head pattern is the best way to distinguish first-year birds from common murres.

Tassonomia

Nome scientifico

Uria lomvia

Autorità

(Linnaeus, 1758)

Ordine

Caradriformi

Famiglia

Alcidi

Genere

Uria

Areale e habitat

The thick-billed murre is distributed across the polar and sub-polar regions of the Northern Hemisphere where four subspecies exist; one lives on the Atlantic and Arctic oceans of North America (U. l. lomvia), another on the Pacific coast of North America (U. l. arra), and two others that inhabit the Russian arctic (U. l. eleonorae and U. heckeri). Thick-billed murres spend all of their lives at sea in waters which remain below 8°C, except during the breeding season where they form dense colonies on cliffs.

Areale riproduttivo

NA, PAL : HolArctic, high and low Arctic (locally temperate) coasts

Areale non riproduttivo

s of pack-ice, to coastal New York

Migrazione

Complete migrants

Conservazione

Although declines have been observed in many parts of their range, the thick-billed murre is not a species of concern as the total population is estimated to contain between 15 and 20 million individuals worldwide.<p>Egg harvesting and hunting of adult birds are major threats in Greenland, where populations fell steeply between the 1960s and 1980s. In the Barents Sea region, the species has declined locally, due to influences associated with polar stations in Russia. Fisheries may also be a threat, but because thick-billed murres are better able to use alternative food sources the effect of over-fishing is not as severe as on the common murre. Pollution from oil at sea exerts another major threat. Murres are among the seabirds most sensitive to oil contamination. Incidental mortality brought on by entanglement with fishing gear is also an important cause of population decline.<p>Thick-billed murres are closely associated with sea-ice throughout the year. Consequently, some scientists believe that climate change may be a threat to this Arctic-breeding species. However the species seems adaptable. Populations at the southern edge of their range switched from feeding on ice-associated Arctic cod to warmer-water capelin as ice break-up became earlier. Dates for egg-laying advanced with the earlier disappearance of ice. The growth of chicks is slower in years when ice break-up is early relative to egg-laying by the murres. In extremely warm years, mosquitoes and heat kill some breeders.

Tendenza della popolazione

rising