confer aggressive intent. For Eastern smooth frogs (Geocrinia victoriana), for example, courtship songs involve shorter notes to attract potential mates, and are followed by longer tones to repel males. Frequency of sounds produced generally negatively correlates with body size both within and among species, and allows competing males to assess body size of vocalizing neighbouring frogs. Male frogs typically approach higher frequency sounds more readily than lower frequencies, likely because the frog producing the sound is assessed to be a smaller, less dangerous competitor. In territorial birds, males increase song production rate when neighbouring males encroach on their territory. In great tits (Parus major), nightingales (Luscinia megarhynchos), blackbirds (Turdus merula) and sparrows (family Passeridae), playing song recordings slows the rate at which males establish territories in an unoccupied region, suggesting these birds rely on song output in establishing territorial boundaries. Experimentally muted Scott's seaside sparrow (Ammodramus maritimus) lose control of their territories to other males. Thus, territorial birds often rely on song production to repel conspecific males. Individual recognition Like the human voice, bird song typically contains sufficient individual variability to allow discrimination of individual vocal patterns by conspecifics. Such discrimination is important to mate recognition of many monogamous species. Seabirds, for example, often use vocalization patterns to recognize their mate upon reunion during the breeding season. In many colonial nesting birds, parent-offspring recognition is critical to allow parents to locate their own offspring upon return to nesting sites. Cliff swallows (Petrochelidon pyrrhonota) have been demonstrated to preferentially respond to parental songs at a young age, providing a means of vocalization-based offspring recognition. Social transmission and learning Learning and development of birdsong Learned vocalizations have been identified in groups including whales, elephants, seals, and primates, however the most well-established examples of learned singing is in birds. In many species, young birds learn songs from adult males of the same species, typically fathers. This was first demonstrated in chaffinches (Fringilla coelabs). Chaffinches raised in social isolation develop abnormal songs, however playing recordings of chaffinch songs allows the young birds to learn their species-specific songs. Song learning generally involves a sensitive learning period in early life, during which young birds must be exposed to song from tutor animals in order to develop normal singing as adults. Song learning occurs in two stages: the sensory phase and the sensorimotor phase. During the sensory phase, birds memorize the song of a tutor animal, forming