Children's story books tell you that's what owls sound like. Well, they're not entirely wrong... 'Tu-whit, tu-woo!' is actually the sound of two tawny owls.
Female tawnies are responsible for the 'tu-whit' (or 'ke-wick') part, while the 'song' of the males is a quivering 'whoooo'. And sometimes both noises can be heard at the same time.
Tawny owls are some of our earliest-breeding birds. They may even be incubating eggs or brooding small owlets right now - they don't like to waste time!
As well as in the countryside, you could hear tawny owls in the suburbs, as they nest in holes in mature trees. If you have somewhere suitable, how about putting up a tawny owl nestbox?