When a male field sparrow sings from the top of a bush in a meadow, it is telling other male field sparrows to stay out of that part of the meadow. Any area that one bird defends against other birds of the same species is called its territory.
Most songbirds are territorial during the breeding season.
In the early spring male birds establish and defend an area large enough to raise a family. The male bird advertises its ownership of the territory by singing from different singing posts throughout the territory. If another male bird of the same species enters this territory, the resident bird attempts to chase the intruder away. This defence takes the form of singing, displaying, chasing, and if all else fails, fighting. The purpose of defending the territory is to attract a suitable mate.