Canterbury ornithologist Niall Mugan had a nasty Christmas Eve surprise when he found the bodies of 50 black-billed gull chicks stoned to death.
The gulls, one of the most endangered gull species in the world and found only in New Zealand, had been nesting on the Ashley River just outside Rangiora, where there are signs asking people not to disturb the birds during their nesting season.
The Ashley Rivercare group, of which Mugan is a member, had been looking after the colony, keeping down predators and erecting signs.
The chicks had been two weeks away from fledging and flying.
"It's quite upsetting," he said.
"Whoever this person was, they did as much damage as they could in the shortest amount of time."
Although police had been notified, Mugan felt it was unlikely the culprits would be found as it was in such a remote area.
He said the gulls had suffered consistently declining numbers because their habitats were being destroyed.
They nest on braided rivers.
"It's pretty much only Canterbury," Mugan said.
"It's such a shame. In two more weeks, they would have been independent.
"I was pretty shocked when I found them. It could have been a case of mistaken identity.
A lot of people complain about gulls keeping people awake, but there are several types of gulls that don't, and it's difficult to tell the difference between them.
"Some are common and some are endangered.
"This person just picked the rarest type there is."
Disturbing protected birds and destroying nests are offences under the Wildlife Act that can result in up to six months' jail or a $100,000 fine.
Department of Conservation (DOC) biodiversity programme manager for Waimakariri, Dean Turner, said the killing was "very disappointing".
He had spoken with Mugan and went to the site on Christmas Eve to collect information.
"They are a totally protected bird; they are nationally endangered," Turner said.
He urged anyone with information to phone the Rangiora police or DOC.
Last month, a motorist allegedly drove over a colony of 3000 black-billed gulls nesting at the Ashburton River, squashing nests and eggs. This was despite prominent "endangered bird" signs along the river.
In 2007, the same Ashburton colony was attacked by vandals, who killed more than 100 birds.
In November 2010, a man and an alleged co-offender killed 23 fur seals near Ohau Pt, Kaikoura, with a galvanised pipe.
Jason Trevor Godsiff, 20, was jailed for his part in the killings but this was changed to home detention after he appealed against his sentence.