In the spring the forest is filled with the singing of birds, but in the summer the singing is not heard. Why?

In the spring, most birds begin the mating season, accompanied by the attraction of individuals of the opposite sex, the construction of nests, and the protection of the feeding area. This is expressed in the singing of birds or other forms of sound signaling between them, fights between males, tournament fights. In the summer, singing is not heard in the forest, as the birds are busy feeding the chicks, obtaining food for them, and training the young.