Free the Animals

In the picture below, there are sixteen square cages arranged in 4 rows of 4. Each cage contains 1, 2, 3, or 4 captured animals, represented by red chips. Your task is to free all of the animals, but there are rules that have to be followed: you must release two animals at a time, but they have to be in different cages, and the two cages have to have a common side.