Another Life: Cuckoos v pipits – next round in the arms race

The insistent, ultimately irritating cu-coo-cu of the collared dove, returning to nest in a mopheaded cypress, prepared me for claims of first cuckoos, prompted by Streptopelia decaocto, the ubiquitous dove from Europe. The migrant male cuckoo, Cuculus canorus, calls cuck-oo, just like it says on the tin.

When climate was relatively steady, Irish ornithologists could write of an "ordinary" first time to hear the cuckoo – say April 16th to 30th – but all bets are now off. As I write, on a sunny Easter Monday, eight cuckoos radio-tagged by the British Trust for Ornithology, having separately moved north from the Congo forests, were pausing to feed on insects at the sub-Saharan fringe. (Track them now at bto.org/cuckoos.)

Transmitters small enough for cuckoos are relatively recent, and, lacking satellite tracking or the survival of the few ringed birds, the travels of Irish cuckoos remain unmapped. The best place to see or hear them is still around the Burren. (You'll find a map at biodiversityireland.ie.) But a severe decline in the northeast – perhaps by half in 25 years – has prompted Cedar, Northern Ireland's record centre, to appeal for reports of the bird (online at nmni.com/cedar).

Study of the cuckoo has been long and disputatious since Aristotle’s observation, 2,300 years ago, that the birds “do not sit, nor hatch, nor bring up their young, but when the young bird is born it casts out of the nest those with whom it has so far lived.”

Today’s students, evolutionary biologists, talk of an “arms race” of adaptation between these “brood parasites” and their targeted hosts.

There are far more of both than one might suppose. Species of cuckoo across the world make up most of the 100 parasitic birds that always lay in other birds’ nests. Their “victims” have ranged over 125 species, from magpies in Japan to fairy wrens in Australia, and at least 16 genetic tribes of cuckoo lay eggs that mimic those of particular host species.

It takes about 10 seconds to nip in and snatch an egg, then lay one in its place – an operation often repeated at many nests in a season, each watched for the right opportunity.

Most female cuckoos now returning to Ireland lay brownish spotted eggs resembling those of the meadow pipit, and they “home” to the pipits’ rough grassland habitats, close to where they were born. This could be why they have grown so scarce in eastern Ulster, where intensive farmland has changed the landscape drastically.

Cuckoos returning to Britain specialise in meadow pipits nesting on moorland and heaths, dunnocks and robins in woods and farmland, and reed warblers in marshes, laying eggs with some degree of match to each species.

Some are rejected, especially if the host bird has seen the cuckoo nearby (this tested by experiments with model eggs and stuffed cuckoos). And Irish pipits, as I’ve watched many times, don’t hesitate to mob the hawk-like cuckoo at its teetering perch on a telegraph wire.

There can be costs, however, in rejecting a suspicious egg in the nest. Experiments by Prof Nick Davies of Cambridge, author of Cuckoos, Cowbirds and Other Cheats (Poyser, 2000) showed the risks to the host's own eggs of trying to get hold of the fake laid by a cuckoo and heave it out.

Recognising the intruding egg brings more variables into play. Birds parasitised with their first clutch have to learn what their own eggs look like, and may accept the cuckoo’s among them. But blackcaps have been hailed as “winners in the evolutionary struggle” for their rejection of cuckoo eggs, both real and model.

The rejection could hang on several factors. The patterns on blackcap eggs vary little within one clutch but quite a lot between clutches. This makes it harder for cuckoos to produce a passable egg for all blackcap nests – and easier for any one pair of warblers to spot the odd egg.

If birds can often tell the difference between the cuckoo’s cheating egg and their own, how is it that so many go on feeding the young cuckoo when it hatches – a big, ugly, noisy chick, often several times their own size, that has heaved their own brood, one by one, over the side of the nest?

As Prof Arnon Lotem, an Israeli zoologist, wrote in the journal Nature, such a picture "challenges evolutionary biologists who explain animal behaviour as adaptive". The answer, as he suggested, again depends on learning and the evolutionary cost of "misprinting". Suppose the birds were beginners, with their first clutch: they could imprint on the cuckoo chick and thus throw out any future chicks of their own as aliens. Safer, perhaps, to accept any chick in the nest that is begging to be fed, even if its squawk of hunger is suspiciously loud from the red gape of just one nestling.

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks