All you have to do is fast travel between the two marked signposts on the map down below and kill the bird at each spot. I found the birds needed for the achievement in these two areas, and figured out that fast traveling between the two spots forces them to respawn. There are probably other spots to shoot them down as well, but traveling between these two areas doesn't cost much hunger/thirst not that it really matters much anyway.

Location: Midlands
Signpost #1: Northeast of Nottingham
Signpost #2: South of Notingham

When you fast travel here, turn and look across the field....that's where the bird spawns most of the time for me. Use your slow ability to snatch it out of the air. If it doesn't spawn right away, listen for the bird calls to point you in the right direction.

Once you fast travel here, you'll want to head a little north to the pond area to the right of Sheriff's Soldiers' Camp (at least that's where I shot the bird the most times), but just listen for the bird calls. Don't listen to the crow as I think it it is just a sound que saying that the bird is in the area. I shot down most of the birds in the marked yellow area on the screenshot down below.

If you find any other areas where you shot down the birds for this achievment, comment down below! I found another spot or two but couldn't remember exactly where they were. Really digging this game!

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