The mother of all freeloaders

The business of procreation is demanding and downright dangerous for birds. While they're raising young they have little time to find food for themselves, and nest sitting, whether at ground level or in the branches of a tree, is far more life threatening than living on the wing.

No wonder the female European cuckoo, cuculus canorus, has devised a way of avoiding such worries. As a nest parasite, she shirks the responsibility of parenthood altogether by duping birds like robins, meadow pipits, warblers and dunnocks to look after her eggs and her young. Meanwhile her mate's distinctive call (quite unlike her bubbling chuckle) has become the familiar spring herald.

The habits of this bird may appear bizarre, but the resulting myth and folklore seem even stranger. Two centuries ago, the disappearance of the cuckoo in late summer led to a popular belief that the bird actually turned into a sparrow hawk. Several types of cuckoo do indeed resemble birds of prey but this has probably evolved as a crafty method of deterring predators.

Many country stories are also told about why small birds can be seen mobbing cuckoos. This happens to the adult female because the small birds know that she is nest hunting and they are only trying to protect their own eggs. The cuckoo spends much time waiting in trees and monitoring the nesting activities of potential hosts so that she can synchronise egg production to coincide with theirs.

Once a newly laid nest of eggs is chosen, the cuckoo takes one egg, flies off and eats it, returning immediately to lay her own. The sight of a cuckoo in flight with an egg in her bill led to one belief that the bird laid its egg away from the host's nest then carried it there when the bird was out feeding. Another theory was voiced in an old children's song which describes the cuckoo as having to suck other birds' eggs to "make her song clear".

The cuckoo's ability to lay an egg on demand is because, unlike most birds, she can retain it inside her body for up to 24 hours, giving it a head start on the host's clutch. She may lay between 10 and 25 eggs in any one season, and to prevent them being detected, they are coloured to match those of the host - cuckoos which inhabit pipits' nests lay spotted eggs, while those using the redstarts' nests in Europe lay pale blue ones.

When the young cuckoo hatches, it will hunch its back and push the legitimate eggs out of the nest. From then on it will have its adopted mother's undivided attention - and it is fed not just by her but by other nearby birds as well. With its deafening "feed me" call and its bright orange throat, the young cuckoo is, apparently, irresistible to most birds - except of course, the real cuckoo mother.

But how does the young cuckoo learn its adult call? And why doesn't it automatically copy the song of its host parent? Some country folk thought the parent cuckoo would sit and teach the young once it had left the nest - yet the theory was difficult to sustain, bearing in mind the cuckoo's silence towards the end of June. In the 19th century it was thought that cuckoos learnt their adult call during winter migration or in the following spring after their return.

However, the greatest question has always been why the cuckoo became such a freeloader in the first place. Some believed the bird to be hermaphrodite, giving this as a reason why two cuckoos were supposedly never seen together. A different, more intriguing theory was put forward by the 18th-century French naturalist, Vaillant. He believed the birds were ardent lovers so they had no time for the niceties of household chores. Sounds reasonable, but personally I'd back the danger-dodging theory.