The Department of Conservation says the massacre of 51 endangered black-billed gulls in a Canterbury riverbed is a disturbing and mindless crime.
The chicks were found stoned to death in the Ashley River near Rangiora on Christmas Eve.
A DoC spokesperson, Dean Turner, says the killing of the birds, who were about two weeks away from being able to fly, is mindboggling.
He says the loss of 51 chicks could cut the Ashley River population by up to 70%.
Mr Turner says black-billed gulls are only found in New Zealand and there are thought to be just 90,000 birds left.
He says they are nationally endangered and anyone harming or killing them could be fined up to $100,000 or jailed for six months.
Ornithologist Niall Mugan says the birds were nesting in the braided river system near Rangiora.
He was surveying the population on Christmas Eve when he found the chicks dead, some with rocks on them.
Mr Turner says DOC staff are really disappointed by attack which will have a significant impact on the local population.
Last summer the department reported that at least 60 of the birds were been found dead at Govers Island, a breeding colony on the Clutha River.
Those birds were found in February 2012, but showed no sign of predation or obvious injury.
More than 100 black-billed gulls were shot dead during the summer of 2010 near Wrey's Bush in Southland.
And in August of this year, Canterbury's regional council reported people driving onto an Ashburton Canterbury riverbed were destroying the nests and eggs of black-billed gulls.