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.” Males may display aggressive behavior towards others in an “ill-defined area” 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. Distribution and 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", 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. Various colonies also live on the North Island, though it was formerly only a "visitor", 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. Important Bird Areas Sites identified by BirdLife International as being important for black-billed gull conservation are: Canterbury region Ashburton River / Hakatere Ōpihi River Opuha Dam Orari River Rakaia River Southland region Aparima River Eyre Creek Mararoa River Mataura River Ōreti River Whitestone River Status In regard to Southland colonies, Rachel McClellan notes that “surveys in the 1970s indicated a population of about 140,000 breeding birds, whereas recent counts gave estimates of only 15-40,000 breeding birds”. This suggests a 6% decline in population every year across thirty years, equivalent to up to 83.6% over the whole period, or 50% if the data are