Wildlife lovers will tally up beaks, claws and feathers as they investigate Mitcham's growing parakeet population this weekend.
On Sunday evening a team is set to count the number of the birds roosting in a line of trees on Mitcham Common, as part of a university's city-wide research project.
The group will count the bright green birds at dusk as they swoop back to their roost, and want more sharp-eyed volunteers to help them with the task.
Project manager Hannah Peck, a student at Imperial College London leading the research for her PHD, said the parakeets' distinctive call made them easier to keep tabs on.
She said: "They're easy to find once you hear the noise - you can just follow your ear."
The last count at the Mitcham roost, in July, suggested there were 1,700 parakeets using it. Ms Peck said this was one of the smaller known roosts in the capital, but seemed to be growing - and because it was unusually close to the ground it offered a great opportunity to see the birds up close.
Ms said it was not known why the birds roosted together, but it could be to communicate information about foraging sites or for protection.
Parakeets are were first spotted living wild in London in the 19th century, but since the 1960s - when more of the birds were bought as pets - the population has grown dramatically.
Ms Peck said: "So little research has been done and the numbers have got so big recently. We are keen to see the impact on other birds, and agriculture."
The birds are roosting in a line of trees behind the BP Connect garage on Mitcham Road. The count will begin at 5.30pm, and last for about an hour.