Amongst all the other bird song this month listen out for the springtime call of the cuckoo. It's becoming a less familiar sound in the UK, but April is the time when this bird returns from Africa to start its notorious nesting behavior. Rather than go to all the trouble of building a nest and raising it's own chicks, the cuckoo simply places its eggs in another bird's nest.
For their size, the cuckoo's eggs are very heavy, and the inside must be like Dr. Who's tardis, because it seems incredible that such a large chick could emerge from such a small container. Either the chick is very well packed inside the shell or it's just instinctively good at Yoga.