Alan Wake

In chapter 5, just after the helicopter crashes and you leave the old woman to investigate, you'll pass through a checkpoint on a cliffside where about 4 groups of birds will attack. After defeating the birds, instead of continuing to the crash site, walk across a log straddling the gap in the cliffs to your right. This log takes you back up to the area prior to the checkpoint. Continue back down the hill towards the crash, through the checkpoint again, and the same groups of birds should respawn and attack just like before. It seems that this loop can be repeated endlessly until the achievement unlocks. I was able to use this method to get the achievement without quitting out of the game or restarting.

Honestly unless you're really impatient to get the achievement there is no need to worry about this one. You will get it as you get the rest of the achievements. During your normal difficulty playthrough you will get between 750 - 850 birds, as a rough average. When you attempt your nightmare difficulty playthrough you will often get slightly less than that amount of birds. Thus over your two playthroughs you will get about 1,500 birds, easily enough to get this even if you run past some of them or just chase them off with flares.

EDIT: According to True Marvelous they were able to get this on one playthrough. The easiest way to do this would be using your flare gun very agggressively to take out flocks and timing flare use so that you catch any birds diving at you. The flashlight is good for finishing off a large group from a distance.

I personally acquired this replay levels to find all of the collectibles but this definitely seems to be doable in one go.

After one play through I had around 860 birds killed.

Replay the end of Episode 2 where Barry is in the house and you have to scare away the birds. Use the flare gun on them, over in a few minutes, check your stats, reload last checkpoint and do until you have the achievement

If like me you have completed the game and cant be bothered to play through Episode 5 until you get to the point mentioned in other solutions you might just want to do this

Choose to play an episode, on normal difficulty. You want

Episode 5: The Clicker
Chapter - Bright Falls Light and Power

Skip the cutscene and you'll be in a yard with the helicopter above you about to fly off, a swarm of birds will come from directly in front of you. Simply kill these, dropping your torch light to the floor ocasionally to make them charge you. Once youve killed 40ish they will spark, and fly off.

Now reload checkpoint and start again. Its about 30-50 a time I found, and I got my final 400 kills in no time.

I find you dont even have to move on this, and if you do move too far after killing the final bird it sometimes saves a checkpoint, so just stay on the spot and kill those birds.

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I got it when I was heading to Mirror Peak lookout in Episode 3. There's a chest with a flare gun in it just before you head along a narrow cliffside path. You'll know you're there when you cross the wooden platforms that fall and there is a ton of birds. If you fall down a mine shaft you went too far. I just reloaded the check point when my flares were gone. You get about 100 to 150 or so per run. I died twice and reloaded once and got it.

Don't worry if you don't get this on your first playthrough, your stats are stored between runs.

Whenever you get attacked by flocks of birds, just make sure to get your torch focussed on them quickly and burn as many as possible. Flareguns, flashbangs can also help, but I found a good torch was the best technique.

Also, if you can, limit the number of directions they can come at you from as this makes it easier for you to spot 'em comin' and to target 'em.

Check your progress under the stats, but you'll probably rack up a good proportion of the kills needed in your first playthrough.

by chapter 5 i had about 600 birds all u do is the area where the helicopter crashes and the birds attack you just keep playing this part over and over i got about 75 each time :)