New facts have emerged to suggest that at the presence of a cuckoo, reed warblers combine various information sources before attacking a cuckoo or its eggs. This finding was published in the journal Scientific Reports by researchers from the University of Cambridge.
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Reed warblers naturally undertake an annual flight of about 5,000 km from their habitat in West Africa to their breeding grounds in the north, where cuckoos try to cheat them by laying eggs in the warblers’ nests. Bad enough, the cuckoo’s egg hatches in the warbler’s nest and the young chick pushes out its host’s eggs and its young ones, and then allows the confused warbler parent to feed it until it gets strong enough and flies away.
The egg of the cuckoo is very similar to that of the warbler in every aspect, but slightly bigger. Understandably, a warbler pushes off the egg of the cuckoo once it suspects that the egg is not its own, but not until it has processed neighborhood information from various sources would the warbler take this decisive action.
The cuckoo deceives the warbler into hatching its eggs because of the great similarities in the eggs, but it also acts fast in laying its eggs – laying only one egg per warbler’s nest, and finishing the business with each nest within 10 seconds. This makes the warbler unsure if the egg is its own or another’s.
But beyond laying eggs in another’s nest, researchers had always wanted to know how reed warblers coped with cuckoos and estimated its risks before acting. Over a 2-year period, Rose Thorogood and Nicholas Davies of the Department of Zoology observed warblers during their breeding season at the Wicken Fen in Cambridge, finding that warblers make use of various sources of information before acting to expel a cuckoo’s egg or chasing away one it sees in its territory.
“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. The warblers needed to be alerted by their neighbors’ behavior that there was a cuckoo at large in the neighborhood and they needed to be aware of a more local and imminent threat, by seeing a cuckoo near their own nest. ” said Thorogood.
This means a cuckoo must come close to a warbler’s nest for it to be considered a real threat, and then the warbler emits an alarm call that carries up to 40 meters away, attracting neighbors who come down to investigate the problem; and finding a lurking cuckoo in the area or close to a warbler’s nest, chase it away before investigating if there are any strange-looking eggs in their nest – with the result that any odd-looking egg obviously belonging to a cuckoo is chucked out of the nest.
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“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 neighborhood,” Thorogood added.