Chuckling cuckoos have the last laugh

When it comes to deception, cuckoos are the masterminds of the avian world. But, researchers at The University of Cambridge have discovered that these birds may be even cheekier than we previously thought. Stevie Bain caught up with Jenny York at her Madingley Wood field site to find out how cuckoos have the last laugh over their favourite host species, the reed warbler… 

Jenny - Cuckoos are probably the most famous example of a brood parasite. They lay their eggs in the nests of other species, so they don’t rear their own offspring, they foist this parental care onto other species. They are incredibly secretive and rapid, so they hide away in a perch up in a tree observing the hosts building their nests. And they wait for this perfect moment to glide down and can, within ten seconds, lay an egg and leave the nest which is just incredibly fast. It’s really important that they do it quickly because they want to avoid the hosts noticing them.

But, after laying their egg in the nest, they give this bizarre chuckling call that’s very conspicuous. It seems, at first glance, a bit of a bizarre thing to do because you’ve gone to all this effort to be secretive and get in and out as quickly as you can so why announce it to everybody as soon as you’ve done your job?

We notice that the female cuckoo’s call is similar to a sparrow hawk call; they’ve both got this sort of high pitch, rapidly repeated, call formation. We wondered whether maybe this acoustic resemblance is useful and beneficial for the female cuckoo because it might distract the host’s attention as they’re returning to the nest. So instead of worrying about the contents of their nest, their defences are misdirected to concerns about their own safety.

Stevie - I’m guessing then you went on a mission to find out what this call was all about?

Jenny - We tried to investigate the reed warbler vigilance, so how worried they are in response to hearing four different call types: the female cuckoos call, the sparrow hawk call, and the male cuckoos call, and the collared dove as a harmless control. We found that reed warblers were much more likely to become vigilant when they heard a female cuckoo call or a sparrow hawk call. This shows that it worries them to hear both of these calls.

So we did an experiment out here up at Madingley in the botanical gardens to investigate whether using the same four calls on great tits and blue tits, which are not hosts of cuckoos in the UK, whether they respond the same way to female cuckoos and sparrow hawks. We found that the great tits and blue tits responded in a similar way to female cuckoo calls and the hawk calls, so that really suggests that there’s something about the call itself that is similar to a sparrow hawk call that triggers vigilance and wariness in small birds.

Stevie - I find that really interesting to think that these animals are so sneaky as to develop these traits.

Jenny - It’s all very well showing that there are similarities between the calls and that host species respond to these with vigilance, but what we wanted to know is whether the cuckoo is better off having created this diversion in terms of successfully parasitising the hosts. So, what we did was we found lots of reed warbler nests and we painted one of their eggs at random to simulate a cuckoo parasitising their clutch.

We found that they were much more likely to accept a cuckoo egg if they’d heard a female cuckoo call or a sparrow hawk call. And this is amazing because this means that by giving this call as she leaves the scene of the crime, she can divert their attention enough that they are paying attention to themselves and their own safety rather than inspecting the contents of their nest, and this distraction might mean that the hosts are more likely to end up rearing her offspring than rearing their own.