Warning: This article contains images that may disturb some people.
Bird protectors arrived at north Canterbury's Waimakariri River to find a horrifying sight on Monday afternoon after five black-billed gulls were brutally killed.
The birds, which are the most threatened species of gull in the world, were found on the Miners Bank Rd-end of the river.
Braided River Aid posted photographic evidence of the gruesome discovery to its Facebook page.
"Gut-wrenching, illegal, and morally abhorrent. Five black-billed gull fledgelings - which were just days away from flying, were stoned to death on the Waimakariri River," the Facebook post says.
"One of the birds has a flattened head and a rock next to it, plus the rest look like they have similar head trauma."
Braided River Aid believes the killings happened mid-Monday morning.
"If anyone has any information about this, or anyone was in the area at the time and believe they have information about who was responsible - please contact firstname.lastname@example.org."
Anita Spencer, a senior ranger for the Department of Conservation, told Stuff a large amount of work is put into ensuring the gulls are protected.
"It is gut-wrenching that they got to this stage for them to be killed."