Cuckoo
The female cuckoo spends a lot of time watching the birds at the nest where she wants to lay an egg. She has to work out the exact time to go and lay her egg while the 'host' parents are not looking. If the cuckoo lays her egg in the nest before the other bird lays, the reed warbler will notice and knows it is not her egg, and so she will get rid of it. If the cuckoo lays her egg too late, when the reed warbler has finished laying, this will be noticed, too.