This is an assumption but i believe uchiha have skill assign to their sharingan/bloodline.Fire style jutsu is automatically assign to uchiha (element) ,ocular genjutsu(genjutsu ) and bird summon (.Both itachi and Sasuke have bird summon ).Therefore they can get another elemental jutsu and summoning jutsu.This is similar to Rinnegan user have automatic access to Gedo Mazo,6 path.