British Wildlife of the Week

Cuckoo

The tale of the cuckoo is a well-known one, liberally sprinkled with deception, suspense – and murder. Famously, the female lays her eggs in the nests of other birds and relies on unwitting foster parents to raise her offspring. It might be tempting, therefore, to view the cuckoo as the ‘bad guy’ – a rogue and a fraudster unfairly taking advantage of an innocent family – but we must avoid making moral judgement through the human lens of what we consider right and wrong. Far from being lazy, selfish birds that shirk all parental responsibilities, cuckoos deserve respect for their ingenious methods.

The Cuckoo’s Call

Few sounds symbolise spring better than the call of the cuckoo. The iconic two-tone ‘song’ of the male, which he uses to advertise his presence on the breeding grounds to potential mates and rivals, is one of the most instantly recognisable of all bird sounds, especially here in the UK. But although many people know the call of the cuckoo, far fewer would recognise the bird if they actually saw one. In fact, the cuckoo is shy and rarely ventures out into the open for very long, so it is seldom seen. To make matters worse, the male’s ‘cuck-ooo‘ is far-carrying and ventriloquial, meaning it can be difficult to locate the bird responsible.

Not only has the distinctive call of the cuckoo heralded the arrival of spring for centuries throughout Europe, but it has also long been surrounded by folklore and superstition, which varies from place to place. For example, hearing your first cuckoo of the year before breakfast was, to some people, considered unlucky – and if you were still in bed when it occurred, it was a sure sign of impending illness. However, if, when you heard one, you turned a coin over in your pocket, it would ensure that money wouldn’t be a problem for the rest of the year. A baby born on the day the first cuckoo is heard, meanwhile, would have good luck all its life.

But although the cuckoo is considered a fundamental part of the British countryside, ingrained in our culture as a harbinger of spring, it is, more accurately speaking, an African bird. Our ancestors believed that the cuckoo turned into a sparrowhawk at the end of summer, thus explaining its absence during the autumn and winter months. This isn’t quite as absurd as it sounds, as the two birds are roughly the same size and share a similar greyish plumage with barred underparts. Today, however, thanks to satellite tracking, we know that the cuckoo spends the majority of its time in the equatorial forests of West Africa and only graces us with a brief visit each year.

Eggs in Many Different Baskets

The cuckoo is unique among British birds in that it always lays its eggs in the nests of other birds, thus assigning the task of rearing its young to unknowing foster parents. Upon arriving in Britain in the middle of April, this feathered homewrecker returns to Africa a mere three months later (sometimes even earlier in the case of the male), unencumbered by family duties and leaving behind a trail of broken homes.

As a species, the cuckoo lays its eggs in a wide range of small birds’ nests. Here in Britain, the reed warbler, meadow pipit and dunnock are its three main hosts. But each individual female cuckoo specialises in just one host 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 instinctively lay mimetic eggs in the nest of the same species as her mother. A female cuckoo raised by reed warblers, for example, is, like her mother before her, genetically predisposed to choose such hosts when she becomes old enough to breed for herself, no matter what nest her father was brought up in.

A female cuckoo must lay her egg within her chosen nest – a warbler nest in this example – when the parents are away. If the warblers detect deception, they may destroy the alien egg, throw it to the ground, or even desert the nest altogether. Once the coast is clear, the female nips in to remove one of the legitimate eggs and lay one of her own. She is able to lay in as little as 10 seconds because she retained her egg within her body for up to an extra 24 hours, long after it was encased in a shell. Since she has no time to properly settle on the nest, her egg may fall a short distance, but it will not break, not even if it lands on another egg, because its shell is twice as thick as those of the host bird.

The cuckoo egg will, of course, be similar in appearance to the original eggs in the nest, for if it is markedly different the warbler parents might notice the substitution. The fact that the cuckoo lays disproportionately small eggs means that any size discrepancies are kept to a minimum and not noticed by the host birds. If everything goes to plan, the reed warblers will return to their nest shortly afterwards and continue incubating the eggs as if nothing has changed.

What the unsuspecting warblers don’t know, however, is that, in all likelihood, only the cuckoo egg will ever hatch. Since the cuckoo egg remained within the female’s warm body for an extra day, the chick inside had already started its development by the time it was laid. In any case, the incubation it needs is one or two days fewer than that required by the hosts’ eggs, so the young cuckoo almost always hatches first.

The cuckoo chick, little more than a pink, featherless, 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 very much larger than the birds they parasitize and they require 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. The newly-hatched chick, therefore, immediately gets to work. Even though it is just a few minutes old and still completely blind, the chick is hardwired to kick the legitimate eggs out of the nest until it is sitting there, quite suspiciously, by itself. If it happens to hatch after the host’s chicks, it doesn’t matter too much – it throws them out of the nest as well.

For the reed warblers, this is a disaster. They have failed to pass on their genes to the next generation – possibly their only chance to do so. Even more tragically, they don’t actually realise this. If parent birds lose an entire brood to predators or harsh weather conditions, they often start afresh. But in this case, they are fooled into thinking that the cuckoo chick is their own precious progeny.

The cuckoo chick begs for food from its foster parents, imitating the sounds made by their own young, only louder and more urgently. The parent birds labour incessantly – which they must do, for a cuckoo chick may consume as much food as ten reed warbler chicks before it fledges. Even when the cuckoo becomes a behemoth, twice the size of its foster parents and bulging absurdly out of the nest, 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. Pliny the Elder saw the foster mother practically putting her head inside a cuckoo chick’s oversized mouth to feed it and thought that she was allowing the chick to devour her as its final meal before it left the nest.

What I find most fascinating about all of this 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 solitary 6,500-km migration to Africa (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 on 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.

Cheaters Never Often Prosper

‘Brood parasitism’, as the habit of laying one’s eggs in the nest of another individual is called, is essentially a labour-saving device. Freedom from physically demanding parental duties enables a female to produce far more eggs than normal. A cuckoo, for example, can lay up to 25 eggs in a single season, several times more than the average for a bird of its size. She can also mitigate the risk of egg loss by distributing those eggs among a number of different hosts. And since she doesn’t need to raise any of them by herself, the cuckoo only needs to stay in Britain for a few weeks – the shortest visit to these shores of any migrating bird.

But brood parasitism can also be a risky strategy. There is always a chance that the host will spot and reject the egg. Over time, natural selection favours hosts that can spot alien eggs in their nests and so they become more successful at preventing parasitism. Those particular species will gradually appear less frequently on the cuckoo’s list of hosts. Pied wagtails, for example, almost always see through the cuckoo’s deception and usually reject any foreign eggs. They may have once been more common victims but, in the arms race between brood parasite and host, they now have the upper hand, and the cuckoo has been forced to move on to less discerning species.

Reed warblers and meadow pipits are probably comparatively recent hosts and have not yet become adept at rejecting cuckoo eggs. The dunnock seems even more clueless and almost always accepts a bogus egg, even if it looks markedly different in size and colour to its own. Perhaps it is a ‘new’ host and has not had the chance to evolve the ability to detect a parasite yet. Presumably, in time, it too will learn to recognise alien eggs and throw them out. But as hosts constantly evolve improved abilities to spot a cuckoo’s egg, the cuckoos themselves must become better at mimicking them. It is an evolutionary race, with hosts developing better defences against cuckoos, and cuckoos, in turn, evolving better trickery.

Strictly speaking, the proper name of ‘our’ cuckoo is the common cuckoo, but it certainly isn’t as common in this country as it once was. In England, its population dropped by 70% between 1995 and 2017. This is probably due to a decline in the hedgerow birds whose nests they parasitise and the destruction of caterpillars (caused by our overuse of agricultural pesticides) that they depend on for food, but recent research indicates that increasing mortality rates along one of their two main migration routes is also a factor. And so the cuckoo’s once-familiar call – that classic sound of springtime – is vanishing from the British countryside. Today, it is being heard by fewer and fewer people. I certainly haven’t heard one since I was a young boy. The silence that remains is a stark reminder of our diminishing natural world.

In the next British Wildlife of the Week, we’ll be travelling to the remote archipelago of St Kilda off the coast of Scotland to look at the Soay sheep, an unusual breed that has changed very little in thousands of years.