Absurd little birds across the world lay their eggs in the nests of other species, leaving the hapless parents to raise a changeling at the expense of their own offspring.
The majority are cuckoos – of the 150 Cuculidae species, 64 are brood parasites.
- Thieves, hostage-takers and wolves in sheep’s clothing – meet nature's cruellest tricksters and most devious schemers
- Meet the laziest animals in the world, who dodge hard work, steal resources and barely move all day
Brood parasitism has evolved multiple times, testifying to its utility as an evolutionary strategy. A range of other birds also engage in this sneaky behaviour, from honeyguides to cowbirds and even one species of duck.
Most birds that foist their young off on other species have no alternative – they simply do not know how to care for them. Their genetic programming seems to have deleted the impulse to build a nest or incubate an egg or bring food to a voracious nestling. They would not have survived were their manipulations ineffective.
Some species of bird lay eggs covered in sophisticated patterns that help to distinguish them from cuckoo eggs – and summarily evict cuckoo eggs that do not match. Sometimes they abandon the nest entirely when an alien egg is found.
In one species, the host has also learned to recognise cuckoo nestlings and will hurl them to their deaths.
But in most cases, once the cuckoo chick hatches, the hosts are duped into raising it. While some cuckoo species are content to share with their adopted siblings, others prefer to be the only young, tossing their nestmates – inside their eggs or even after they have hatched – over the side.
This suggests that they are hardwired for resource competition. Imagine committing fratricide before you can walk.
Once the deed is done, some species mimic the calls that their victims would have made. Others do not bother with mimicry, preferring instead to engage in exaggerated begging behaviour that appears to override any specific vocal cues and motivate the parents to feed them more than they might have fed their own hatchlings.
Some cuckoo chicks even look like the chicks of their host species, though others do not – and may be significantly larger.
The deceit only persists until the cuckoo leaves the nest, though. Any pretence of familial feeling is abandoned after fledging. Adult cuckoos do not imitate the calls of their foster parents or appear to show any affinity for them – aside from seeking out nests of the same species.
A common cuckoo raised by a reed warbler does not think it is a reed warbler. It calls like a cuckoo and mates with other cuckoos – though a female common cuckoo will imitate the call of a sparrowhawk, for example, to scare a potential host away from its nest so she can swoop in and deposit an egg.
