Are you looking for real birds flying in your 3d scene ? You can find some here.
If you choose MULTIPLY mode for this layer, you´ll get great final result.
We hope you like it this footage for several reasons:
1 – When you need, it´s very difficult to find them.
2 – Film then during enough time
3 – Special weather conditions to get best results
4 – Fine postproduction work did it for you, so, ready to use.
Too much late for Hitchcock but soon for you 