Do not clip any hedges with nesting birds in them, it would disturb them greatly.
Before clipping hedges it is mandatory to check for nesting birds beforehand.
Fast-growing hedges need clipping throughout the growing season! You could cut a cat out of your hedge: http://bit.ly/m0ASV9
Leave nesting birds undisturbed in garden shrubs and hedges.
Leyland cypresses and other fast-growing hedges should be clipped as necessary throughout the growing season.
Always check for nesting birds before you start clipping hedges.