Scientists are fitting tiny backpacks containing trackers onto one of the UK's most well-known birds to study their movements.
The bird experts are worried about the cuckoo, as there are only a third of the number there were 25 years ago.
They will tag five male birds with trackers in tiny backpacks, with straps that fit around the cuckoos' wings.
This is the first time cuckoos in the UK have been tracked and it should show where the birds like to live.
The trackers weigh just 5g and feeds back information to a computer which tracks the birds as they migrate.
The birds' call is very distinctive
The birds only spend from mid April to mid July in the UK, and the team hopes that the information generated by the project will shed light on what is causing their decline.
Dr Chris Hewson, from the British Trust for Ornithology, said they'd like to have tagged more but each tracker costs £2,000!
He said: "At the moment, we don't know what cuckoos do when they leave Britain - how they move around, where they go and when."
Cuckoos are well-known for their habit of laying their eggs in other types of birds' nests.