The Cuckoo: Ireland's Most Scandalous Bird!
By Richard Collins
''The evil that men do lives after them but the good is oft interred with their bones' declared Mark Anthony in Shakespeare's Julius Caesar. The Bard might have said the same of Cuckoos; their contempt for the ordinary decencies of family life makes them pariahs and nobody sees their positive side. But cuckoos are by no means the only birds which lay their eggs in other birds' nests. Of the world's 8,900 bird species, as least 80 foster their young on others. This figure does not include those which occasionally lay in the nest of another member of their own species. DNA profiling has shown that this reprehensible practice is fairly common. For example, up to 30% of the eggs in Starling nests, in some colonies, had not been laid by the lawful owner. By pawning off the odd egg on a neighbour, a bird is saved the effort and expense of feeding the chick. It’s not of course an infallible strategy; neighbours are likely to do the same to you. Also, adding an egg could put a clutch at risk; there may not be enough food to go around.
The first cuckoos show up toward the end of April when they feature in newspaper letters columns. The earliest records, however, are of the 'April fool' variety; boys like to mimic the Cuckoo, sending their sentimental elders into a tizzy. The earliest ‘respectable’ date is the 2nd April, but even that seems a bit shaky. Its origin is Richard Ussher and Robert Warren’s Birds of Ireland, published in 1900. They said that ‘the ordinary time of arrival is from the 16th to the 30th of April, but it has frequently been noticed in the first half of that month, from the 2nd onwards’. The Irish Bird Report mentions one at Cape Clear on the 4th April 1969 but no observer's name is given.
Cuckoos spend the winter in sub-Saharan Africa. We don’t know where in Africa our Irish Cuckoos go. Only a handful of Cuckoos have been ringed in Ireland and none was ever heard from again. Cuckoos are loners by disposition and they probably fly to Ireland on their own. They are not strictly territorial and, when they get here, both males and females patrol a ‘home range’. The home range overlaps those of other Cuckoos. Some young males don't bother with a home range and adopt an itinerant lifestyle. During the weeks leading up to laying, females will have casual encounters with males, mating with several of them.
The female Cuckoo's next task is to choose potential victims for her confidence trick. Over 100 different host species have been recorded in Europe, but in each region there is a favourite. The main Irish species is the Meadow Pipit, hence its name 'Banaltra na Cuaice', the Cuckoo’s nurse. The second choice is the Dunnock, followed by the Robin and the Pied Wagtail. Over much of Europe, the bird of choice is the Reed Warbler.
A Cuckoo's egg resembles that of its host. Egg colour is genetically determined and there are Meadow Pipit and Reed Warbler Cuckoos. Egg matching implies that Cuckoos return to the place where they themselves fledged. If an Irish Cuckoo were to go elsewhere, it might have difficulty finding enough Meadow Pipits to exploit and its eggs would be mismatched for anything else. A baby Cuckoo is imprinted on its foster parents and seeks out hosts of that species when it comes to breed.
The female Cuckoo is the supreme bird-watcher. She may lay as many as 25 eggs and must monitor the activities of at least that number of Meadow Pipit pairs. She will lay when she knows that the pipit is away from the nest. The pipit produces an egg each day and starts incubating as soon as all are laid. The Cuckoo tries to deposit her egg before the pipit's clutch is completed. She lays every two days, so placing all her eggs in nests at the right time is a formidable task. She flies to a nest, usually in the evening, removes one or two eggs, and lays her own, all in about ten seconds. She leaves immediately and eats the stolen eggs.
If the pipit notices the offending egg, she will either remove it or desert the nest. It was thought that host birds recognised their own eggs and so some were able to spot the rogue one. However, a recent experiment with Garden Warblers showed that this may not be the case. The eggs in the warbler's nest were replaced with those of another species. The warbler returned, laid another egg, but promptly threw it out of the nest. She had, apparently, no innate ability to recognise her own eggs; she just rejected the odd one out.
A Cuckoo's egg has been slightly incubated within the oviduct by the time it’s laid. This ensures that it hatches sooner than the host's. The young Cuckoo heaves the other eggs, or chicks, over the side of the nest. It has to do this, if it is to get enough food. The Cuckoo, at fledging, will weigh more than an entire brood of its hosts'.
As soon as their eggs are laid, the adult Cuckoos grow a new set of feathers and leaves for Africa. The youngster will have to follow them on its own, the route being hardwired into its brain.