Syntactic rules predict song type matching in a songbird

Song type matching has been hypothesized to be a graded signal of aggression; however, it is often the case that variation in matching behavior is unrelated to variation in aggressiveness. An alternative view is that whether an individual matches a song is determined mainly by syntactic rules governing how songs are sequenced. In song sparrows (Melospiza melodia), two such rules are the cycling rule, which directs that a bird cycles through its song types in close to the minimum number of bouts, and the bout length rule, which directs that a long bout of a song type is followed by a long interval before that song type is sung again. The effect of these rules on matching is confirmed here for a population of eastern song sparrows. Territorial males were challenged at the end of a recording session with playback of one of their own song types. Logistic regression showed that the probability of matching the playback song type increased with the length of the interval since the subject had last sung that song type, as predicted by the cycling rule. The probability of matching decreased as prior bout length increased, as predicted by the bout length rule. In a multivariate logistic regression, interval length and prior bout length were both associated with matching and together correctly predicted matching in 81.3% of cases. The results support the syntactic constraints hypothesis, which proposes that matching is a non-signaling by-product of internal rules governing the ordering of song type sequences. Significance statementVocal matching has attracted widespread interest in large part because it seems an effective method of directing an aggressive message at a particular recipient. Here, we show that in an eastern population of song sparrows, decisions on whether to match another bird are largely determined by internal rules of syntax governing how a singer sequences its song types, rather than by variation in aggressiveness or other individual traits. These results support the view that vocal matching is an incidental byproduct of internal mechanisms controlling the ordering of vocalization types and so is not a signal at all. This hypothesis may be broadly applicable to vocal matching in other species.