̽��ֱ�� of Cambridge - reed warblers

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

amb206 — Mon, 22 Feb 2016 10:45:59 +0000

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?’ ̽��ֱ��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.

̽��ֱ��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.

̽��ֱ��lecture Games Animals Play will take place in the Lady Mitchell Hall, Sidgwick Site, ̽��ֱ�� 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 (https://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).

Professor Nick Davies, who gives this week’s Darwin Lecture, has been studying reed warblers for more than 30 years – and has unlocked many of the secrets of their interactions with the cuckoo. His work shines light on the evolutionary games played out in nature as species compete with environmental pressures, with other species, and with the opposite sex, to pass on their genes.

I get most of my ideas by watching animals and simply asking ‘I wonder why they’re doing that?’ ̽��ֱ��key to research is coming up with a good question and devising an experiment to answer it.

Nick Davies

Richard Nicoll

A reed warbler feeds a cuckoo fledgling

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Neighbourhood watch and more: how reed warblers watch out when there’s a cuckoo about

amb206 — Fri, 22 Jan 2016 12:00:00 +0000

It’s a risky business being a reed warbler. Not only do these tiny birds embark on an annual migration of some 5,000 km from their West African winter quarters to breeding grounds in the north, but they are also ‘hosts’ to the cuckoo, a species that lays its eggs in other birds’ nests and takes no further part in raising its offspring. When the cuckoo chick hatches, it pushes the reed warbler eggs and young out of the nest. As sole occupant, it tricks its warbler ‘parents’ into supplying its voracious appetite until it fledges.

Cuckoos are expert tricksters: their eggs mimic those of their hosts in pattern though they are a little bigger. If the reed warbler detects an alien egg in its nest, or spots a cuckoo nearby, it may eject the odd-looking egg. But cuckoos are so swift in laying their eggs (only one is laid per nest and the process is over in as little as 10 seconds), and so clever at disguising their eggs, that warblers are often uncertain whether an odd egg in the clutch is a cuckoo egg or one of their own.

Research into the relationship between cuckoos and reed warblers has to date concentrated on the behaviour of individual birds and their interactions with cuckoos, described as parasites. A new study published today (22 January 2016) in Scientific Reports looks at wider interactions between neighbouring communities of reed warblers, their strategies for coping with cuckoos, and, in particular, how warblers assess levels of risk by gathering information from a variety of sources.

After two years of observation of warblers that spend the breeding season at Wicken Fen in Cambridgeshire, authors Rose Thorogood and Nicholas Davies (Department of Zoology) reveal that a kind of “neighbourhood watch” exists out in the reed beds, keeping birds up-to-date with the latest threats. Using a series of controlled experiments, involving model cuckoos and broadcasts of reed warbler alarm calls, the researchers revealed that reed warblers factored information gathered from close surveillance of the neighbourhood into their decision-making when assessing whether or not to eject an egg.

When reed warblers spot a cuckoo, they may mob it and emit alarm calls that carry up to 40 metres. These alarm calls attract neighbours, who come to investigate the cause of the commotion. But the sound of neighbourly mobbing of a cuckoo alone is insufficient to prompt warblers to eject a suspect egg from their own nests. They also need clues that suggest a more close-up and personal threat.

“We found that warbler pairs ejected an odd egg only when there was strong evidence that it might not be one of their own. For action to be taken, the clues had to add up. ̽��ֱ��warblers needed to be alerted by their neighbours’ behaviour that there was a cuckoo at large in the neighbourhood and they needed to be aware of a more local and imminent threat, by seeing a cuckoo near their own nest. ” said Thorogood.

“Neither personal encounters nor social encounters alone were sufficient to stimulate egg rejection. Instead, information was combined from both these sources. This is fascinating because we have assumed previously that animals favour one type of information over the other – for example, experiments show that some fish species will ignore where their shoal mates forage if they already have information about the location of food themselves, even when it is less profitable. Here we show that combining information is the best way to take the most appropriate course of action.”

̽��ֱ��use of multiple sources of information has important consequences for cuckoos too. With their neighbourhood abuzz with information, cuckoos need to be wary of alarming potential hosts.

“Because the information warfare between cuckoos and their hosts extends well beyond individual interactions, there’s pressure on cuckoos to be increasingly secretive, not only to avoid alerting their target host pair, but also other host pairs in the local neighbourhood” said Thorogood.

Cuckoo numbers have declined by as much as 60% in the past 30 years for reasons that remain unclear. At Wicken Fen, where several hundred warblers arrive to breed each May, between 10% and 20% of reed warblers nests were used by cuckoos. Today only 2% of warbler nests at Wicken host cuckoos. This rapid drop in cuckoo numbers, which contrasts with a stable warbler population, has enabled Thorogood and Davies to track how the warblers have dropped their defences in concert with the dramatic decrease in cuckoo threat.

Davies has been researching cuckoos and their hosts at Wicken Fen since the 1980s. He said: “Reed warblers are much less likely to eject an egg from their nest today than they were in the 1980s. This makes complete sense. They have matched their behaviour to the changing level of risk. Most reed warblers have just one or two summers in which to breed. So every opportunity to mate, construct a nest and raise a clutch of eggs is precious. If a pair of warblers mistakenly identifies one of their own eggs as a cuckoo egg and chucks it out, or deserts the nest, the loss is great. Our work shows how they match their defences to the risk of parasitism.”

A study of reed warbler behaviour reveals for the first time that in assessing the risks posed by cuckoos the birds combine information from multiple sources. An ‘information highway’ provides one set of clues and personal encounters another. Only when both add up, do the birds take defensive action.

Warbler pairs ejected an odd egg only when there was strong evidence that it might not be one of their own. For action to be taken, the clues had to add up.

Rose Thorogood

Richard Nicholl

A cuckoo chick ejects a reed warbler egg from a nest

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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