The reed warbler and the cuckoo: an escalating game of trickery and defence

Reed warblers are a little smaller than sparrows and each one weighs no more than a large envelope. As autumn begins they migrate some 5,000 km from Britain to West Africa, a journey they might make just two or three times in their short lives. In April they fly north to breed in the watery landscapes of northern Europe where they raise their young in nests suspended from reeds. Sometimes they are tricked into raising cuckoo chicks which grow to four times their size. 

In his book  Cuckoo - Cheating by Nature, Nick Davies (Department of Zoology) describes what it’s like to watch reed warblers at the Cambridgeshire nature reserve of Wicken Fen. He carefully parts the reeds until he can see a pair of warblers feeding their young in a nest. He senses the parents’ urgency in collecting insects for their chicks while keeping them warm and staying alert for signs of danger. When several hours later he stands up, the intimate world of the warbler disappears into the great expanse of fenland and the wide East Anglian skies.

Observation remains vital to learning more about the world, believes Davies. “There’s still plenty more to learn from going out into nature and watching carefully,” he says. “I get most of my ideas by watching animals and simply asking ‘I wonder why they’re doing that?’ The key to research is coming up with a good question and devising an experiment to answer it.”

Davies, who gives this week's Darwin Lecture (Games Animals Play), has been studying reed warblers at Wicken Fen for more than 30 years. He thinks of them as ‘his’ warblers and calls his interest in their lives, and their fragile niche within a changing environment, a kind of obsession. In the process of countless early mornings, and dozens of experiments, he and his colleagues have gradually unlocked some of the secrets of warblers’ interactions with cuckoos, who ‘parasitise’ other birds.

In an endless game of trickery and defence, the cuckoo and its hosts engage in an ‘arms race’ involving mimicry of many kinds – from the patterning of eggs to the demanding twittering of chicks – as the two species weigh up the risks of being duped and discovered. Reed warblers sometimes reject eggs that don’t look like their own – but what evidence does a warbler need before it takes such drastic action?  Recent research reveals that warblers eject suspect eggs from their nests only when local information is reinforced by signals from a wider ‘neighbourhood watch’.

Davies’s fascination for birds stems from a childhood spent on the Lancashire coast where the skies were full of skeins of pink-footed geese and the sand dunes were home to croaking natter jack toads. He got his taste for patient observation, for asking difficult questions (why, for example, does the reed warbler accept a cuckoo chick so obviously different to one of its own?), and interest in detective work from Niko Tinbergen, a pioneer of scientific studies of animal behaviour. As an evolutionary biologist, Davies is also respectful of the observational studies of the early naturalists who laid the foundations for subsequent experimental work.

The remarkable insights explored so vividly in Cuckoo - Cheating by Nature would have been impossible without research collaborations, often international. Birds migrate vast distances: to understand them, and how they’re shaped by evolution, requires an investigation of every aspect of their lives. Within the same species, there are behavioural variations which offer clues to their evolutionary pathways. To get a picture of the different ‘races’ of cuckoos (categorised by the species they parasitise to host their young) Davies has worked with biologists across the world.

He says: “Some of the most exciting discoveries are now being made in Africa by Claire Spottiswoode and in Australia by Naomi Langmore. In both places, the arms race between cuckoos and hosts has been going on much longer and has escalated to new levels. For example, in Australia some hosts reject chicks unlike their own and their cuckoo has combated this by evolving a mimetic chick. And in Africa, cuckoo hosts have the most remarkable egg signatures in the form of individual spots and squiggles which makes it easier for them to detect a foreign egg.”

In the accompanying podcast, Davies talks about the games animals play with particular reference to the dunnock, a small brown bird with a surprisingly inventive sex life.  

The lecture Games Animals Play will take place in the Lady Mitchell Hall, Sidgwick Site, University of Cambridge, on Friday, February 26, 2016 - 17:30 to 18:30. No booking required, no charge. Arrive in good time to secure a seat.

Main image: a reed warbler feeds a cuckoo fledgling (http://www.richardnicollphotography.co.uk/) Inset images: a clutch of reed warbler eggs with a larger cuckoo egg (Nick Davies); a meadow pipit feeds a cuckoo fledgling (Charles Tyler).