When is the Right Time to Cut My Hedges?
For a few reasons, generally in the autumn and winter period between October and March. (Flip that if you're in the Southern Hemisphere!)
Importantly, at this time of year there aren't nesting birds in the hedges. This is important because if you knowingly disturb an active nest then in the UK this is a criminal offence. You can still cut hedges during nesting season but it is easier and safer to cut them at a time of year when the nests are not active.
Should you decide to cut during nesting season and find a nest before you have finished the job, then you have to down tools and stop immediately which is inconvenient. Also you can't resume until the eggs are hatched and the babies have fledged and left the nest. As a result of this you also run the risk of having a half cut eyesore of a hedge for anything from a couple of weeks to very possibly several months.
Thirdly hedges take more kindly to being cut whilst dormant over the Autumn and Winter periods as it is less stressful for them and they are able to divert their energies into recovering in the spring when the majority of their growth takes place rather than in the summer when different species are variously producing fruits, nuts, flowers etc.
Lastly there is less mess to clear up when deciduous species have shed their leaves and nobody wants too much mess to clear up now do they?