A bird deemed a pest by most people is now being classified as ‘nationally critical'.

The black-billed seagull is now on the threatened species list – the most serious category usually reserved for the rarest birds because of the rate of expected decline.

The black-billed gull is 'nationally critical”.

According to a report on the bird by the Department of Conservation, numbers are predicted to drop by more than 70 per cent in the next 30 years.

There were an estimated 180,000 to 200,000 of the birds in 1977. There are now thought to be 60,000 to 70,000.

There are now fewer than 100,000 red-billed gulls and the numbers are expected to fall by 50 to 70 per cent in the next 30 years.

As reported by Stuff, DOC bird scientist Hugh Robertson says the reason for the declining numbers is unknown.

Predators such as wild cats, stoats and ferrets were thought to be partly to blame, but global warming could also be at play, says Hugh.

It influenced changes in currents at sea, which affected food supply, and the water flow in rivers, says the report.

'The lower the flow, the islands [in braided rivers] are less defendable. It makes them more accessible to predators,” says Hugh.

'You've got weeds growing on the gravel so there's more cover for predators. They can get closer before they're detected.”

Humans destroying habitats through four-wheel driving and other such activities is also a problem leading to the decline in seagull numbers, explains Hugh.