The black-billed gull (Chroicocephalus bulleri), Buller's gull, or tarāpuka (Māori) is a Near Threatened species of gull in the family Laridae. This gull is found only in New Zealand, its ancestors having arrived from Australia around 250,000 years ago. Taxonomy 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. He then took up Prince Napoléon Bonaparte’s "playful" genus name Bruchigavia (literally, "Bruch's seabird") as a provisional name for New Zealand gulls. But because Buller's proposed species name melanoryncha (literally, "black-billed") had already been given to another gull species, Frederick Hutton suggested the name bulleri, in honor of Buller, in 1871. Buller accepted the offer and followed others in adopting the "larger and better-defined genus" of Larus. The alternative common name Buller's gull also retains the connection to Buller. 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. Description 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. 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. 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. 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." Observations suggest the gull is sexually dimorphic, but there is a lack of published data to support this. There is likewise a lack of data in regard to geographical variation. 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. F1 and F2 hybrids between the two gulls have been observed, both hybrids exhibiting dark red bills. Behaviour 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