Use of vocalizations to establish species limits in antbirds (Passeriformes: Thamnophilidae)

-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. Received 16 September 1997, accepted 2 February 1998. THE CONSISTENT DEFINITION OF SPECIES is a necessary foundation for systematic, zoogeographic, ecological, and conservation research. The Neotropics are home to more species of birds than any other region and support the most diverse ecosystems on earth (see Stotz et al. 1996). Among Neotropical birds, recent studies (Robbins and Ridgely 1992, Willis 1992, Prum 1994, Whitney et al. 1995, Bierregaard et al. 1997, Isler et al. 1997, Krabbe and Schulenberg 1997, Zimmer 1997) exemplify a growing concern that many taxa presently considered subspecies are more appropriately recognized at the species level. Additionally, undescribed but diagnosable populations are known to exist (Fjeldsa and Krabbe 1990, Whitney 1994, Brumfield and Remsen 1996). One of the numerically most important families of Neotropical birds is the Thamnophilidae (sensu AOU 1997), repre3E-mail: piantbird@aol.com sented by 35 to 50 sympatric species in Amazonian forest localities. As with other Neotropical birds, their current ranking into species and subspecies was established by Hellmayr (Cory and Hellmayr 1924), Zimmer (1931 and subsequent papers), and more recent authors solely on assumptions of the significance of differences and similarities in plumage and external measurements. Furthermore, as Mayr (1982) noted, "every isolated 'species' was ... scrutinized for the possibility that it was simply a geographic representative of some other species, in which case it was reduced to the rank of subspecies." The resulting judgments were unavoidably inconsistent and sometimes highly arbitrary. Since these decisions were made, and especially over the past 20 years, knowledge of Neotropical birds has been augmented greatly. New specimens clarify our understanding of morphological variation and distribution. Molecular evidence regarding genetic variation