Songbirds have the capacity for species-specific recognition and individual recognition. The mechanisms of species-specific recognition generally involve recognition of invariable song features rather than variable ones. For example, though European robins produce hundreds of songs, they all follow the same syntactic rules. The robins’ songs must be composed of different phrases, phrases must alternate in pitch, and during bouts, all of the songs must be different. Experiments were performed where the speakers played songs using sounds that robins can’t produce but followed their song’s syntactic rules and they responded as if the speaker were another male. However, the robins didn’t respond to the speaker when the song was changed to include only low-pitched phrases. This supports the inference that the environment may degrade pitch and different individuals will use differently pitched notes, but that syntactic rules will remain the same, allowing for conspecific recognition. Another example of this invariability phenomenon is in the indigo bunting, which recognizes conspecifics by element composition. Indigo bunting song consists of a single element repeated quickly to produce a trill. As opposed to the robin where manipulating syntactic elements changes response rate, changing the element results in much lower conspecific response in the indigo bunting. Individual recognition is a more complicated story, and is very species specific. Colonial birds tend to have the ability to recognize individual calls, as is the case with bank swallows and emperor penguins. As these species live in colonies, parents have to be able to recognize the calls of their offspring in order to feed them. In the zebra finch, females seem to recognize their father’s calls, as they tend to choose mates which have similar but not identical songs, a behaviour that is likely to have arisen in order to avoid incest. Songbirds have the capacity to recognize individuals, and not only respond differently to neighbours and strangers but have different levels of response to different neighbours. A male will respond less strongly to a neighbour’s song from a familiar location than a stranger’s song from that same location. Should the neighbour’s song be played from an unfamiliar location, the male will respond just as strongly to it as it would a stranger’s song. This response seems explicable from a territoriality point of view. The male will respond more aggressively to new individuals who pose threats as opposed to neighbours with pre-established boundaries, and will respond aggressively to expansionary neighbours.
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