Nobody knows for sure why cuckoos have been declining in numbers
The unmistakable sound of a cuckoo's song is becoming more and more infrequent, as bird numbers in the UK get fewer and fewer.
"For every 10 cuckoos that were around 10 years ago, there are now only four," said David North, principle ecologist for monitoring at the BTO in Thetford.
With help from the BTO, BBC Springwatch was able to bring viewers pictures of young cuckoos in the wild in 2009.
The birds were found nesting in Thetford Forest.
The exact reason for the steep decline is unclear, but it's thought that there are three main factors contributing to the cuckoo's downfall.
"We think one of the reasons is that they are dependant on hairy caterpillars, which are in decline," said David.
"Cuckoos are choosing to spend more time in Africa due to climate change and because they are classed as parasites, and are dependant on other animals, if those other animals are in decline there is a knock on effect," he added.
Figures for East Anglia show that cuckoo numbers have seen a 57% decline between 1994 and 2007. The only way to gage numbers is by asking people to help in a bird survey.
In May 2009, BBC Springwatch teamed up with the BTO and asked the public to help get an up to date snapshot of how the cuckoos are doing.
Within just a few weeks, more than 12000 people responded and helped plot the success of cuckoos in the UK.
The BTO have produced a map plotting
cuckoo sightings in East Anglia.
Much of the interest has been generated by the BBC 2 wildlife programme, which this year managed to film a cuckoo chick who took over a reed warblers nest.
The programme followed the young bird for almost three weeks as it grew from a tiny pink chick, to a little monster.