Niantic is rolling out an update to Pokémon Go that should make it easier to track down rare Pokémon. With the update, trainers will receive a bonus when they catch enough Pokémon of a certain type. For instance, if you collect 50 water-type Pokémon, you'll get a Swimmer medal with a "catch bonus" that improves the chances of finding rare Pokémon.
We are adding a new feature which grants a catch bonus when you earn medals based on catching certain types of Pokémon (Kindler, Psychic, Gardener, etc.). These new bonuses will give you a better chance of capturing Pokémon with a related type. For example, as you reach a higher tier for the Kindler Medal, your bonus to catch Fire-type Pokémon such as Charmander, Vulpix and Ponyta increases.
Trainers can work their way to a new tier by catching many Pokémon of the same type. If a Pokémon has multiple types, your bonus will be the average of your bonuses for each type. For example, Pidgey is both Normal and Flying type. Your bonus in this case would be the average of your Normal-type and Flying-type bonuses.
The screenshots detailed by Niantic show that catching more Pokémon of the same type increases the corresponding bonus for each tier. There's no mention of when the update will go live, but we'll let you know once it does.