Birds
Cuckoo
Cuculidae · also called cuckoos
The cuckoo is famous for a con. Around sixty species lay their eggs in the nests of other birds and vanish, leaving a chick that hatches first, shoves every other egg and nestling over the edge, and then begs so convincingly that its tiny foster parents feed a bird several times their own size. The other ninety-odd cuckoo species, including the roadrunner, raise their own young like anyone else.
Overview
Cuckoos are the family Cuculidae, around 150 species, on every continent except Antarctica. They are slender, long-tailed birds with zygodactyl feet (two toes forward, two back), mostly insect-eaters with an unusual taste for the hairy caterpillars other birds refuse. Fewer than half are brood parasites; the rest — the roadrunners, anis, coucals, and most American cuckoos — build nests and raise their own chicks.
The parasite’s playbook
A female common cuckoo belongs to a line, a gens, that targets one host species and lays an egg matched to that host’s eggs, slipping into the nest to lay in about ten seconds. Her egg hatches early, and the blind chick heaves the host’s eggs and young out of the nest. Alone, it begs with a call and gape that mimic a whole brood, so the foster parents feed it at full pace even after it dwarfs them.
An arms race in eggshells
Hosts learn to reject odd eggs, so cuckoos evolve better mimicry, faster laying, and puncture-resistant shells; some tropical cuckoo chicks even mimic the host chicks’ colour and call. It is one of the best-studied coevolutionary arms races in nature.
Status
Cuckoos as a family are Not Evaluated, trend decreasing. Many tropical species are common, but the migratory common cuckoo is down about 30 per cent or more across Europe since the 1980s, tracking declines in its host birds and in the moths whose caterpillars it eats.
Cuckoo: common questions
Do all cuckoos lay their eggs in other birds' nests?
No. Brood parasitism is what the family is famous for, but only around 40 per cent of cuckoo species do it, and they are concentrated in the Old World. The common cuckoo of Europe and Asia, the various bronze and hawk cuckoos, and the koels are obligate brood parasites that never build a nest. But the roadrunners of North American deserts, the anis of the American tropics, the coucals of Africa and Asia, and most of the American cuckoos build their own nests, incubate their own eggs, and feed their own chicks like any other bird. Some anis even nest communally, with several females laying in one large shared nest.
How does a cuckoo trick another bird into raising its chick?
Through a sequence of adaptations, each answering a host defence. The female cuckoo watches a host nest, waits until the owners leave, and flies in to lay a single egg in about ten seconds, often removing one host egg to keep the count right. Her egg is coloured and patterned to match that host's eggs, because hosts learn to throw out eggs that look wrong. It hatches quickly, ahead of the host young, and the newly hatched cuckoo instinctively shoulders the other eggs and chicks out of the nest so it has no competition. It then begs with an exaggerated call and gape that mimic the sound and signal of a whole brood, driving the foster parents to feed it at full pace.
Why do the host birds not notice a giant chick that looks nothing like them?
Partly because the cuckoo chick exploits the host's hard-wired rules for responding to a begging nestling, chiefly a loud rapid call and a wide brightly coloured gape, and it produces both at superstimulus intensity, so the parents respond even though the chick is the wrong shape and size. Partly because small songbirds have limited ability to count or to compare a lone huge chick against a mental image of their own species, especially inside a dim nest. And partly because rejecting a chick is riskier than rejecting an egg, since a bird that abandons a nest late in the season may not breed again, so selection for chick recognition is weaker. Some hosts have nonetheless evolved to abandon or starve a suspicious chick.
Is a roadrunner really a cuckoo?
Yes. The greater and lesser roadrunners of the deserts and scrub of the southwestern United States and Mexico are ground-dwelling members of the cuckoo family, Cuculidae. They have the family's long tail, zygodactyl feet with two toes forward and two back, and slender build, but they have taken up a fast-running, largely terrestrial, predatory life, sprinting at over 30 km per hour to catch lizards, snakes, insects, rodents, and small birds. Unlike the parasitic Old World cuckoos, roadrunners build a nest of sticks in a cactus or low tree and raise their own young. The cartoon roadrunner's speed and its ground-running habit are, unusually for cartoons, based on the real bird.