depends on the birds and the time of year, birds sing for many different reasons, and have different bird noises some are warnings, of predatirs. some are mating calls.
this is my tree you fuck off, is not just what bird song is about it is not all territoria; , that is too simplistic.
"The distinction between songs and calls is based upon complexity, length, and context. Songs are longer and more complex and are associated with courtship and mating, while calls tend to serve such functions as alarms or keeping members of a flock in contact.[3] Other authorities such as Howell and Webb (1995) make the distinction based on function, so that short vocalizations, such as those of pigeons, and even non-vocal sounds, such as the drumming of woodpeckers and the "winnowing" of snipes' wings in display flight, are considered songs" quote. so it also depends on how you define" Bird song"