THE once-widespread sound of the cuckoo on sunny spring days – particularly in early morning and evening – is now at a premium.
In recent years there has been a population crash and the bird is now on the red list of endangered species.
Count yourself lucky if you hear one, even if you live in an area where you've always heard them.
A number of reasons for the cuckoo's decline has been suggested.
Climate change is thought to have made their migration routes from central Africa more hostile; the decrease in "host" species such as reed warbler and meadow pipit is also believed to have had an impact.
A more interesting theory is that host species are becoming better at identifying and disposing of alien eggs in the nest.
In my opinion, it is likely to be a combination of these factors and others that conservationists have yet to identify.
The cuckoo has a curious life cycle. Male birds arrive in Britain significantly before females and begin their characteristic "cuck-oo" call.
After locating a male and mating, the female identifies the nest of a host species before depositing her egg in it (sometimes removing and eating the host parent's eggs in the process); her job is then complete and, shortly afterwards, she begins her return migration to Africa.
The egg is then incubated by the host parents, and the chick – which ejects any remaining eggs or chicks when it hatches – subsequently makes exhausting demands on them for food.
When the chick fledges, it makes its way to Africa alone.
How does it know the way? That is one of the mysteries and miracles of nature.
The male's song varies throughout its vocal period.
When they begin in late April, the "cuck-oo" is at a musical interval of a major second; this rapidly widens into a minor third then, more gradually, to a major third, perfect fourth and, finally, towards the end of June, an augmented fourth.
After that, the bird croaks for a few days and then falls silent. Listen for the changes.
Among the places you can hear cuckoos at the moment are Bookham Common, Holmwood Common and Inholms Nature Reserve.