From Opus

Pishing is a technique, first developed in the USA, for attracting birds in woodland. By making high-pitched squeeking noises it is possible to attract many small passerines. Some people achieve this by noisily kissing the back of their hands. Another technique which can produce good results is to rub a piece of polystyrene on we glass. Yet another approach is to make insistent 'pssssss, psssss, pssss' noises. In the UK, great tits Parus major are often the first to respond. If they are agitated they will start to make 'mobbing' type noises themselves. You can end up with quite a crowd of small birds excitedly squeeking and chipping. Occasionally this will attract predators, such as sparrowhawk.