Croydon's parakeets are perched in a precarious position as Government chiefs consider a cull.

But London's leading nature expert claims it could already be too late.

As numbers of the exotic bird have increased, traditional favourites such as the robin, nuthatch and starling - whose numbers have dropped more than 60 per cent in the last 25 years - have come under threat.

Last week the Department for Environment, Food and Rural Affairs commissioned a study of the exotic birds. In recent years the ring-necked parakeet's colourful feathers have been spotted in Norwood Grove and Crystal Palace Park, especially in the trees around the cafe and play area.

The birds are common in parts of south west London, Surrey and Kent. Because they nest in January, long before native birds, the parakeets occupy vacant tree-holes and with nearly 30,000 nesting in the south east - rising to an expected 50,000 in three years - a cull appears to be the only solution.

South London nature expert Tony Drakeford has warned action to curb the bird's numbers is needed quickly. He said parakeets were very invasive and likened them to the problem of Japanese knotweed. He said: "There is not an ecological niche for parakeets in Britain and they are upsetting a fragile ecological balance that has taken thousands of years to form. It maybe too late already."

He said action was needed but a cull would not be easy as the birds prefer to roost in tall trees. Access to prick or inject eggs is limited and gassing or netting could kill other species.

Shooting the birds seems to be the only viable option, but will inevitably cause anger among animal groups, and, as the birds disperse rapidly, holds no guarantee of success.