Chestnut-sided warblers use rare song types in extreme aggressive contexts

Songbirds of a number of species use song repertoires that contain some song types that are uttered only rarely, and are therefore unlikely to be heard by the typical listener. Singing patterns that hide song types are difficult to reconcile with putative repertoire functions that require listeners to evaluate the size or diversity of singers' repertoires. Perhaps in species with repertoires that include rarely used song types, the rare types serve a function that is required only rarely. Based on observations of chestnut-sided warblers, Setophaga pensylvanica, I hypothesized that in this species, rarely used song types are reserved for use in extreme aggressive contexts. I experimentally simulated such a context (a prolonged territorial intrusion by a male competitor) by presenting song playback and a taxidermic mount on chestnut-sided warbler territories. The targeted males were much more likely to use their rare song types during the simulated intrusions than during natural baseline singing. A similar but smaller effect was evident in an analysis of natural singing that compared songs associated with fighting and chasing with songs that were not. Use of rare song types by experimental subjects did not predict their likelihood of attacking the simulated intruder. If this lack of predictive value is not an experimental artefact and extends to natural interactions, it suggests that despite their association with highly aggressive contexts, rarely used song types might not represent the strongest possible signal of aggressive motivation in chestnut-sided warblers. Perhaps use of rare types instead represents the penultimate level in a graded series of aggressive signals that culminates with a signal variant that went undetected in this experiment.