Birds
Common Cuckoo
Cuculus canorus · also called European cuckoo, common cuckoo
The common cuckoo is the bird that outsourced parenthood. It builds no nest, incubates no eggs, and feeds no young. Instead the female slips a single egg into the nest of a much smaller bird, and her chick does the rest — including murdering its nest-mates within a day of hatching.
Overview
Common cuckoos breed across Europe and temperate Asia in a wide range of open and wooded habitats, and the European population winters in central and southern Africa, making two Sahara crossings a year. Adults are slim, grey, and long-tailed, and in flight their pointed wings and level glide make them look remarkably like a sparrowhawk — a resemblance that helps scare hosts off their nests.
Brood parasitism
The cuckoo is an obligate brood parasite: it can only reproduce by exploiting other species. A female spends the season laying up to about 25 eggs, one per host nest. She favours a single host species — reed warbler, dunnock, meadow pipit, robin, and others depending on the region — and females form host-specific lineages called gentes, each laying an egg that mimics that host’s eggs in colour and markings. She lays with extreme speed, in seconds, after first removing one host egg.
The chick
The cuckoo egg hatches first, after a short incubation. The chick is blind and naked but has a hollow back and a shoving instinct: over its first hours and days it works each host egg or nestling onto its back and heaves it out of the nest. Left alone, it is fed by the exhausted foster parents until it is several times their size, its begging call loud enough to imitate a full brood and even to draw food from passing birds that are not its parents.
An arms race
Hosts fight back — mobbing adult cuckoos, rejecting strange eggs, deserting parasitised nests — and cuckoos respond with better mimicry and hawk-like looks. It is a textbook case of co-evolution. But once a cuckoo chick is in the nest and begging, most hosts cannot help feeding it.
Diet
Adult cuckoos are among the few birds that eat large numbers of hairy and toxic caterpillars, including species other birds leave alone, periodically shedding the stomach lining that the irritant hairs accumulate in.
Status
The common cuckoo is Least Concern globally, but it is declining steeply in western Europe — UK numbers have roughly halved since the 1980s — probably through a combination of falling host populations, fewer large caterpillars, and hazards on the African migration.
Common Cuckoo: common questions
How does the cuckoo trick other birds into raising its young?
A female cuckoo watches a host pair — commonly a reed warbler, dunnock, meadow pipit, or robin — build their nest and start laying. When the hosts are away, she flies in, removes one of their eggs, lays one of her own in under 10 seconds, and leaves. Her egg often closely matches the host's eggs because female cuckoos form host-specific lineages, each specialising on one host species and laying a matching egg type. The cuckoo egg also has an unusually thick shell and a short incubation period, so it typically hatches first.
Why does the cuckoo chick throw the other eggs out?
To monopolise all the food. A newly hatched common cuckoo, still blind and featherless, has a hollow in its back and an instinct to shovel. It works around the nest, manoeuvring each host egg or nestling onto its back, climbing the nest wall, and tipping it over the edge, until it is the only occupant. The foster parents then direct their entire feeding effort to the single giant chick, which grows far larger than they are and begs loudly enough to sound like a whole brood.
Why don't the host birds notice?
Some do. Many hosts have evolved defences — mobbing adult cuckoos, learning to recognise and reject odd-looking eggs, or abandoning parasitised nests — and cuckoos have evolved counter-adaptations like better egg mimicry and hawk-like plumage that frightens hosts off the nest. It is a co-evolutionary arms race. Once the chick has hatched, though, most hosts seem unable to stop responding to its begging, apparently because a gaping, cheeping chick in the nest is too strong a stimulus to ignore, whatever its size.
Is it true you can tell the seasons by the cuckoo's call?
The famous two-note "cuck-oo" is made only by the male, and only on the breeding grounds, so across much of Europe it is a classic sound of spring — traditionally the first cuckoo heard marks the season's turn. The birds arrive from Africa in April, call through May and June, and fall silent and depart by July, with the adults leaving well before that year's young, who must find their own way to Africa on instinct alone.