On May 23rd, 2018 Niantic announced via Twitter that Articuno would be the next Pokemon to be available via Research Breakthrough. On June 1st at 1:00PM PDT Niantic will rotate out Zapdos and bring in Articuno, as well as change research task rewards to be Water-type themed. See below for Twitter post.
Get ready for more Field Research tasks and exciting rewards in June! On June 1 at 1:00 p.m. PDT, a brand-new set of Field Research tasks focusing on Water-type Pokémon will become available. pic.twitter.com/raWVpEUaGO
— Pokémon GO (@PokemonGoApp) May 24, 2018
