You might think that the female host bird, be it a robin, dunnock, or reed warbler, would spot the imposter cuckoo egg.
Sometimes she does, and she uses her beak to put a hole through the shell and then ejects the egg out of the nest. But surprisingly often, that's not the case. And there's a number of reasons for why the cuckoo egg doesn't get clocked as an imposter and remains in place.
When the female cuckoo visits the nest to lay her egg, she will also remove one of the host's eggs within the nest (she may even eat the host's egg). Thus, when the female host returns, she will have exactly the same number of eggs. The cuckoo's visit to the nest is also very quick, to reduce the chance of being spotted by the host, and she is only there for about 10 seconds.
The egg looks remarkably like the host's eggs. Each race of cuckoo is specialised in targeting a specific host species, and will produce eggs that look like that species' eggs. In an evolutionary arms race, the hosts have gradually added designs to their eggs, but in response, so have the cuckoos.
And that's not all. When the cuckoo chick hatches, it goes on a bit of a murder spree. Despite being newly hatched, featherless and with its eyes still closed, it gradually works its way around the nest, shifting an egg onto its back, balancing it carefully and then chucking it over the edge, before moving onto the next one. If one of the other eggs has hatched before the chick, the other chick also gets thrown overboard. Until the cuckoo chick is the only one remaining in the nest.
But what of the host? Does the female bird not realise that this cuckoo chick is not one of her own, especially as it gets bigger ... and bigger ... and bigger? Somehow it does not. And to help with this, the cuckoo chick has another trick up its sleeve – it is able to mimic the sounds of a whole nest of host chicks, tricking the bird into bringing enough food to raise it.
- It's the ultimate master of deception, able to impersonate 13 other animals, from deadly sea snakes to flamboyant lion fish - quite the mimic
- "After getting the go-ahead, it will drop its act and use its large canines to tear chunks of flesh off its unsuspecting client." Nature's 10 biggest, greatest liars and con artists
