The best place to accomplish this is in the final reserve (Tanzania
) and the final level there (Morogoro
). The first power-up you find will be a slow-mo one, shoot the herd rampaging away from you, the few that come from the right (last one's a trophy legend) then a few more that run away from you. Swap to shotgun and activate the power-up. There's a great chunk of birds that comes from right to left you can blast. Don't swap back until the next checkpoint as you can keep pumping shots into the birds in top center/right area while you avoid the barrels coming down the incline. Later in that level when you are offered a choice of paths, make sure you choose the left hand path
as there is another flock of birds here (no slow-mo though). Once you reach the end of the level, choose restart and repeat.
Things to make note of:
This is cumulative, all birds you bag throughout all game modes count. There is no counter anywhere in game for how many birds you've shot unfortunately, so you just have to keep plugging away.
This is likely to be one of your last achievements in the game. Having the Shotgun Spread and Powerup Mania perks fully unlocked and upgraded (and chosen as your 2 active perks) will make this much faster. The double blast from the maxed shotgun and wider spread are a godsend, and the extra duration on powerups is a bonus.https://www.trueachievements.com/TheFlamingChode.htm
suggests using the rifle as it is easier to shoot more birds each run and although you are penalized for using the rifle, the kills still count toward your total.
Thanks to https://www.trueachievements.com/CanuckLefty.htm
for the heads-up on the sweet level.