Suppose that there were lots of pigeons in that park, and a few sparrows, too. Let's imagine that one day it appeared to you that there seemed to be fewer pigeons than there used to be. Eventually, as you continued watching, you realized that you hardly ever saw any pigeons any more, and that most of the birds in the park were sparrows. Some more time went by, and the pigeons began to reappear in the park. Being a naturally curious person, you decided to investigate to find out what happened.
Pigeons are big, tame, and slow.
Sparrows are small, easily frightened, and fly away quickly. The boy was able to shoot the pigeons, but seldom hit the sparrows. Eventually he got tired of shooting birds. He got a real gun, joined a gang, and was killed in a drive-by shooting. With the absence of the predator, the pigeons gradually returned to the park.