It’s not just the personal experiences animals have that can select for new traits in another species; it’s also the information they share amongst each other. It broadens our understanding and opens our eyes to the complexities of the animal world.
Dr Rose Thorogood
Funded by the Natural Environment Research Council (NERC), a study has appeared in the journal Science
today which offers an explanation for the curious colouring of some female cuckoos. The common cuckoo, Cuculus canorus
is notorious for its parasitical approach to bringing up offspring, laying its eggs in another bird’s nest to be raised by unsuspecting host parents from another species.
One host species commonly targeted by cuckoos across Europe is the reed warbler, but these birds have developed ways of fighting back against the threat of their nest being parasitized. While the familiar grey morph sometimes has the advantage of scaring hosts into thinking the cuckoo is a hawk, the warblers’ first line of defence is to attack. Some female cuckoos have evolved a brownish-red (rufous) colour morph to avoid detection.
Research carried out by a team at the University of Cambridge has demonstrated how reed warblers observe their neighbours mobbing cuckoos and subsequently become alert to the threat of cuckoos – but only to the colour morph that their neighbours fended off. They placed models of both colour morphs near the nests of warblers and recorded the behaviour of neighbouring members of the species.
I spoke to the University of Cambridge’s Dr Rose Thorogood, co-author of the paper along with colleague Professor Nick Davies, who was able to describe the implications of their findings in greater detail…What was the overall aim of your study?
It stems from long-term research we’ve been doing which tries to understand how cuckoos manage to fool their hosts into raising their young on their behalf and understanding how host species have evolved to fight back using various different defences. A lot of work has been done which looks at cuckoo eggs. We know that cuckoos lay eggs which look very similar to those of their hosts to trick the host into accepting the egg and thinking it’s their own.
Cuckoo in flight
More recently we’ve been interested in what happens before
the cuckoo lays its egg and whether the host is able to defend itself then too. We’ve turned our attention to what is called ‘mobbing’ behaviour, which is when the hosts attack the cuckoo when they see it at their nest. We’ve always known that cuckoos come in two colour morphs. The male is always grey, with yellow eyes and feet. The female either looks the same as the male or comes in this drastically different rufous colour. In the United Kingdom the rufous morph is quite rare, but in other parts of Europe it’s much more common.
It’s called polymorphism, when you have these different colours within species. While the phenomenon is quite common across cuckoo species, it’s very rare amongst birds on the whole. It has always been a bit of a puzzle as to why the two morphs exist. We hypothesised that the polymorphism has something to do with allowing the cuckoos to get closer to the nests of host birds.Do we know of any reason why polymorphism is rare across the bird kingdom?
No, it’s another big puzzle. Most of the work on polymorphism and mimicry has been done with butterflies. We therefore understand quite a lot about what’s happening at the genetic level in various butterflies, but we still have few clues as to what is happening in birds.What’s the most exciting aspect of your study and its findings?
The most exciting thing about it is realising how much information the reed warblers share with each other about the threat of cuckoos and just how specific that information is. We’ve always assumed that when they watched their neighbours mob a cuckoo it would make them more alert to cuckoos in their environment and cause them to mob anything that resembled a cuckoo, because it’s such a dangerous threat. We were quite surprised that the effect was so specific and that they only learn about grey cuckoos or rufous cuckoos depending on what their neighbours have mobbed.
This shows that it’s not just the personal experiences animals have that can select for new traits in another species; it’s also the information they share amongst each other. It broadens our understanding and opens our eyes to the complexities of the animal world and where their information is coming from.
Does this kind of social learning exist in other bird species?
Above, the more common grey morph; below, the rufous morph
Yes. Social learning is a field of study that has been around for some time now, not only in humans but in many different animal species. Within birds, it has been looked at mostly in the context of predators and especially learning about predators from your neighbours. This study is different because we’re looking at how social learning has taught animals about brood parasites and also about mimetic defences, which is something that hasn’t been looked at before.
The unique aspect of our paper isn’t the demonstration of social learning, but the context of how social learning is being used and how it can select for a change in another species. That hasn’t been considered before.What are the next steps for your research?
At the moment cuckoos are in drastic decline across the UK. Their numbers have fallen by 60 per cent in the last 30 years. One of the things we’re interested in looking at next is trying to understand how the different defences of the reed warblers might change as cuckoos become less and less common. We’ll be able to chart the changes in the reed warblers’ response to cuckoos alongside the cuckoo’s decline. People have predicted that if you have populations where there is no parasitism, ie no cuckoos visiting, the hosts will not retain their defence behaviours.
The next step is to try to understand how the cuckoo populations tie in with changes in the sparrowhawk and reed warbler populations and to look at how mobbing and socially learned behaviours change. It’s not just the frequency of the cuckoos and reed warblers that is important; the behaviour could also be tied to the frequency of sparrowhawks. If there isn’t a sparrowhawk to scare the reed warblers in the first place, the fact that the cuckoo looks like a hawk is no longer very useful.
As sparrowhawks increase in numbers you would expect that cuckoo populations could also increase because they would have the enhanced protection of looking like a hawk, but sparrowhawk numbers have actually been increasing dramatically since the 1980s, which seems counter to our predictions. It looks like it’s a lot more complicated for the cuckoo than simply looking like a hawk. Their decline probably has more to do with other changes to habitat such as food supply.