Keep in mind each level ends when your creature comes into contact with the other creature in the level so collect as many buttons as possible before this happens.
A level can somtimes be completed by moving the 'goal' creature towards your creature. Consider sometimes bringing the goal to your creature.
Be sure to collect as many buttons before your creature comes into contact with the other creature at the end.
Unlike many other physics-based aspects of the game, the green bird blocks first encountered in the second game world move at a relatively quick speed. Be sure you have your timing down and plan set before you release them.