Cuckoo (S27E07)

The sound of the cuckoo is to many the very essence of spring, yet behind the magical call is a bird that is a cheat, a thief and a killer. Just how does the cuckoo trick other birds into accepting its eggs and raising its young? Why don't the duped foster parents react as they watch the baby cuckoo destroy their own eggs and chicks? And why do they work so relentlessly to feed a demanding chick that looks nothing like them and will soon dwarf them? (BBC)