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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