Take a step out your front door and you'll be instantly confronted with many species of bird. But have you ever wondered, just how many different ones are there. Simple enough questions, just grab a clipboard and start counting... Except no. It turns out this is one of the trickiest problems in biology, and isn't helped by the fact that we don't even have a consistent definition for a species is. This week we're going to dive in and try to untangle this problem, and see if we can't come up with a count of just how many birds are in this big old  world. Or maybe we'll get side-tracked along the way. Who can say?