Planet Earth - Cuckoo Trickery
Ben - Wicken Fen, a National Reserve in Cambridgeshire, is one of Europe's most important wetlands. A vast expanse of meadows, sedge and reed beds. It's home to more than 8,000 species of plants, dragonflies and birds. Nick Davies, a Professor of Behavioural Ecology at the University of Cambridge has been doing fieldwork on cuckoos at Wicken Fen for more than 25 years. They're known as tricksters, cheats, and even parasites. The cuckoo is one of those birds that enthrals nature lovers and it provides scientists with continual insights into its complex behaviour. Planet Earth podcast presenter Sue Nelson joined Nick beside towering reeds on a windy day on the fens to find out more.
Nick - The way to find cuckoos on Wicken Fen is to go looking with a stick for reed warblers' nests because here on the Fen a reed warbler is the host that the cuckoo targets. And right here in the reeds, if I just part them with this stick, we can see a reed warbler nest...
Sue - Oh yes.
Nick - ... which is a beautiful cup.
Sue - That's tiny. It would fit in the palm of my hand.
Nick - It would and it's suspended between four new reeds. These keep the reed warbler eggs nice and snug, even on a strong windy day like this. If I just tilt the nest slightly, you can see that it contains four reed warbler eggs. So, so far, this nest has avoided the attention of the cuckoo.
Sue - You're specifically interested in the behaviour of cuckoos and this seems to be an area that is bringing up more and more new insights every year it seems.
Nick - It is. It's been known for a very long time that one of the tricks the cuckoo uses is to lay an egg which mimics the host egg, in other words an egg which looks just like the host egg. Now there are several sub-species or host races of cuckoos in Europe. The ones that we find here on Wicken Fen are specific to reed warbler hosts and they're genetically programmed to lay a green egg which matches the green egg of the reed warbler. It's very well known that this egg matching is really important to fool the hosts, and we've shown that by experiment. If you put a model egg of a different colour into a host nest they'll throw them out. Another form of trickery occurs at the chick stage. When the cuckoo chick hatches it throws all the host eggs out of the nest so it's raised on its own. Here it employs the most wonderful vocal trick. It begs at a fantastic rate which sounds just like a whole brood of hungry host young.
Sue - Now more recently I read about research that involved hawks as well. I would never have put cuckoos and hawks in the same sentence.
Nick - That's right. The tricks I've described so far occur at the egg stage and at the chick stage and we wondered whether the hosts have got a defence early on to try and thwart the cuckoo from laying in the first place. What we discovered is that if reed warblers see a cuckoo at their nest they're much more likely to throw out an egg. We showed by experiment that presenting them with a stuffed adult cuckoo, or a model wooden cuckoo we made out of wood, they became much more fussy if they saw this model close to their nest. They were alerted to the possibility of parasitism. The cuckoo's done two things in response. One trick is actually to mimic hawks by having under-part barring and grey plumage and hawk-like behaviour too with a dashing flight. We think the cuckoo benefits from looking like a hawk because the hosts would then be reluctant to approach this cuckoo or hawk-like object close to their nest. So the cuckoo then has time to slip in and lay without being detected. Now to test whether this cuckoo barring really is the key we presented our model cuckoos either with barred under parts or with unbarred under parts. What we showed is that if the model had barred under parts the reed warbler hosts were much more frightened of it, much more reluctant to approach closely. So actually, the big problem now for the host is that if the cuckoos are so secretive, how can they gauge whether they're likely to be within a cuckoo area and so likely to be targeted as a host? Instead of just relying on their own perception of cuckoo activity, what we've discovered is that the reed warblers also listen out for their neighbours' responses to cuckoos. So it's a bit like a neighbourhood watch system.
Nick - Whenever a reed warbler sees a cuckoo in its territory it mobs with a rasping sound, sort of skrrrr, krrr call. And as soon this rasping call takes place, all the neighbours will quickly come over to see what's going on. And you can see the reeds twitching as the neighbours fly often 20, 30 metres to come and have a look at the source of all this alarming. We've shown by experiment that if they see neighbours mobbing a cuckoo they then up their defences at their own nests. So the cuckoo's really got a double reason for being secretive.
Ben - Professor Nick Davies from the University of Cambridge talking to Sue Nelson at Wicken Fen.