Chuckling cuckoo imitates predator

Cuckoos mimic predatory hawk calls to trick other birds and increase nest invasion success, according to researchers at The University of Cambridge.

When it comes to deception, cuckoos are the masterminds of the avian world. They surreptitiously lay their eggs in the nests of other birds to evade the responsibilities of parental care. These unsuspecting hosts end up raising cuckoo chicks, often at the expense of their own offspring.

In order to carry out their cunning deed, female cuckoos must be furtive to avoid being attacked by hosts and having their eggs rejected from the nest. This is why female cuckoos are generally much quieter than their male counterparts, whose songs are an audible indicator of springtime.

However, researchers noticed that after laying an egg in a host’s nest, female cuckoos make an unusual ‘chuckling’ call. At first glance, this behaviour seems bizarre; why bother being stealthy whilst laying the egg if you are going to declare your presence immediately afterwards?

Jenny York, lead author of the study, was particularly interested in the acoustic similarity between this cuckoo ‘chuckle’ and the call of the sparrowhawk. Sparrowhawks are predators of reed warblers, which are a favourite host species of the cuckoo, and York hypothesised that this similarity may not simply be a coincidence.

She explains, ‘We thought that maybe the cuckoo is imitating the sparrowhawk to distract the reed warbler’s attention from its nest to its own safety, increasing the cuckoo’s chances of successfully parasitising the nest’.

To test this, York and her team assessed the behaviour of reed warblers in response to four different call types - a female cuckoo call, a sparrowhawk call, a male cuckoo call and a non-threatening collared dove call.

The results, published in Nature Ecology and Evolution, show that the reed warblers respond to both the female cuckoo call and the sparrowhawk call with fear and vigilance. The call of the male cuckoo and collared dove does not have any noticeable effect on reed warbler behaviour.

To find out whether or not cuckoos are mimicking the sparrowhawk call, the same four bird sounds were played to great tits and blue tits. Since cuckoos do not parasitise tit nests in the UK, they should not respond to a female cuckoo call in the fearful way they respond to a sparrowhawk call. However, the results show that the female cuckoo call does evoke a fearful response in these birds demonstrating that the cuckoo is successfully imitating the sound of a predatory sparrowhawk.

York and her team then wanted to know how this mimicry benefits the cuckoos. To do this, they planted fake cuckoo eggs into the nests of reed warblers and played the four bird calls again. They found that reed warblers are more likely to accept a cuckoo egg in their nest if they hear the cuckoo chuckle and the sparrowhawk call. These fascinating results demonstrate a clear advantage of making this seemingly counterintuitive chuckling sound after laying.

Mimicry is a very common phenomenon in the animal kingdom and although the female cuckoo call is not identical to that of the sparrowhawk, the natural world is so fast-paced that only a few key features are required to trick reed warblers into this misidentification.

York points out, ‘When it’s a matter of life and death, it makes sense that the warblers would opt to be vigilant, even if the mimicry is imperfect, because if they wrongly assume something is not a threat then the consequence is often death.’

The cuckoo's masterful deception shows that, in the animal kingdom, sometimes it pays to be sneaky.