Gabbiano dal becco nero

Identificazione

A healthy adult black-billed gull is typically 35–38 cm long, with a wingspan of 81–96 cm, and a weight of around 230g.: 545  The head, body, and parts of the wings are white, with silvery grey on the saddle and wings, as well as black edging on the wings.: 546  The gull also undergoes some seasonal color change. While typically black from February to June, the orbital ring is orange-red, red, or dark red the rest of the year.: 556  The legs, too, change from black to dark red and even bright red as the breeding season progresses, "possibly stimulated by presence of begging chicks and juveniles.": 556  Observations suggest the gull is sexually dimorphic, but there is a lack of published data to support this.: 557  There is likewise a lack of data in regard to geographical variation.: 557 <p>Easily mistaken for the red-billed gull, the black-billed gull is distinguished by its black bill and is described as having a "more delicate appearance", a "more buoyant and graceful" flight, and being "generally less noisy", despite having a similar call.: 546  F1 and F2 hybrids between the two gulls have been observed, both hybrids exhibiting dark red bills.: 556

Tassonomia

Originally named Gavia pomare in 1855 by Carl Friedrich Bruch, the name was rejected by the New Zealand ornithologist Sir Walter Lawry Buller because it was already being used for another species.: 58–59  He then took up Prince Napoléon Bonaparte's "playful" genus name Bruchigavia (literally, "Bruch's seabird") as a provisional name for New Zealand gulls.: 60  But because Buller's proposed species name melanoryncha (literally, "black-billed"): 545  had already been given to another gull species, Frederick Hutton suggested the name bulleri, in honor of Buller, in 1871.: 59  Buller accepted the offer and followed others in adopting the "larger and better-defined genus" of Larus.: 59  The alternative common name Buller's gull also retains the connection to Buller.<p>The species is now considered to belong within the genus Chroicocephalus. The holotype is in the collection of the Museum of New Zealand Te Papa Tongarewa.

Nome scientifico

Chroicocephalus bulleri

Autorità

(Hutton, FW, 1871)

Ordine

Caradriformi

Famiglia

Laridae

Genere

Chroicocephalus

Areale e habitat

The black-billed gull is endemic to New Zealand. Up to 78% of the total population is estimated to be living in the Southland region, on the southern end of the South Island. In breeding season the gull is found on major rivers, especially braided rivers, lakes, and farmland. It generally prefers estuaries and coastal areas outside of the breeding season, though some can be found at breeding sites all year round. The gull is also attracted to urban areas, and "anywhere refuse of scraps available",: 547  such as rubbish dumps and freezing works. In 2019 some gulls established a colony of around 300 birds in Christchurch Central City. The species has been sighted occasionally on Stewart Island and The Snares, as well as at altitudes of up to 1700 MASL on the mainland.: 546–47  Various colonies also live on the North Island, though it was formerly only a "visitor",: 547  the first recorded breeding taking place at Lake Rotorua in 1932. Some South Island birds cross the Cook Strait after breeding season to winter in the North Island.: 548–49 <p>Sites identified by BirdLife International as being important for black-billed gull conservation are:

Areale riproduttivo

AU : mostly on braided rivers, North and South Is. (New Zealand)

Areale non riproduttivo

some move to coasts

Migrazione

Complete migrants

Comportamento

Colonies are formed around the first pair that begins nesting. Where more than one pair forms the initial nest, clustering around these sites takes place within the colony. Colonies are frequently tightly packed, with "Gulls little more than pecking distance from each other and nests often touch, leaving little room for taking off and landing".: 550  Males may display aggressive behavior towards others in an "ill-defined area": 550  for a few minutes before leaving the area and forgetting it. Nonetheless, not all male aggression is linked to defending a particular area, and some pairs without chicks will show aggression towards those with chicks. Fights are rarely prolonged, usually consisting of a single attack by the aggressor, employing the bill for pecking, the wings for beating, and the legs for scratching. The target typically retreats right away. Adults roost at the breeding colony or feeding sites, though the latter is more common.: 551–52

Conservazione

In October 2018, the Department of Conservation began a trial cull of black-backed gulls along the Hurunui River to control predation of black-billed gull chicks and other threatened bird species. In August 2019, after a successful trial, the department announced a five-year programme extending to the Waiau Uwha River that aims to reduce the black-backed gull population in the areas by at least eighty per cent.<p>The black-billed gull placed 30th, with 441 votes, in Forest &amp; Bird's 2018 Bird of the Year competition.

Tendenza della popolazione

unknown