Every spring, across Britain, people vie to be the first to hear that distinctive, two-toned call which heralds the growing season. They write to newspapers about it, and rejoice that a new year has truly begun.

Everyone knows all about the cuckoo, of course they do. Our parents taught us as children, and we pass it on in our turn. But what of the bird itself? How many can say that they have ever actually seen one, or indeed would recognise one if they saw it?

The Low-down

Of course, the cuckoo lays her eggs in the nests of other birds. But think about it. How does she know where the nests are? Surely the victims keep their nests well hidden? Why don't the eggs get recognised and thrown from the nest?

In fact, as a family, the cuckoos are among the least understood of birds. Even the common cuckoo, Cuculus canorus, whose habits were described by Aristotle in 4BC, is still something of a mystery.

The sexes in fact look pretty similar. Both are large birds, a little under the size of a Jay, and have slate blue backs and a typically predator-like underside of white with dark stripy dapples. In fact, some researchers have surmised that the bird's somewhat sparrowhawk-like appearance serves to temporarily scare off the parents of the target nest for long enough for the Trojan¹ egg to be laid. Be that as it may, the cuckoo appears sufficiently raptor-like that it triggers a mobbing response in smaller birds, a typical reaction to a large predator that works because the target becomes confused by the sheer quantity of available food animals and can never decide which one to strike. In the case of the cuckoo, the mobbers are quite safe, as the adult has a fairly harmless diet of caterpillars, insects, spiders, seeds and grasses. The young, of course, eat whatever their foster-parents give them.

Eggs and the Young Cuckoo

In spring, the females arrive first from Africa to reclaim their old territories, defending them fiercely against all comers, so that by the time the males arrive all the borders are already drawn. Not that the males care much, for they wander from territory to territory mating as they go; although the females establish the territories and do the fighting, it is the males that give the famous cry; the song of the female is more of a low bubbling sound.

Each female cuckoo has a particular favourite host species, usually the one in whose nest she herself was brought up, and her eggs, when laid, will tend to resemble those of the chosen host. The cuckoo, however, has no magical control over her egg colouration. If a particular cuckoo's eggs look like robin's eggs, they will still look like robin's eggs even if she is forced, perhaps by lack of robins, to lay in a sparrow's nest.

Within her territory, each female cuckoo locates as many nests as she can that belong to her chosen host. She does this by following birds whenever she sees them carrying nesting material. She remembers where they fly to and where they fly from, eventually building up a map of all the suitable nests in her territory. She then mates with one or more passing male cuckoos, who have no effect on egg coloration, and waits for her chosen hosts to start laying their own eggs.

Species favoured by our common cuckoo are the hedge sparrow, reed warbler, meadow pipit, pied wagtail and robin. These are all birds which lay their eggs at a rate of one a day until the clutch is complete, only then returning permanently to the nest to incubate them, so in the laying period it is fairly easy for the cuckoo to nip in and deposit its own egg while the parents are away foraging. The cuckoo only lays a single egg in each nest, finding a new nest for each of her 15 to 20 eggs.

The chosen hosts are always smaller birds, ensuring that the young cuckoo will not be crowded out by its larger nest-mates, but this means that quite often the mother cuckoo cannot fit into the nest. This is particularly a problem with birds such as warblers which build covered nests with only a small opening. Country lore has often suggested that the cuckoo lays her egg on the ground and then carries it to the nest in her beak, but sadly this pretty story has never been officially recorded. What she actually seems to do is cling to the outside of the nest and stick her tail inside. If there is not enough room in the nest, though, she is quite capable of pushing some of the original eggs out with her beak.

One Flew Over The Cuckoo's Nest

Once the young cuckoo is born, it finds itself in a nest containing other young birds and eggs belonging to the host species. Within a few hours, the naked, blind chick develops a strong urge to evict them, which it does by crawling underneath each egg or chick, and hoisting it over the side of the nest using the depression between its shoulder blades. Soon it has the nest to itself, and the undivided attention of its host parents.

The parent cuckoo shows no further interest in its offspring. The hapless host parents, on the other hand, find themselves presented with a huge gaping mouth, ever hungry for more and more food. The chick soon exceeds the size of its hosts, but parent birds have an in-built instinct to feed gaping mouths in their nests, and the insistent demand of this deeply instinctual signal far outweighs any oddity that they might otherwise perceive.

Eventually, however, the cuckoo leaves the nest and learns to fly. Its real parents have already left for Africa; a few weeks later, the young ones follow in their invisible migratory footsteps, to winter in the warmth of warmer climes, ready to return the following spring and start the cycle all over again.