Vocal learning in Savannah sparrows: acoustic similarity to neighbours shapes song development and territorial aggression

Abstract

Vocal learning is a biologically rare adaptation that underpins both human language and the songs of songbirds. The adaptive value of vocal learning in birds is still poorly understood, but a growing body of literature suggests that vocal learning allows songbirds to gain a fitness advantage by adopting songs that are structurally similar to the songs of individuals in neighbouring breeding territories. In this study, we investigate patterns of song development, acoustic similarity, and territorial aggression in Savannah sparrows, Passerculus sandwichensis. Four years of field data reveal that Savannah sparrows routinely overproduce songs during development; more than half of young males sang more than one song type early in their first breeding season, before their repertoire underwent attrition to a single song that males maintained throughout the remainder of their lives. We also found that the attrition of song types is a selective process, with males retaining songs that are similar to the songs of their territorial neighbours. Males that sang songs that were more similar to their neighbours may have faced lower levels of territorial aggression, as indicated by lower numbers of aggressive calls. Our results provide support for the hypothesis that vocal learning in songbirds allows animals to produce songs that match territorial neighbours, possibly providing a benefit in terms of decreased aggression during territorial defence.