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".

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.

Up to 78% of the total population is estimated to be living in the Southland region, on the southern end of the South Island.

The gull is also attracted to urban areas, and "anywhere refuse of scraps available",[9]: 547  such as rubbish dumps and freezing works.

[9]: 546–47  Various colonies also live on the North Island, though it was formerly only a "visitor",[9]: 547  the first recorded breeding taking place at Lake Rotorua in 1932.

[17]: 26, 174  Higher population sizes in the 1950s and 1960s were potentially the result of increased agricultural activity, but the correlation is tenuous.

[17]: 13  Populations at this time were of legendary proportions, with anecdotes telling of farmers who "were forced to wear raincoats so as not to be coated in droppings".

[6]: 90 In her study of Southland colonies of black-billed gulls, Rachel McClellan found that eighty per cent of observed chick deaths resulted from predation.

[17]: 105  Introduced mammals, namely ferrets, stoats, cats, and hedgehogs, constitute the "primary factor influencing productivity", that is, nest success, in the colonies.

Largescale shootings of black-billed gulls, such as a 2009 "massacre" of around 200 birds in North Canterbury,[22] are particularly detrimental to the species' future.

[6]: 88  Morris suggests such actions in part result from confusion with the more common and disliked red-billed gull (Chroicocephalus novaehollandiae scopulinus), which is nonetheless also endangered in New Zealand.

[6]: 88, 94–94  Although no studies have been conducted in this area, previous DDT usage on farms may affect the breeding success of black-billed gulls and herbicides that remain in use could have consequences that are yet unknown.

[25] In October 2019, Mike Turner responded to a proposal for commercial rafting along the Mataura River, citing concerns for the welfare of black-billed gulls.

[27] 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.