Sometimes it pays to be a lay-about

The cuckoo seems to go out of its way to behave like an automaton. That inane door-chime of a call, repeated interminably from here to Japan; the indifferent, robotic flight, as if nobody's inside; the dressing-up as a hawk to enrage the little birds and bring them up to mob, thus giving their nests away - all of it seems such a virtual-reality outfit, a charmless, gene-driven masquerade. Nobody has actually proved the bit about the nests, but nobody would put it past the cuckoo either: its whole life is based on clever trickery. And in this, it is far from alone in the bird world. On the very day the summer's first cuckoo flew across the acre, trailing a harassment of angry meadow pipits, the postman brought a new and magnificently-marshalled study of the evolution of con-birds across the world.

Cuckoos, Cowbirds and Other Cheats (Poyser, £24.95 in the UK) is by Nick Davies, Professor of Behavioural Ecology at Cambridge, a name to be bracketed with giants such as Niko Tinbergen and Konrad Lorenz. The book happens to be about birds that fool other birds into hatching and rearing their young, and in this it is deeply enjoyable (and beautifully illustrated by David Quinn). But it also offers, quite incidentally, an exceptionally seductive tutorial on the workings of natural selection.

As with a good many people, the cuckoo was always something of a sticking-point in my initiation into Darwin. Just as creationists point to the human eye as something too complex and perfect to have evolved bit by bit, the elaborate perfidy of the cuckoo had seemed to me simply too devious to have evolved by chance and natural selection. But that was before I knew how it fitted with the wider picture and the history of devious bird behaviour.

The common cuckoo of the Old World is one of 100 species of birds - 57 of them other sorts of cuckoos - that never raise their own young but use other host species to do all the work. A further 185 kinds of birds, and probably a whole lot more to be discovered, will cheat on their own kind by laying in each other's nests.

Starlings, for example, sometimes behave like cuckoos and replace another bird's egg with one of their own, especially if nestholes are in short supply. Moorhens, and several ducks and geese, will lay extra eggs in their neighbour's nests before getting on with their own.

Swallows, too, sometimes lay extra eggs in the nest next door, or carry one in with their beak. And there is one report of a swallow chick, introduced experimentally into a magpies' nest, behaving exactly like a cuckoo chick, balancing an egg on its back between its wing stubs and heaving it out of the nest. So "laying around" is not so unusual, with a mixture of motives, and sibling rivalry between hungry nestlings provides plenty of precedents for a ruthless infancy. The common cuckoo of the Old World has had 100 million years in which to evolve into the ultimate parasitic specialist.

It has now evolved several races, or "gentes", that target specific songbirds and imitate their eggs. In Ireland, the meadowpipit, with its open, accessible nest, is the outstanding host among a dozen that include skylark, blackbird and linnet (in the wetlands around Cambridge, the reed warbler is the favourite host and figures heavily in Davies's experiments).

In Irish museum collections, almost all the cuckoo eggs from meadow-pipit clutches are good-to-perfect imitations of the host's brown-speckled eggs. This owes nothing to the cuckoo's own talents, but results from the host's continual rejection of eggs that look noticeably different from their own - literally a process of natural selection.

Each cuckoo may lay up to 25 eggs in different nests and over time, their slight variations are refined by the survival of eggs with the most acceptable spots and scribbles. Also, a pipit has to learn what its own eggs should look like, and a first clutch that happens to include a cuckoo's egg is automatically accepted.

Once hatched, and having heaved out the other eggs, the cuckoo nestling is served by the pipit's compulsion to feed any begging mouth in the nest, even if it belongs to a huge impostor with the wrong-coloured gape. The cuckoo keeps its foster-parents working by continuous calls that sound like a whole brood of hungry chicks. It imprints on pipits as it grows, and looks for the same species when it returns from Africa the following year.

Rejection of "wrong" eggs is a defence mechanism in what Davies calls the "evolutionary arms race" between the Old World parasites and their hosts. His experiments in Iceland show that meadow pipits, which don't know about cuckoos and the cost of getting stuck with one, are much less ready to throw out odd-looking eggs than pipits in Britain.

In North America, millions of songbirds spend their summers raising cowbirds because their defences haven't caught up with this species' cuckoo-type behaviour. Cowbirds are small, black birds that once thronged the great plains of America, foraging between the hooves of buffalo herds. When European settlers cleared the eastern forests and opened up the west, cowbirds spread dramatically along with the cattle.

They met a whole new range of species to parasitise, especially woodland birds, most of which passively accept the cowbird eggs, however different they look. The cowbird chicks don't clear the nest of competitors but brazenly monopolise the food: their parasitism is blamed for hastening the decline of several threatened species.

Even Davies sounds apprehensive that cowbirds (which migrate in flocks) will ultimately cross the Atlantic and colonise Eurasia: a male was feeding with starlings on the Scottish island of Islay, just across from northern Ireland, in 1988. Davies quotes the warning from a North American ornithologist that any further cowbirds reaching Europe should be "instantly shot, without debate or delay". Meanwhile, the cuckoo seems to be on the decline in Ireland, especially in the south and east, as their meadow pipit hosts lose their rough grassland habitats. But cuckoos can sometimes target new hosts with surprising speed. In Japan, for example, there has been a dramatic spread of the azure-winged magpie and the cuckoos have swooped in to take advantage. Some magpie populations have suffered severely; others are slinging out the eggs or deserting their nests. It's a battle we could do with in Ireland.