When Pidgey evolves?

Pidgey evolves into Pidgeotto starting at level 18, and then Pidgeotto evolves into Pidgeot starting at level 36.

Pidgey is #16 in the National Pokedex, and was introduced in the Generation I Pokemon titles (Red, Blue, and Yellow) where it could be found on Routes 1, 2, 5, 6, 7, 8, 12, 13, 21, 24, and 25; Routes 3, 14, and 15 (Red and Blue); and Route 11 and Viridian Forest (Yellow). In Generation VI (X and Y), it can be found on Routes 2 and 3.