When the black-billed gull started nesting in the bowels of an earthquake-damaged building in central Christchurch, it was hailed as a sign of the city’s rebirth after the 2011 disaster. Some 300 black-billed gulls – once the most threatened gull species in the world before recovering to be classed as “near threatened” – have been nesting in a half-demolished office block on Armagh Street in central Christchurch since November 2019, surprising conservationists and delighting local birders. However, the colony’s unusual choice of nesting site is holding up development on a new Catholic cathedral, for which the site was earmarked last year – and frustrating local business owners. The site of the former PricewaterhouseCoopers building has been cordoned off since it was damaged in the 2011 earthquake. Today it resembles an odd zoo exhibit, with flyers papered to the barriers advising of the birds’ status. The black-billed gull (in te reo Māori called tarāpuka) went from nationally endangered to nationally critical in 2013, after stronghold populations declined rapidly by as much as 80%. It has recovered to “near threatened” status but remains the most threatened of the three gull species that breed in New Zealand. The species is affected mainly by predators, habitat loss and human disturbance, breeding mostly in the braided riverbeds of Canterbury and Southland. But the Armagh Street colony seems to be flourishing in its new urban environment as it nears the end of its second successful breeding season. A department of conservation (DoC) ranger who inspected the colony last month reported that it was “pumping out the birds”, with both chicks and fledglings evident. Vanessa Mander, a DoC community ranger, said the colony was believed to have originated from the nearby Charlesworth Reserve, but was unusual for having opted to nest in a manmade site. “Certainly there is no other city site across the country that has the tarāpuka. … We were somewhat unprepared.” During the 2019 breeding season DoC designed and installed floating platforms to act as life rafts for chicks that were falling from their nests into the car park’s flooded basement. Under the Wildlife Protection Act, once the birds start nesting it is illegal for them to be disturbed until the end of the nesting season. Penalties can extend to imprisonment or a fine of up to $100,000. But with only a few nests remaining at the site, Mander said the birds were at the “very tail end” of their fledgling season. “We would expect the colony to be moving out of that site very soon.” For one local business owner, it can’t come soon enough. Bink Bowler, the owner of Belle cafe facing the site on nearby New Regent St, said the gulls harassed diners, scavenged off diners and befouled his building. The cafe was running a $600 crockery and glassware bill per month, he said: “A lot of it is them … They scare off people, almost.” Bowler questioned holding up the cathedral’s rebuild over them. “Where’s the precedent? … It’s become a big problem.” He said customers had suggested he scare them off with water pistols, but he had so far resisted. “When I tell tourists [that they’re protected], they’re just gobsmacked.” Mander said that once the colony had moved out of Armagh St Doc would work with Carter Group to prevent it from returning as it would have more breeding success elsewhere. “It’s not an ideal site, for a large variety of reasons … We would really prefer them to be moving into these more natural systems, which are really designed for species like them.” In the meantime, she suggested, the urban colony was an unusual opportunity to learn about a rare native bird in close quarters. “There’s a lot to love about them.” This article was amended on 11 February 2021. A previous version incorrectly referred to the black-billed gull as the world’s rarest seagull.