How does a cuckoo know it’s a cuckoo?

Chris - No one knows for absolute certain but one theory is that the cuckoo's parents may pay a visit to the nest that they cuckolded and visit their chick as it's reared so that it imprints on them because one of the things about birds is that they imprint - in other words, they recognise objects that are nurturing them as their parents - and you can actually make them imprint on inanimate objects. If you have a bird and you put it in contact with the door that opens and closes, it can end up thinking that the door is its mother and it will follow a door around bizarrely and so, one theory is that the cuckoo's parents do occasionally pay a visit so the cuckoo recognises that it is genuinely a cuckoo. That or there's some other genetically programmed innate behaviour that's in there.