Cuckoos lay their eggs in other birds’ nests and when the cuckoo chick is born it kicks all of the eggs out of the nest and monopolizes the care and attention of the cuckolded parent.  Fairy wrens have evidently evolved a countermeasure:

Diane Colombelli-Negrel from Flinders University in Australia has shown that mothers sing a special tune to their eggs before they’ve hatched. This “incubation call” contains a special note that acts like a familial password. The embryonic chicks learn it, and when they hatch, they incorporate it into their begging calls. Horsfield’s bronze-cuckoos lay their eggs too late in the breeding cycle for their chicks to pick up the same notes. They can’t learn the password in time, and their identities can be rumbled.

Which is incredibly cool.  An ingenious solution and a testament to the resourcefulness of the evolutionary invisible hand.  Especially when you notice that the cuckoo chick “is a huge, grey monster that looks completely unlike a warbler chick.”  Apparently evolution favors the complex system of teaching and repeating a singing password rather than the boring solution of just staring at the invader and noticing that he looks nothing like a cute little fairy wren.