Another Life: Evolutionary trick that has other birds working for the cuckoo

A farmer friend once showed me how to summon a cuckoo from the sky. As its call chimed tirelessly from beyond the trees around his house he spat on the palm of one hand, then cupped them both and brought them to his lips. He blew, as into a trumpet, his fingers uncurling to shape the notes. The bird duly appeared above our heads, its door chime pausing in the vain search for a rival.

My friend, sadly, has now passed, so such a folk skill may have disappeared with him, along with the many Irish names he knew for his fields. The cuckoo, too, is in apparent decline. It may already be here from Africa, ahead of the traditional influx nearer the end of the month, but, erratic as ever in its timing and travelling, its presence now warrants the first attempt at counting.

Don’t tell me, please, that you’ve heard the cuckoo. Instead, send a record online to the North’s Centre for Environmental Data and Recording, which will share it with the Republic’s biological records centre in Waterford. The website, habitas.org.uk/ records/submit-cuckoo-record, asks for the habitat the cuckoo was in, and obviously needs to be sure that you weren’t hearing the collared dove, with its similar call.

In Britain, where numbers of cuckoos have halved in 20 years, at least one of its many mysteries – its migration routes to and from Africa – is beginning to yield to technology. Since 2011 the British Trust for Ornithology has been attaching solar-powered satellite-tracking devices to cuckoos summering in East Anglia, Scotland and Wales. Many have subsequently gone missing in action, perhaps in unseasonal hailstorms or in the spreading expanse of the Sahara, but enough have survived to show the birds making U-turns to find more food, and taking very different routes across the Mediterranean to wintering grounds in the Congo and elsewhere. You can follow the travels this year of nine surviving birds at bto.org/science/migration/tracking-studies/ cuckoo-tracking.

'Brood parasitism'
More than enough mystery remains, however, in the details of what science calls "brood parasitism" and Gilbert White, with more feeling, termed "a monstrous outrage on maternal affection". That the cuckoo lays its egg in a small bird's nest to get the host parents to feed and rear its big, ugly chick is common knowledge. Not so widely known is the chick's incredible behaviour in emptying the nest of all the competition.

Working blindly and instinctively, apparently in response to any pressure on a sensitive hollow patch on its back, the cuckoo chick gets underneath an egg or fellow nestling, backs up the side of the nest and tips it over the edge. Then it begs for food with a rapid call that mimics the whole brood of missing chicks. The baby cuckoo imprints on its foster mother as it grows, then on its return as a breeding adult looks for the same species to use.

Brood parasitism is an evolutionary trick shared globally with 57 other sorts of cuckoo and some 180 other bird species. Even among Irish birds, moorhens and several ducks and geese will lay extra eggs in their neighbours' nests before getting on with their own. Swallows, too, may lay extra eggs in a neighbouring nest or carry one in with their beak. One Spanish experiment, quoted by Richard Dawkins in The Selfish Gene , found a swallow chick, introduced into a magpie nest, heaving out eggs lodged between its wing stubs ("an extreme case of what must go on in any family," judged Dawkins).

Deception
Different races, or "gentes", of cuckoo have evolved to target specific songbirds for deception, imitating the size and pattern of their eggs. In Ireland the meadow pipit, with its open, accessible nest, is first choice, among a dozen that include the blackbird, skylark and linnet. The loss of suitable rough habitat for the pipit may help explain the cuckoo's decline in Ireland, especially in the intensive farmland of the eastern and northern counties. In the UK the reed warbler is often preferred, and the loss of wetlands may show the same kind of link.

The female cuckoo lays a lot of small eggs during its breeding season – perhaps 25 in different nests – and they are not always good enough copies to deceive the pipit’s eye. In Irish museum collections almost all the cuckoo eggs retrieved from pipits’ nests are good-to-perfect imitations.

Pipits need to lay at least one clutch of eggs to know what they look like, however, so first-year pairs are more likely to accept the cuckoo’s fake. Experiments in Iceland have suggested that pipits who don’t know about cuckoos, and the labour cost of getting stuck with one, are more likely to be fooled than the cuckoo-wise pipits of Britain, who may throw out the fake egg or abandon the nest.