Photo: © World. Dove-sized and ash grey with grey barred underparts, the European cuckoo can be frequently seen across the UK, although numbers are higher across southern and central England. Note how it perches low on a branch with wings drooped, due to its zygodactyl feet 

Famous for its signature behaviour of tricking other birds into rearing its young, the cuckoo isn’t always seen in the most favourable light, says BILL NAYLOR. But this fascinating bird is so much more than just a lazy parent.

OVER its extremely large breeding range the call of the European or common cuckoo (Cuculus canorus) is universally known, however few people recognise our secretive spring visitor. Often seen in isolation from other cuckoo species, it does seem an oddity. When flying overhead it could be mistaken for a male sparrowhawk, whose shape and plumage it is said to mimic. When a cuckoo lands, its small zygodactyl feet force it to perch low on a branch, wings drooped.

The sexes are difficult to distinguish. Plumage varies, but it’s mainly ash grey with lighter underparts and grey barring. The curved beak is yellow at the base, matching its eyes and feet.

The European cuckoo shares the barn swallow’s standing as a harbinger of spring. But it has a more impressive CV. No other native bird shirks parental duties by laying its eggs in another bird’s nest. Nor does any other British bird have such a recognisable call.

When cuckoos arrive in the UK in April, males immediately start calling. A school pal would comment when he heard a cuckoo: “That sounds to me like a male.” And he was right. The iconic call belongs to the male, which changes in June from “Cuck-oo” to “Cuck-cuck-oo”. The female by contrast makes a bubbling-type call.

In context of the cuckoo family, the European or common cuckoo is not unique. In total, 49 of the 135 cuckoos are brood parasites. One per cent of other bird species are also brood parasites, including cowbirds, honeyguides, indigobirds and whydahs. But since Biblical times the cuckoo has been denounced as deceitful in duping smaller birds, even wrens, to rear its young at the expense of their own.

The hen cuckoo’s habit of pairing with a number of males prompted the coining of the word “cuckold”, meaning unfaithful. But it’s now known that males and females of a number of bird species don’t prioritise fidelity. A regular percentage of nestlings don’t share the DNA of both parents. 

The young cuckoo, that outgrows its foster parents’ nest was also once a target of derision and superstition. Ungainly on leaving the nest, it tries to coordinate its small feet and large wings and squawks continuously for food. “Gawk” the derogatory old Norse name for cuckoo became popular. Later “cuckoo” became synonymous with stupid, and this lives on in everyday speech. An impractical person is still accused of living in “cloud cuckoo land”.

Although the cuckoo family has rich variety of species, it has been largely neglected by aviculture. Orphaned European cuckoos have been hand-reared to adulthood many times, but adult parasitic species are usually hard to maintain, their feeding preference being mainly hairy caterpillars, which often have rigid spines and contain toxins. The spines can perforate the bird’s stomach lining, prompting it to periodically shed its stomach lining. It also eats buds and fresh leaves to neutralise toxins. Pellets of insect and vegetable residue are then regurgitated. Female cuckoos are usually based near a host’s nesting location and males will visit her there.

The African didric cuckoo (Chrysococcyx caprius) and emerald cuckoo (C. cupreus) were occasionally kept in the UK before the import ban. The non-parasitic South American guira cuckoo (Guira guira) has frequently been imported into the UK. It was first bred here in 1911. An intelligent bird with an extrovert personality, it was exhibited in some British zoos. Apart from two species of roadrunner and the guira cuckoo, the 28 species of coucals are the most commonly kept cuckoo species. The white-browed coucal (Centropus superciliosus) was bred at Exmoor Zoo in 2007.

Sometimes cuckoos destroy host’s nests, especially if they are discovered with young. The cuckoo then waits until the owner builds a new nest with eggs.

The hen usually lays her eggs in the nests of the species that reared her. For a long time it was believed she laid her egg on the ground and carried it in her beak to the host’s nest. In The Cuckoo’s Secret, Edward Chance discovered that the egg carried by the cuckoo actually belonged to the host. Taking seconds to lay her own egg, the cuckoo then flies away with the host’s egg, which she eats later. In a season she can lay as many as 25 eggs. Inheriting the gene that dictates design and colour from her mother, each female cuckoo can be identified by her identical eggs.

Descriptions of egg mimicry in the common cuckoo are often inaccurate. They are frequently poor imitations of the host’s egg in colour and size. A host prioritises the number of eggs in her nest. The size and colour seem to be of secondary importance, but unusual eggs are sometimes ejected. 

Egg mimicry, however, seems to be perfected where one host species is parasitised. The great spotted cuckoo (Clamator glandarius) in Northern Europe parasitises magpies. Koels parasitise the Indian house crow (Corvus splendens). Eggs of both of those species mimic the foster host’s egg perfectly. Only after being weighed and measured can the “counterfeit” egg be identified.

In the UK, cuckoo’s eggs in dunnock (Prunella modularis) nests – the most popular host, followed by the reed warbler (Acrocephalus scirpaceus) – barely resemble the host’s egg. But on the Continent, redstart’s eggs are usually accurate imitations. Overall, egg mimicry for the common cuckoo seems to be a work in progress. If shoddy mimicry fools the host species, it is unlikely to improve.

To gain a headstart on the host’s nestlings, incubation begins inside the female cuckoo at least 30 hours prior to lying. On hatching, the young blind naked cuckoo is driven to eject everything from the nest. Its flattened back is designed to lever eggs and young over the nest edge. If the eggs or young can’t be ejected due to the design of the nest, the cuckoo will be reared with the host’s nestlings.

The young cuckoo thrives on whatever food the host species provides and begs even after being fed. Insectivorous birds are almost exclusively parasitised, but Indian red-backed shrikes (Lanius collurio) rear cuckoos on a carnivorous diet.

On leaving the nest, the young cuckoo is fed by its parents for a few weeks. It calls loudly and almost non-stop and is more interested in food than safety, which is the reason mortality in young cuckoos is high. Cuckoos are soft feathered like their relatives – the turacos – and youngsters easily become rain soaked.

The wide reddish gape and begging cries stimulate passing birds other than its foster parents to feed it, including cuckoos! The young cuckoo has a white patch on the rear of its head, like young raptors ­– further proof, it is said, of raptor mimicry. Young birds are heavily barred and first-year birds can be identified by the head spot. At the end of the summer, relying on a genetic GPS, the young cuckoo migrates to Africa and temporarily stops over in Europe. In Africa, common cuckoos undergo a complete moult and remain largely silent. Males won’t utter their iconic call until they return to Europe again in the spring.   

Bill Naylor has enjoyed a 40-year career in ornithology and aviculture.

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