About Chroicocephalus bulleri (F.W.Hutton, 1871)
A healthy adult black-billed gull (Chroicocephalus bulleri) 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, and black edging along the wing margins. This species undergoes seasonal color change. The orbital ring is black from February to June, and becomes orange-red, red, or dark red for the remainder of the year. The legs also change color, shifting from black to dark red and even bright red as the breeding season progresses; this change is "possibly stimulated by presence of begging chicks and juveniles." Observations indicate that the black-billed gull is sexually dimorphic, but there is no published data to confirm this claim. There is also a lack of published data regarding geographical variation across the species' range. The black-billed gull is easily confused with the red-billed gull, but can be distinguished by its black bill. It is also described as having a "more delicate appearance", a "more buoyant and graceful" flight, and being "generally less noisy" than the red-billed gull, despite having a similar call. F1 and F2 hybrids between the black-billed gull and red-billed gull have been observed, and both types of hybrid have dark red bills. The black-billed gull is endemic to New Zealand. An estimated up to 78% of the total population lives in the Southland region, at the southern end of the South Island. During the breeding season, the gull is found on major rivers (especially braided rivers), lakes, and farmland. Outside of the breeding season, it generally prefers estuaries and coastal areas, though some individuals remain at breeding sites year-round. This gull is also attracted to urban areas, and "anywhere refuse of scraps available", such as rubbish dumps and freezing works. In 2019, a colony of around 300 black-billed gulls established itself in Christchurch Central City. The species has occasionally been sighted on Stewart Island and The Snares, as well as at altitudes of up to 1700 meters above sea level on the New Zealand mainland. Multiple colonies are also now found on the North Island; it was formerly only a visitor to the North Island, with the first recorded breeding taking place at Lake Rotorua in 1932. Some South Island black-billed gulls cross the Cook Strait after the breeding season to winter in the North Island.
