Aug 30

A Cuckoo in the Nest

Being a parent is tough. All you have to look forward to is less sleep, less time to do your own things, and generally one or more hungry mouths to feed. Raising young is a very energetically demanding activity, so the cuckoo has done away with it altogether. It appears in the British Isles in the summer and, only a few weeks later, after leaving behind a trail of broken homes, foster-children and infanticide, it returns to Africa again, having contracted out the job of looking after its young to an unknowing third party. Far from being a lazy, selfish bird that shirks all parental responsibilities, though, the cuckoo deserves respect for its ingenious methods.

The common cuckoo as a species lays its eggs in a whole range of small birds’ nests, but each individual female specialises in just one particular species, whose eggs her own closely match. Genes that regulate egg colouration are passed exclusively down the maternal line so a female cuckoo will be able to lay mimetic eggs in the nest of the same species that her mother, whom she never met, also victimised.

Crafty Cuckoos

The female cuckoo will only lay her egg in the nest of the host bird – a reed warbler in this example, a very common victim – when the parents are away. The cuckoo swoops down, pushes out one of the original eggs and then quickly lays her own. She is able to lay her own egg within as little as ten seconds because she has retained it within her body for up to twenty-four hours, long after it was encased in a shell and ready for laying. The cuckoo egg will be similar in colour to the original eggs in the nest, for if it were markedly different the warbler would either peck a hole in it or throw it out. If everything goes to plan, the reed warblers will return to their nest shortly afterwards and continue incubating the eggs as if nothing were wrong.

What the reed warblers don’t know, however, is that only the cuckoo egg will ever hatch. Since the cuckoo egg remained within the female’s body for an extra day, the chick inside has already started its development by the time it has been laid. And, in any case, the incubation it needs is one or two days less than that required by the warblers’ chicks, so the young cuckoo always hatches first.

Photo: Mike Richards

Brood Parasite

The cuckoo chick – little more than a pink, naked, squirming blob of flesh – is just like a spoiled child who wants all the attention – or, in this case, all the food. Cuckoos, after all, are much larger than reed warblers and need a lot of insects to fuel their growth, so the chick can’t afford other youngsters being in the nest and taking some of its much-needed food. So it gets to work. Even though it is just a few minutes old and still completely blind, it kicks the legitimate eggs out of the nest until it is sitting there, quite suspiciously, by itself.

For the reed warblers, this is a disaster. Not only have they failed to pass on their genes to the next generation – possibly their only chance to do so – but, even more tragically, they don’t actually realise this. If parent birds lose an entire brood to predators or uncooperative weather conditions, they can start afresh. But in this case, they think that the cuckoo chick is their own progeny and they continue feeding it. Even when the cuckoo chick becomes a behemoth, twice the size of the adult birds and bulging out of the nest (as seen in the picture below), the warblers don’t seem to notice anything unusual with the situation. So programmed is the instinct to pass food into the cuckoo’s massive gape that they sometimes wear themselves out collecting insects to satiate its never-ending hunger.

Photo: Per Harald Olsen

What is arguably even more impressive about cuckoos, though, is that none of this behaviour is learned. Since a cuckoo chick never meets its own parents, almost everything it does is instinctual. From the moment of its birth, as it kicks all other eggs from the nest, to its great solitary migration back to Africa to spend the winter (without having ever made the journey before, or even having older birds to follow), all of its actions must be pre-programmed within its brain. Whereas some birds, such as ducks and geese, imprint upon the first living thing that they see when they emerge from their eggs, cuckoos obviously have an innate recognition of their own species. It’s a miracle they don’t have an identity crisis.

But the thing I respect the most about cuckoos? The fact that they are living proof that cheaters can – and, indeed, do – prosper.

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