Science Matters – an arms race of springtime trickery

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The cuckoo’s distinctive call heralds spring, but this migrant bird has a darker side.

Arriving in mid-April, female cuckoos seek out host nests to parasitise. When the first host egg is laid, she swoops down, lays her egg and flies off with the host egg in her beak. She continues this subterfuge until setting off on the 5,000km flight to Africa in mid-July, confident that her fledglings are well provided for.

The advantages of parasitism for cuckoos are clear – with no brood to raise they can lay more eggs and migrate earlier. But loss of host species offspring has engendered a vigorous fightback.

An arms-race of trickery and defence has been ongoing between cuckoos and hosts ever since their parasitism began some 2,000 years ago.

Hosts would normally attack birds seen laying in their nest, but female cuckoos look scarily hawk-like and are stealthy and quick. Spying an unattended nest she lays her egg in just 10 seconds! Parent birds generally reject foreign-looking eggs, so cuckoos have developed mimicry. Separated into three races, each specialises in copying the egg colour and intricate markings of one common UK host species – reed warblers, pied wagtails or meadow pipits.

While cuckoo chicks are large and pink, host chicks are small and black, so the former could be recognised and rejected on hatching. But no, thanks to some astonishing adaptations, cuckoo chicks survive. Hatching earlier than host chicks, they have a strong instinct to eject host eggs and any early host hatchlings.

So, blind and naked, cuckoo chicks hump eggs and chicks out of the nest. Now, reigning supreme, a final deception prevents revealing the chicks’ identity. Cuckoo chicks’ calls mimic a nest full of host chicks, inducing hosts to feed them juicy insects until maturity – by then eight times the foster-parents’ weight.

Victorian naturalists regarded the extraordinary behaviour of cuckoos as scandalous; Gilbert White – Author of The Natural History of Selbourne 1789 – calling it ‘a monstrous outrage on maternal affection’.

But to modern ecologists this struggle between parasitic and parasitised host species is a beautiful, ongoing example of Darwinian evolution.


A young cuckoo being fed by pied wagtail. Photo: Alastair Forsyth. NO_B32july03