So it turns out that the entire sound track of Hitchcock's 'Birds' is synthesised, including all the bird sounds (and flock textures). I was always impressed by the sound design, now I know why! 

http://www.bbc.co.uk/iplayer/episode/b03bm2fy/Sound_of_Cinema_The_Music_that_Made_the_Movies_New_Frontiers/