This is a very big question.
By "nature animals" I think you probably mean wild animals. There are different types of eaters in nature. Some wild animals eat meat and hunt other animals. They are called carnivores. Some wild animals graze on grass or eat berries and roots, they are called herbivores. Some animals can be both.
Birds are animals but they are not mammals. Birds generally eat insects, berries, seeds, and in some cases small animals like mice, lizards, frogs, and chipmunks. Some birds eat dead meat.
A good way to study this subject is to chose one type of animal at a time and get to know their habits.