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Building the farm

Note: See if there is a tent, sauna or house nearby that you can access so the animals can respawn.
To build a bird/reindeer farm you first have to find a spawnpoint for the birds/reindeers. You can find the general locations of animal spawnpoints on the map below.
If you know who made this map create an issue on GitHub so we can give proper credits.
You then can simply try to place an item there and when you see a red circle similar to the one below you know that you found a spawnpoint. Now all you have to do is circle the red circle with fences and leave a wide pointing opening that gets smaller towards the end. This will make the birds/reindeers run into the opening and get stuck there.

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Farming

Now just grab a chainsaw and start killing the birds/reindeers. Once all the animals are dead enter and leave a tent, sauna or house that is nearby and the animals will respawn instantly.