Crows in the sky means the area beneath them is Infested. Infested zones will have several Freaker nests in them and will continually spew out freakers unless cleared. These zones cover a large area and you won’t be able to Fast-Travel if the route to your destination passes through the zone.
To clear an infestation zone, you have to find and destroy all Nests in the area. The game will not automatically mark their Locations until you're very close to it. Nests can be identified by structures filled with branches, twigs, and other materials similar to bird nests. Use Molotovs to set these nests on fire. Materials for creating molotovs are conveniently found near nest locations.
Once the fire starts, hibernating freakers will come out and attack. It’s recommended to throw the molotov from a high ground or from cover to avoid getting overrun by freakers. Once they’re out, you can proceed in eliminating them using any method you prefer. Fewer swarmers will appear if you burn their nests at night.
Cleared nests will always have a container inside them that contains items, crafting materials, ammo, etc. Clearing infestation zones are also part of storylines and will raise the Trust level of an area and give you XP as well.
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