Rare tree sparrows have been fitted with red plastic rings in a national project to study them.
Nearly 400 chicks hatched at Beddington Farmlands this year, and experts are eager to find out what happens once they leave the colony.
The sewage treatment site is home to the biggest single colony of the birds in the south-east, making it one of the most important sites in the UK.
Site managers Viridor have teamed up with the Beddington Farm Bird Group, MKA Ecology, Sutton Council and the RSPB to form the London Tree
The group hopes the ring fittings will help solve the mystery of where the chicks go each year, and help to identify areas alternative colonies could be created to secure the birds' future.
Tree sparrows, the cousin of the larger and more famous house sparrow, are red-listed as its population is falling.
Councillor Colin Hall, Executive Member for Environment and Climate Change on Sutton Council, said: “Tree sparrows are of real concern due to their declining numbers across the country, but we have
one of the largest colonies in the UK right in our back yard.
“By working to protect the birds hatched in Beddington, we can make a real difference to the species’ numbers.”
Although Beddington Farmlands is a traditional sewage treatment site, the way in which sewage is managed has changed.
The large settling pools are no longer needed, meaning the sparrows’ habitat on site will change.
Dr Richard Black of the RSPB said: “Londoners can help by putting up bird boxes, putting out specific food, such as red and white millet, and letting the Partnership know if you spot any blinged-up
tree sparrows with red rings, anywhere in the capital.”
Reported sightings can be emailed to: LTS@rspb.org.uk