Getting their own back on cuckoos: Australian fairy-wrens have the last laugh

A team of scientists from Cambridge University and Bristol University, led by Dr Naomi Langmore of the Australian National University, has found that some Australian birds are one step ahead of their British counterparts in their ability to avoid being victimized by cuckoos.

Cuckoos exploit other bird species by laying their eggs in the nests of other birds. Soon after hatching, the cuckoo chick kills host young by tipping them out of the nest. The foster parents then work hard to rear the imposter in their nest, for which they gain no genetic reward.

Not surprisingly, host species go to great lengths to avoid being exploited. British hosts of the common cuckoo, such as reed warblers, are extremely good at spotting odd-looking eggs laid in their nest, which they quickly remove. In response, cuckoos have evolved mimetic eggs that closely resemble host eggs, and so have a higher chance of being unnoticed. However, despite their fine egg recognition skills, British hosts persist in feeding the cuckoo chick even though it looks utterly unlike their own young and can be six times bigger than the adults feeding it.

Now it seems that the superb fairy-wren, which is host to Australia’s Horsfield’s bronze-cuckoo, goes one better. The Horsfield’s bronze-cuckoo lays such a mimetic egg that it is never spotted or removed by hosts. Within 48 hours of hatching, the cuckoo chick evicts host fairy-wren young from the nest. However, two days after that, roughly 40 % of host fairy-wren mothers have the last laugh. They abandon the nest, leaving the cuckoo chick to starve to death, while they renest. Cuckoo chicks are recognized partly because they are alone in the nest, and partly through their odd sounding begging calls.

“We don’t really know why these Australian hosts are so much better at deserting cuckoo chicks than their British equivalents”, says Dr Rebecca Kilner, of Cambridge University’s Zoology Department. “One possibility is that the Australian breeding season is longer, meaning that birds have more to gain from abandoning a breeding attempt at such a late stage. Australian birds have plenty of time for renesting which simply isn’t available to the British hosts”.
The research is published in the March 13th issue of Nature.