The cuckoo: A cheat, a thief and a killer

The cuckoo's call is probably better known than that of any other bird in Britain – an instantly recognisable signal that spring is here. But the owner of the call is a cheat, a thief and a killer – wiping out the families of other birds and tricking them into raising its young instead.

For centuries, nature-lovers have puzzled over the cuckoo's strange behaviour. It dupes other birds into caring for a chick that will quickly dwarf it in size after it has destroyed any eggs or babies. Now the cuckoo's secrets are about to be revealed in a remarkable TV documentary tonight on BBC2 narrated by Sir David Attenborough.

Cuckoo shows exactly how the bird spends its brief annual visit to Britain. Using a combination of archive footage, a re-enactment of a study carried out in the 1920s, footage using new techniques by the award-winning cameraman Alastair MacEwen; and the latest findings by Professor Nick Davies, of Cambridge University, who has been studying cuckoos on the National Trust's Wicken Fen, and elsewhere, for more than 20 years.

Professor Davies has shown the quite exquisite adaptations and counter-adaptations that have evolved among cuckoos and their hosts. For example, by using model eggs he has been able to test which cuckoo "host" species are discriminating about foreign eggs in their nests, and which are not.

In the case of a bird which is not discriminating – such as a dunnock, or house sparrow, which will "sit on anything" – the cuckoos that parasitise it have not needed to evolve a special egg (different groups of cuckoos tend to stick to one host species). But among species which are very choosy, such as the meadow pipit or the reed warbler, the cuckoo groups which parasitise them have evolved eggs that resemble the host species' egg very closely.

An even more stunning discovery by Nick Davies, shown in the film, was of how a comparatively tiny host species such as a reed warbler is tricked into working doubly or trebly hard to feed a cuckoo chick many times its size. The answer is that a cuckoo chick makes a noise which is not just like one reed warbler chick asking for food – it resembles a whole brood of reed warbler chicks demanding loudly to be fed. The parent reed warbler's instinct simply makes it work overtime on hearing the call.

The cuckoo's parasitic behaviour puzzled biologists for years. Aristotle, in 4BC, was well aware that cuckoos foisted their eggs on to other birds. Subsequent observers struggled to add significantly to cuckoo knowledge until the 1700s, and even then they found it hard to figure out what was really going on. Some believed that cuckoos laid their eggs on the ground, swallowed it and then regurgitated it into the host's nest. It was not until the 18th century that anyone noticed cuckoo eggs were often very similar to those of the species they parasitised. Rather than suspecting that to be the way cuckoos duped their hosts, the similarity was thought to occur because cuckoo and hosts ate the same food. Even after ornithologists (well, egg collectors actually) had latched onto the idea of egg mimicry, they still had difficulty making sense of it.

The way a young cuckoo ends as the sole occupant of a nest was another puzzle. One idea was that the cuckoo chick simply ate its nest mates. Another was that the mother cuckoo returned to remove (or eat) the host young. The truth was discovered in 1788 when Edward Jenner (of inoculation fame) watched a young cuckoo in the process of evicting a nestful of host young.

Tim Birkhead is professor of behaviour and evolution at the University of Sheffield. His most recent book is 'The Wisdom of Birds, a new history of ornithology'

Switch your Gas & Electricity and save up to £380 with Independent Compare