The white-fronted tern colony on Hairini bridge is under attack from humans, and fears are held for the colony's future.

The bridge colony lives on the pile frames of the old wooden bridge, beside the current concrete structure, giving pedestrians a close view of tern life – and putting them in easy range.

One of the dead birds on Hairini bridge. Photos: Andrew Campbell.

Three nesting birds were killed last week, and biologist Dr Ian McLean has genuine concerns for the colony's safety.

The trio of dead birds were still sitting on their newly laid eggs when Ian saw them last week and they had been dead for several days.

'Entire tern colonies can be abandoned after a disturbance, and the birds may not return in subsequent years,” says Ian.

'We can only hope that the remaining birds will persist with the nests that are still being established.”

'The mates of the dead birds are still visiting and sitting with them. The eggs lie there exposed and ignored.”

Two years ago, the nesting habitat for terns at the bridge was reconstructed when the wooden piling framing moved in a storm. The reconstruction is considered a remarkable success, with 16 nests counted in the 2013/14 season – the largest number recorded.

'Those terns are regularly visited by photographers who come from all over the country,” adds Ian.

'It is probably the most accessible nesting tern colony in the country, and very charismatic because photographers and passers-by can look down upon the nesting birds.

'Unfortunately, the terns nest very close to the bridge footpath. For the five years that I have been monitoring them, the public have respected them and enjoyed them.

The bridge's pile frames are home to the colony.

'They are also very tame because they are so used to people going past on the bridge. If you were that close to a tern colony and not used to people every day, they would be gone.”

He believes it's the only such tern colony within a New Zealand city, as they normally nest on offshore rocky stacks.

'It's quite unusual for them to be nesting well inside an estuary like this,” says Ian. 'It's unusual to have them so close to people.”

The first eggs were laid just before the birds were killed a couple of weeks ago. Incubation is 25-30 days and the first chicks from any remaining eggs are expected to hatch around Christmas.

He says the birds were killed by people, who were probably throwing rocks at them from the bridge.

White fronted terns normally nest out of reach of people, and do not defend themselves and respond like gulls do, says Ian.

Ian's never seen the terns attacked by natural predators like hawks or rats, and there's no chance three terns on new eggs died at the same time, he adds.

Attempts to contact the Department of Conservation in Tauranga have been unsuccessful, says Ian. When SunLive called today the response was to call tomorrow when their bird specialist is back in the office.

As well as stone-throwing humans, the bridge colony faces a longer term threat from plans to four-lane the Hairini bridge.