The death of three "at risk" native birds in a Tauranga colony is devastating and suspected to be the result of foul play by idiots, a local biologist says.

Ian McLean, a professional Ornithologist, had monitored the small colony of White-fronted Tern on the Turret Rd bridge for the past five years and suspected the birds had been killed when he discovered them dead last Friday.

"It's just abominable. I had a very sleepless night."

The flock had made the wooden trestles under the bridge their home for a number of years with people coming from all over the country to photograph them, Mr McLean said.

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"This is not just any tern population. It is the most accessible tern population in New Zealand."

The accessibility had been an attraction for the colony but Mr McLean suspected it may have led to the death of three of their mating adults.

"In the five years I've been monitoring them I've only ever seen one dead ... All of a sudden you get three birds at once.

"Those birds were very healthy, they've just finished egg laying."

He suspected they had been targeted by people throwing rocks, as he had seen the same sort of behaviour from people at a Rotorua lake recently.

The death of the three terns came at a similar time to 30 little blue penguins being killed near Dunedin.

The NZ Herald reported that rangers and animal control were on the hunt for a mystery killer after 21 bodies were recovered from the rocky coast at Doctors Point on Saturday, another seven on Sunday and two on Tuesday.

Jason Blair from Tauranga SPCA was not aware of the death of the birds but said they had received calls time to time of people throwing things at birds.

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"People throwing rocks and what not or luring seagulls into the path of cars with bread.

"It's mindlessness really ... Other people are just silly and not thinking about it."

He said the SPCA had noticed a "disturbing trend of recreational cruelty" growing on social media such as Facebook and YouTube where people were sharing videos of animal cruelty.

White-fronted Terns
• New Zealand status: Native
• Conservation status: Declining
• Most common tern on the New Zealand coastline
• Mainly a marine species that is seldom found far from the coast.
• The name "white-fronted" refers to the "frons" or forehead, where a thin strip of white separates the black cap from the black bill.NZ Birds Online