bird song is thought to have evolved through sexual selection. female songbirds often assess potential mates using song, based on qualities such as high song output, complexity and difficulty of songs, as well as presence of local dialect. song output serves as a fitness indicator of males, since vocalizations require both energy and time to produce, and thus males capable of producing high song output for long durations may have higher fitness than less vocal males. it is thought that song complexity may serve as an indicator of male fitness by providing an indication of successful brain development despite potential early-life stressors, such as lack of food. social transmission of songs allows for development of local dialects of song, and female songbirds also typically prefer to choose mates producing local song dialects. one hypothesis for this phenomenon is that selecting local mates allows the female to choose genes specially adapted to suit local conditions.