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In what's being called an evolutionary arms race, bronze-cuckoo appear to be another step ahead of their hapless hosts.

Using techniques that provide a bird's-eye view of the world, Dr Naomi Langmore from the Australian National University and colleagues have discovered that bronze-cuckoo chicks have evolved to appear almost identical to the chicks of their hosts.

The research, published today in Biology Letters, could also raise the alarm about how an otherwise well-balanced natural system could lead to extinctions in the future due to climate change.

The researchers studied three species of cuckoos living in Darwin, Cairns, Canberra and the Kimberley in Western Australia. The little bronze-cuckoo parasitises the large-billed gerygone; the shining bronze-cuckoo parasitises the yellow rumped thornbill, and Horsfield's bronze-cuckoo parasitises superb fairy-wrens and purple crowned fairy wrens.

After the cuckoo chicks hatch, they evict the host bird chicks from the nest.

Despite the cuckoo's ingenuity, many host birds retaliate by throwing the cuckoo chicks out of the nest. Researchers wondered whether some cuckoos were fighting back by producing chicks which looked like the host's own chicks.
Seeing chicks in a different light

To find out, they removed cuckoo and host chicks from the nest and compared them using a spectrometer. This is an objective way to measure colour in the red, blue, yellow and ultraviolet range. Birds, unlike most mammals, can detect ultravoilet light.

Lead author Naomi Langmore says the measurements showed that the cuckoo chicks overlapped the same colour space as their host.

"It was just a way of telling us what is already obvious from the photos, which is that the cuckoo chick does mimic the host, but we confirmed that from a bird's eye view."

They found the dark pin feather-covered skin of freshly hatched little bronze-cuckoos was a near perfect match to that of gerygone chicks, while the shining bronze-cuckoo chicks were the same vivid yellow as their host thornbill chicks.

The researchers also found a good match for the rictal flange, the brightly coloured border of the chick's mouth which alerts the parents where to put the food.

The mimicry only lasted for eight days, after which the chicks began to look like their own species, but it was long enough to fool the hosts.
Difficult for parents

Researchers have always puzzled over why some birds are willing to raise cuckoo chicks when they appear so utterly unlike them - and often so much larger. Langmore says the problem for the parents is that chicks grow so fast, it's cognitively very difficult for parents to 'learn' what their chicks look like because they are changing every day.

One strategy hosts use is to detect minute differences between a cuckoo egg and their own egg. This works in Europe, where nests are cup shaped and open, but in Australia, nests are domed-shaped and dark. As a result, Australian hosts are more likely to reject cuckoo chicks after they hatch.

As a consequence, Langmore argues, cuckoos have taken the next step in the arms race by impersonating the chicks of their host. Australian hosts may have a better idea of what their newly hatched chick should look like, and the cuckoo uses this to its advantage.

"If it weren't for changes to the environment, it would be a very balanced system," says Langmore.

"The problems begin with climate change, so for instance the Australian Koel (another cuckoo) is moving its range south now. Supposing it were to come into contact with the regent honeyeater, (which isn't currently parasitised by cuckoos), that would be a very ideal host - but it's also highly endangered. So if you have a host species with no defences against cuckoos, then you could then start to see real problems for the host."

Dr Leo Joseph who is Research Director and Curator of the CSIRO's Australian National Wildlife Collection says: "it's another equally fascinating instalment in the research story that Naomi and her colleagues have been building. They are calling it the evolutionary arms race between cuckoos and their hosts.

"If the eggs don't get detected, the hosts have another card up their sleeve. They can abandon the nest or throw the cuckoo nestling out. So the cuckoo has been responding to that selective pressure, it seems, by developing visual mimicry of the host nestling, providing yet another challenge to the host."

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