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 a great final result.
Reasons to buy it here
1 – It´s very difficult to find that kind of stuff out there
2 – Royalty free with unmatched price
3 – Full collection with different birds and groups or just a clip
4 – Please, try to do it by yourself. I´m sure that you´d prefer it get it here
Reasons because it´s difficult to find this stuff
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.