Five young black-billed gulls believed to have been stoned to death were found near a Canterbury river on Monday afternoon.
The "gut-wrenching" discovery was made near the Waimakariri River about 12.30pm, close to the end of Miners Bank Rd in McLeans Island.
It was reported on the Braided River Aid Facebook page. The organisation is involved in river management and conservation, with a focus on protecting critically endangered birds.
"Five black-billed gull fledglings, which were just days away from flying, were stoned to death on the Waimakariri River out from the end of Miners Bank Rd," the post read.
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"Gut-wrenching, illegal, and morally abhorrent.
"One of the birds has a flattened head and a rock next to it, plus the rest look like they have had similar head trauma.
"They were found around 12.30pm (on) 17 February, and were still soft, ie the killings must have happened sometime around mid-morning," said the post.
Anyone who has information about the deaths, was in the area at the time or may know who was responsible, should email the Department of Conservation.