While they still number in the thousands their total numbers are rapidly declining. The majority nest in colonies in the South Island, primarily Southland, from August-February. Colonies often change from year to year on braided rivers through to single-channel rivers and streams with gravel beds. Occasionally birds resort to nesting on adjacent farmland after major flood events or when they can find no suitable breeding habits on riverbeds. In the North Island, the most well-known colony is on the shores of Lake Rotorua. Elsewhere in the North Island, black-billed gulls nest on coastal shell banks and sand spits. Recently, the breeding range has expanded in the North Island as far north as the Kaipara Harbour. After the breeding season, most South Island birds migrate to the coast, though movement patterns are poorly known (distribution map – somewhat dated but still useful).
Endemic (unique) to New Zealand, the black-billed gull (Larus bulleri) also know as Buller’s gull, tarāpuka, tarapuka (Māori), seagull, black-billed gull, and black billed gull, is a medium-sized gull measuring 35-38cm and weighing 230grams. Unlike other common gulls, black-billed gulls are not normally scavengers, and so they are less likely to be found in towns and cities.
The wings and back are pale grey, with white-tipped black margins to the main flight feathers. The eye ring is black and iris is white. The legs are black and the black bill is a relatively long and straight. Non-breeding adults have a bi-coloured bill, reddish at the base with a black tip and a line through the middle.
Rebergenb, A. (2016) Changes in the number of black-billed gulls (Larus bulleri) nesting on the Ruamahanga River, Wairarapa, between 1998 and 2012. Nortonis, Vol 53: 50-53 (PDF)
McClellan (2009) The ecology and management of Southland’s black-billed gulls : PhD thesis (links to abstract)