What’s the password?
Superb Fairy-Wrens (Malurus cyaneus) from southeastern Australia are often exploited by the Horsfield’s bronze-cuckoo (Chalcites basalis), who lay their eggs in a fairy-wren’s nest to pass on parenting duties to unwitting foster parents. In most species, this intrusion is either sorted out before the cuckoo eggs hatch, with the parents recognising and ejecting the eggs, or parents are just resigned to feeding the chick anyway. But in 2003, Naomi Langmore found that fairy-wrens will abandon 40% of nests with a Horsfield bronze-cuckoo chick in it, which means that somehow, they can recognise the intruders. By keeping nests under constant audio surveillance, Diane Colombelli-Negrel from Flinders University found out how they do it: a very neat evolutionary trick. Starting about nine days into the eggs’ 14-day incubation, the mother wren sings a two-second tune to them every four minutes. This tune contains a unique note that’s literally a password—when the eggs hatch a week later, the chicks do what’s natural: beg for food. Their call contains this special note, and the parents know it’s their chick. Colombelli-Negrel found that if she swapped eggs early in the incubation period, the hatched chicks’ calls matched their foster parents’ calls, not their biological parents, suggesting that this isn’t an innate ability—it’s something they learn. Cuckoo eggs are usually dropped into a fairy-wren nest late in the incubation period, so when they hatch, cuckoo chicks haven’t learnt the password. The parents realise something is wrong, and as a Langmore found, 40% of the time they abandon the nest and go make a fresh start at a family elsewhere. It isn’t a clear win for the wrens—maybe they’re getting worse at recognising the signature note, or maybe cuckoos are getting better at mimicking. Either way, it’s a pretty interesting adaptation in the long-running battle of the birds.