There is a good chance that you can get this in an entire playthrough of the game. I managed to pass the 1,000 mark in the closing hours of the game. My advice, don't run off from the birds, stick around and try to take them all out. If that fails and you don't make it by the end, simply load up the end of episode 2 where the birds are attacking Barry in the house, and just rinse and repeat. Or you could easily do the bird attack at the end of episode 2 and then restart checkpoint to kill them all again. If you're doing 2 playthroughs though (Normal and Nightmare), then this should be easy anyway.
Good tactics? Obviously the boosted torch is effective, but so is a direct hit with the flaregun or a quick flare when they are near-by.