It is actually a very efficient way for cuckoos to have their offspring raised, because if they had kept their offspring they would constantly need to feed and take care of it, but instead of that they rely on other birds to do it, which is very efficient because now they can focus on themselves. When they lay their eggs in another bird's nest that bird thinks that those eggs are in fact its offspring so it takes care of it. This does in some way affect the other birds offspring of course because they could probably only do so much for so many babies. If you want more in depth just ask :)
