Where have all the cuckoos gone?

ADULT CUCKOOS have left our shores and soon their youngsters will bid farewell to their devoted foster parents and head for Africa.

For years, we have lamented the decline of the famous bird.

This summer, however, more cuckoos were reported than usual, at least in some parts of the country. It has even been suggested a comeback is underway. The claims are “anecdotal”, the in-term nowadays for “unreliable” or “exaggerated”. So what do we know, for sure, about our cuckoos and their prospects for the future?

Between 1968 and 1972, bird watchers visited every nook and cranny of Britain and Ireland. The locations of singing birds, nests and fledged chicks were noted and a distribution map prepared for each species. The cuckoo was found in every Irish county and, according to the Atlas of Breeding Birds in Britain and Ireland, there were between 17,500 and 35,000 pairs of them in these two islands.

The census was repeated between 1988 and 1991. It found that cuckoos still bred in every Irish county but that the numbers had declined. The New Atlas of Breeding Birds gave an estimate of 16,000 to 32,000 pairs, 3,000 to 6,000 of them in Ireland. This may not seem much of a reduction on the previous estimate, but cuckoos were no longer found in 237 of the 943 Irish 10km squares in which they had been recorded in the earlier census. They had colonised only six new squares. But how have cuckoos fared since 1991?

David Glue of the British Trust for Ornithology has examined the results of bird-monitoring schemes such as the Common Bird Census and the Breeding Bird Survey. He estimates that cuckoo numbers in Britain fell by 57% between 1967 and 2003 and that the decline is continuing. The 2005 season, he claims, was a particularly bad one for cuckoos. Numbers were down by about a third on those of the previous summer. There has been much speculation as to the causes of the decline.

Cuckoos, as everybody knows, depend on other birds to raise their young. In Ireland, the main host is the meadow pipit, a little brown bird which looks like a thrush and lives on grasslands and open country. Indeed the tit-lark, as we used to call it in Limerick, is known in Irish as banaltra na cúiche, “the cuckoo’s nurse”.

Foisting your eggs on other birds is hard work. A cuckoo can lay up to 25 eggs, one every two days, although most only manage about eight or nine. To do so, she must watch the local pipits, discover where they are nesting and choose the optimum moment to strike. Pipits lay an egg each day and don’t begin incubating until the clutch is completed. The cuckoo waits until a pipit starts laying. Then, when the pair are away from the nest, she approaches quietly, removes one of the pipits’ eggs and lays her own. With one cuckoo’s egg to each nest, a cuckoo needs an area with plenty of pipits if it is to breed successfully. The decline in cuckoo numbers, therefore, could be caused by a general reduction in pipit numbers.

Meadow pipits are much more difficult to census than cuckoos and we can’t be too confident about population estimates. In the 1968 to 1972 census, however, there were thought to be over 3,000,000 pairs of meadow pipits in Britain and Ireland. By 1988 to 1991, the number had fallen to 2,800,000 pairs. Pipits disappeared altogether from 5.5% of the 10km squares which they had occupied during 1968 to 1972.

Of course, a cuckoo will lay in the nests of other birds, if she can’t find enough pipit ones. With plenty of alternative hosts available, a drop in pipit numbers should mean diversification on the part of cuckoos rather than a massive decline in their numbers. There must be other factors at work.

The cuckoo travels to Africa, south of the equator. In recent decades, the Sahel region, north of the Sahara, has suffered from prolonged droughts. This has increased the size of the desert which migrating birds must cross. The longer journey may prove too much for some birds. Young cuckoos, migrating for the first time, could be especially vulnerable.

David Glue raises a third possibility; habitat degradation in Africa. The land in parts of Africa has been rendered unusable for farming through over-exploitation and climate change. Bird numbers have been reduced by these changes and cuckoos may have suffered. This argument is, however, somewhat speculative since we don’t know where in Africa our Irish cuckoos spend the winter. Of the 5,894 cuckoos ringed in Britain and Ireland up to 2002, only one has turned up in Africa. This was a bird ringed as a nestling in Britain and found subsequently in Cameroon. The only other such record was of a cuckoo ringed in Holland which turned up in Togo.

The decline of the cuckoo is probably due to a combination of factors and part of a much wider decline in the numbers of countryside birds generally. Habitat destruction both here and in Africa, the abuse of pesticides, intensive farming methods and, in all probability, global warming are the likely culprits. Cuckoo numbers may have increased slightly this year, but the jury is still out as to whether any significant recovery is underway.

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