But they need the public to help them out in a survey.
The Woodland Trust and the British Trust for Ornithology want the public to let them know when they hear one.
Woodlands, where most cuckoos find their favourite nibble - the hairy caterpillar - are being damaged by climate change.
And as more land is used up by humans, there are fewer places for the birds to nest.
Here are some other things they want you to look out for too:
Register your sightings at the Nature's Calendar website.
- swifts and swallows
- spring flowers including bluebells, oxeye daisies and dog
- insects, like bumble bees, queen wasps and seven spot