Use of Vocalizations to Establish Species Limits in Antbirds (Passeriformes: Thamnophilidae)

Abstract

We introduce an empirically derived methodology for the definition, measurement, and application of vocal characters in assessing species limits in the numerically important Neotropical family Thamnophilidae. On the basis of available evidence, we assume that vocalizations are innate and that all vocal characters have a role in maintenance of species integrity in thamnophilids. Vocalizations of eight syntopic pairs, the members of which resemble one another in both plumage and voice, were analyzed to identify diagnosable vocal characters. To be diagnosable, characters had to distinguish members of a pair unambiguously. Three of the eight pairs were diagnosable by three vocal characters, four pairs by four characters, and one pair by seven characters. Vocalizations differed most consistently by characters reflecting loudsong note structure, loudsong pace, and note structure of calls. In establishing species limits for allopatric antbird populations, we suggest that three vocal characters, the minimum number distinguishing the syntopic pairs, be used as a point of reference. Keeping this point of reference in mind, we recommend that multiple diagnosable vocal characters be present when vocalizations are a major factor in determining species limits in thamnophilid antbirds. To the extent that populations differ strongly in other characters (e.g. morphological, molecular, other behavioral), fewer vocal characters may suffice. The identification of multiple vocal characters as a point of reference, as opposed to individual characters (e.g. note shape) or vocalization types (e.g. the loudsong), allows for greater latitude in the diagnosis. A focus on the number of characters is appropriate given the possibility that the role and importance of vocalization types in species' repertoires may differ across groups of taxa. The methodology and results may also have application in the study of other avian groups in which vocalizations are innate, especially the suboscines.