Gary Oldman doing sound effects?

That bit where theyre in the corridor and Guildenstern is listening to Palonious telling the king and queen of hamlet's letter to ophelia and suddenly Rosencrantz starts doing animal impressions, including a crow, a lamb, a cuckoo and at the very end an automobile horn! I have always wondered watching that bit, was Gary actually doing all those impressions or were the noises dubbed in? I know Gary is a very talented impressionist but i haven't really heard him do those sounds live so I'm not sure. ?