Once a bird has reached the maximum level for its current evolution stage, you can evolve it, if you have enough evolution materials of the same colour as the bird. To evolve a bird, go to the village, tap “Your team” and select the evolution tab in the top right of the window. Drag the bird you want to evolve into the slot on the left and tap “Evolve”. Four and five star birds have two variants for the last evolution, you must select which one you want before you evolve the bird!
To evolve your four and five star birds for the second time at level 50, you will also need Evolution essence in the bird's colour. You can earn Evolution essence from the dungeons.