Another Life Michael Viney
'Thou mordrer" said Chaucer's merlin to the cuckoo. Well yes, but most of us just take pleasure in the call - that limpid descant at the edge of one's consciousness; the very grace-note of summer. I like to hear it in a landscape of close, leafy spaces, where "cuck-oo" comes flutey and teasing: Leitrim, say.
The bird is quite scarce in west Mayo. On my own bare hillside I seem to see the cuckoo more often than I hear it, teetering on a wire or flying low over the walls with one or two meadow pipits in pursuit. Do the pipits mistake it for a sparrowhawk - it looks and flies a lot like one - or are they only too aware of the cuckoo's ruthless designs? Either way the mobbing by brave little birds brings back Chaucer's epithet: "mordrer".
In my early plain-man's grappling with evolutionary biology, I used to have trouble with the cuckoo. For just one sort of bird to perfect such an elaborate and fiendish way of perpetuating its genes, all by chance, seemed past belief.
To recap on what happens . . .
The cuckoo flies in from southern Africa in April, the males arriving first and calling incessantly, sometimes even at night. "Cuck-oo" is loud and simple - loud because cuckoos are scarce and need to find each other, simple because the call is programmed by inheritance: baby cuckoos don't get the chance to learn from their mothers.
The female stays silent until she is ready to mate, then utters a cry described as "water-bubbling" (I have never heard it). After a fairly perfunctory rendezvous with the male, she goes off alone to begin what science calls her "brood-parasitism".
She ranges widely over a large territory, looking for nests under construction. She usually picks those of the same species that reared her - perhaps because of imprinting on her foster-mother as a nestling. On moorland above 240m, it is usually the pipit; down to 50m the dunnock; below that the reed warbler. But robin, wheatear or pied wagtail are also among the victims.
That the cuckoo's egg so often (not always) manages to mimic those of her host, both in size and colour, still poses what Edward O Wilson, doyen of sociobiology, calls "a first class scientific mystery". It is extraordinary enough that European cuckoos should be divided into co-existing "host races" - pipit-cuckoos, dunnock-cuckoos and so on. But a male cuckoo often mates with females of more than one "race", and how the genes keep the specific egg-mimicry going within the female line remains baffling.
Whichever egg the cuckoo is mimicking, the egg she lays is unusually small for a bird her size, so she can lay a lot of them. In one study, a female was seen to lay a total of 61 eggs over four breeding seasons, 58 of them in separate meadow-pipits' nests.
The usual pattern is to lay one egg every second afternoon, having chosen the nest soon after dawn. She waits until the host bird is away, then glides down, and, in a 10- second flurry, lifts out an egg in her bill and lays her own in its place - or rather, drops it in: it has a specially thick shell. If a host nest has developed too far for successful parasitisation, the cuckoo may destroy the eggs or nestlings to force the bird to lay another clutch. She has to get the timing right, so that her chick hatches out (in 12½ days) in advance of the other eggs, or when the nestlings are very small. Then the baby cuckoo empties the nest of all the competition.
Working blindly and instinctively, apparently responding to any pressure on a sensitive hollow patch on its back, it gets underneath an egg (or fellow-nestling), backs up the side of the nest, and tips it over the edge.
It seems an incredible piece of behaviour to evolve by natural selection, however many million years it took. But there is evidence that it is not confined to baby cuckoos. In The Selfish Gene, Richard Dawkins notes some research from Spain in which a baby swallow was introduced experimentally into a magpie's nest. It began ejecting the magpie's eggs in exactly the same way as a baby cuckoo. With typical relish, Dawkins concluded that "the ruthless behaviour of a baby cuckoo is only an extreme case of what must go on in any family".
We are always ready to accept the remarkable if examples can be multiplied; brood parasitism is practised by about 80 bird species worldwide, 50 of them within the cuckoo family. In Ireland the cuckoo is our only example of this lifestyle, but in South America five kinds of cowbird are parasites like the cuckoo. And in India the male koel approaches a crow's nest with loud cries and lets itself be driven off - while the female koel nips in to lay her egg.
This article was first published in June 1992. Michael Viney is on leave
EyeOnNature
Since the dry spell became established, a birdbath in my suburban garden is a much-visited attraction. I have noticed that magpies make use of it by bringing dry crusts of bread, dropping them in the water and then agitating them until soft enough to eat. Today one did this with some Monster Munch crisps!
Susan O'Flynn, Dundalk, Co Louth
One morning recently I observed a pair of bullfinches, along with a number of greenfinches and goldfinches, feeding on the seeds on one of the many patches of dandelions in my garden. Was this a finches' breakfast meeting?
Marian Carthy, Drumshanbo,
Co Leitrim
The seeds may have been riper on that patch.
For a few weeks we have had a pair of collared doves in the garden. When I went to the petshop to buy bird seed, I was told that collared doves arrived from India on grain ships. Is this true?
Anne Burns, Galway
Collared doves arrived here from eastern Europe about 50 years ago, and have spread all over the country since then.
Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo. E-mail: viney@anu.ie. Please include a postal address