PATTERN, PROCESS, AND RIGOR MEET CLASSIFICATION

Auk, The, Apr 2005 by Remsen, J V Jr

THAT CLOSELY RELATED, sympatric bird species have different voices has been known since the earliest days of ornithology and was presumably known to early humans. Indigenous peoples routinely distinguish similar-looking species by vocal differences. On the other hand, age, sex, individual, and ecological differences in vocalizations are also known, in addition to regional dialects, which themselves may change through time. So, the critical question is: how different must vocalizations be to represent barriers to gene flow? In terms of speciation models, how divergent must vocalizations be to facilitate reproductive isolation?

To address this question, Isler et al. (1998) compared vocal characters between eight pairs of syntopic antbird (Thamnophilidae) species to search for objectively definable differences in vocalizations associated with an absence of gene flow. They studied congeners, presumed sister species in many cases, having both minimal plumage differences and superficially similar primary vocalizations. Their goal was to determine whether closely related taxa ranked as species by any criterion showed common patterns of vocal differences. They analyzed all available vocalizations in a species' repertoire and quantified spectrographic differences between homologous vocalizations in each species to generate an objective index of diagnosability for each character. From those comparisons, they determined that no single character was diagnosably different in all eight species pairs, but that within each pair, the members were diagnosable in at least three characters of the primary song ("loudsong") or calls. The broader goal of Isler et al. (1998) was to establish an objective framework for taxonomic ranking of allopatric antbird taxa. Their strategy was to take their findings from known "good" species and apply them in a comparative framework to the thorniest problem of the Biological Species Concept, the ranking of allopatric taxa (Mayr 1942). This methodology allows an objective, reproducible classification of allopatric thamnophilid taxa (e.g. Krabbe et al. 1999; Isler et al. 2001a, b, 2002; Isler and Isler 2003; Whitney et al. 2000).

A severe test of their system is the appropriately christened Variable Antshrike (Thamnophilus caerulescens), which shows extreme geographic variation in color, with a dramatic change in male plumage from mostly black to gray-and-white within 400 km in Bolivia. Although Cory and Hellmayr (1924) considered the Bolivian populations to represent three distinct species, subsequent classifications from Peters (1945) to Zimmer and Isler (2003) have treated the entire complex, from Peru to Argentina, as a series of subspecies-level taxa constituting one biological species because of the step-clinal nature of plumage variation; the nature of contact between adjacent subspecies, however, has never been studied. Because the taxa are not allopatric, but parapatric, and levels of gene flow can be measured and compared with vocal diagnosability, the Variable Antshrike provides a test of the Isler-Whitney classification scheme.

In this issue of The Auk, Robb Brumfield and Mort and Phyllis Isler (Brumfield 2005, Isler et al. 2005) present their analyses of genetics and vocalizations of the Variable Antshrike across the steep geographic gradient of plumage types in Bolivia. They analyzed recordings from 154 individuals and sampled mitochondrial DNA (mtDNA) from 126 individuals within that gradient, a remarkably rigorous sampling regime. Their composite findings represent the most detailed assessment of fine-scale geographic variation in a tropical bird, perhaps surpassed only by work on the White-bearded Manakin (Manacus manacus) complex (e.g. Parsons et al. 1993, Brumfield et al. 2001, McDonald et al. 2001). Briefly, vocalizations and mtDNA show concordant patterns of transition between parapatric taxa, and those patterns are, in turn, concordant with plumage patterns. In other words, each plumage-based subspecies of T. caerulescens differs slightly in its genetic and vocal characters; nonetheless, a comparison of the geographic extremes reveals no species-level differences in vocalizations, as defined by the criteria of Isler et al. (1998).

Isler et al. (2005) have also shown, for the first time, that vocal characters can vary clinally in a suboscine passerine and that this variation parallels a step-cline in plumage characters. Even within a subspecies, vocalizations vary between sample points in a manner consistent with points along a cline. That finding provokes questions concerning the pattern and mechanism of inheritance of vocal characters in suboscines, especially as to whether songs are inherited as a single character or as a series of separate characters each with its own genetic basis. From the practical standpoint of assessing species limits in suboscines by comparing vocalizations, the message is clear. Broad geographic sampling is required to assess questions of species limits, and, as with other kinds of geographically variable characters, it is not acceptable to compare vocalizations from distant localities and then assume that they represent a taxon as a whole, much less the intervening localities.