Do vocal mimics learn their mimetic songs from heterospecifics or conspecifics?

Vocal mimicry can develop either through imitation of heterospecific models (primary mimicry) or through imitation of conspecifics already producing mimetic song (secondary mimicry). Distinguishing primary versus secondary mimicry is important from ecological, evolutionary, and neurobiological perspectives. We outline four empirical strategies for researchers to detect the usage of secondary mimicry: (1) model selection strategy (heterospecific model commonly mimicked but rare or absent locally), (2) usage frequency strategy (syllable types commonly mimicked but rarely used by the model), (3) acoustic structure strategy (acoustic divergence in how a syllable type gets produced by mimics versus models), and (4) syntax strategy (consistent variation between mimic and model in the ordering of syllable types). We then use these strategies to produce evidence for secondary mimicry in northern mockingbirds (Mimus polyglottos), focusing mostly on the mimicked songs of northern cardinals (Cardinalis cardinalis) and tufted titmice (Baeolophus bicolor). As further evidence of the mechanisms by which secondary mimicry might occur, we also demonstrate that mockingbirds match countersing much more frequently in response to conspecifics versus heterospecifics. Deeper questions about the precise extent of primary versus secondary mimicry in mockingbirds and other mimicking species still need answers.