After a full year of Pokémon GO, many fans have completed their PokéDex and are currently anticipating the arrival of Gen 3 Pokémon that may be introduced similar to the way Gen 2 Pokémon were.
In order to balance out the PokéDex world-wide, Niantic had to create a way to add variation to each location around the world.
During this concept creation they came up with the Nest Migration, which changes frequent or common nests to different Pokémon. Giving the Pokémon world a small shuffle. When Gen 3 Pokémon come out, the Nest Migration feature will play a huge part in creating a fair variation across the world.
Many people seem to mistake spawns and nests, often claiming they have no nests because they never search for them, or just have no clue where to look. Most of the nests around the world can be found at Parks, which is where they first started popping up. A lot of the time you can also find them around water or different biomes.