Cuckoos are among nature's laziest parents. These birds are brood parasites that lay their eggs in the nests of other species. If undetected, the cuckoo chick will throw out the host's own chicks, obtaining exclusive care from its foster parent. Alternatively, if the host catches on, the cuckoo chick will be abandoned and left to die. While some birds endure high rates of cuckoo parasitism, others, such as Australia's superb fairy-wren, appear to have gained a leg up on their cuckoo parasites. How have they managed this feat? New research published in Current Biology by an international team of researchers led by Sonia Kleindorfer at Flinders University in Australia has identified the fairy-wrens' unexpected mechanism of cuckoo detection: egg whispering.

In contrast to most species, which reject cuckoos on the basis of visual cues, typically at the egg stage, superb fairy-wrens instead appear to reject cuckoo chicks using acoustic recognition. Until now, however, how a fairy-wren mother distinguishes its own chicks from that of a cuckoo has remained a mystery.

Using detailed audio-visual monitoring, Kleindorfer and her colleagues spied on fairy-wren females while they set up their nests. During egg incubation the researchers discovered a novel ‘incubation call’ that mother fairy-wrens produce at a high rate. And embedded within this call is a ‘signature element’, or password, that chicks repeat following hatching. Strikingly, each mother produces a different password, and the chicks of each nest learn only the appropriate message of that nest. That is, fairy-wrens are not born with a password hard-wired into their brains, but rather they have to learn it as an embryo from within the egg.

But how does this help fairy-wrens to thwart cuckoos? Simply, by only heeding the begging calls of chicks that repeat the correct signature element, parents can ensure that they are stuffing food into the mouth of their own fairy-wren chick instead of the gaping maw of a hungry cuckoo.

Less clear is why cuckoo chicks do not also learn the secret code in ovo. The answer to this question, the authors believe, lies in the timing. In a normal nest, fairy-wren eggs are exposed to their mother's lessons for around 5 days. And the more a mother teaches – that is, the more often she repeats the password – the more reliably the chick repeats it upon hatching. By contrast, cuckoo eggs are only exposed to the password for around 2 days, and this is apparently not long enough for a cuckoo embryo to get the hang of it. Thus, when the cuckoo hatches and fails to provide the secret ‘handshake’, the mother assumes that the chick is not her own and leaves it to die.

Cuckoos are remarkably plastic in their ability to mimic their hosts. This new escalation in the co-evolutionary arms race by superb fairy-wrens might, however, prove too tough an egg for the Australian Horsfield's bronze-cuckoos to crack. As with all evolutionary arms races, only time will tell.

M. E.
F. J.
Embryonic learning of vocal passwords in superb fairy-wrens reveals intruder cuckoo nestlings
Curr. Biol.