It may seem at first that the Pokémon that you come across in Pokémon GO are randomised, but that isn't the case. Depending on where you live and what you live near, the 'usual' Pokémon can be wildly different from what someone might expect to see somewhere else in your country. These are something that the fans have dubbed 'biomes'. While this means you might be more likely to run into similar Pokémon over and over again, there is in fact another way to find Pokémon that you wouldn't normally come across. These are locations that are known as Pokemon Nests.
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Pokemon Nests are particular locations that can, and often, spawn Pokémon that normally wouldn't show up all that often. They're typically tied to locations like public parks, and can dramatically shorten the time it takes to evolve your Pokémon. Some may spawn a few Pokemon every hour, while others can spawn multiple, all in one location.
What Pokemon appear at Nests?
While Nests can spawn a huge variety of different Pokémon, there are a few limitations. The main one is that Nests will only spawn basic-stage Pokémon, meaning you can't get a nest of Gyarados, for example. That's not to say that evolved Pokémon won't appear in the vicinity, but they'll be spawned in by the local biome and not by the nest.
There is also a limitation in what basic-stage Pokémon can appear as nests. Currently, Pokémon that you find in 10km eggs won't appear as nests, though this could change in future updates. As an example, Scyther, Pinsir, Kabuto and Omanyte all used to be found in 10km eggs, yet they appeared as Nests. Mantine, Gligar and Miltank, who are currently 10km egg Pokémon, cannot be found as nests.
Where are the Pokemon Go Nests?
Unfortunately, there's no surefire way of knowing where a nest can appear in-game, though there are some tools that you can use to help narrow it down. The Silph Road's Nest Atlas is one of the best ways to learn about Nests that have been found by other local players. Even if the Nest Atlas isn't up to date, you'll still be able to see historical locations for local nests, allowing you to pay them a visit to see if anything interesting is currently hiding out.
Nest Migrations in Pokemon Go
Every two weeks, Niantic refreshes Nests all over the world. This typically happens on a Thursday, so check local Facebook groups and Discord channels to see if anyone is reporting on what the Nests change to at those times. Also, make sure you get what you need from the Nests while they're around!