Split Frederickena unduligera into two species

Proposal (431) to South American Classification Committee

Effect on South American checklist: This would split an existing species on our list (Frederickena unduligera) into two species, F. unduligera and F. fulva.

Background: The Undulated Antshrike, Frederickena unduligera (Pelzeln 1869) is a relatively poorly known, low-density species that is widely, but seemingly patchily distributed through much of Amazonia. It was considered monotypic until J. T. Zimmer (1944) described three new subspecies (fulva, diversa, pallida), primarily on the basis of female plumage characters. Zimmer’s arrangement has been followed by taxonomists ever since. A potential problem with the taxonomic status quo is that Zimmer used a small series of specimens in describing his subspecies, and these specimens were from widely scattered localities. Unfortunately, there are still relatively few specimens from the unduligera complex, and the patchy nature of its apparent distribution, combined with the uneven collecting effort that is typical of Amazonian fieldwork, means that it is difficult, with any certainty, to distinguish individual or clinal morphological variation from subspecific variation. This difficulty is compounded by the inherent difficulties in distinguishing between juvenal-plumaged male unduligera from adult females, thus confounding possible age/sex variation with geographic variation.

Parker et al (1991) were the first to note geographic variation in vocalizations within the unduligera group, and suggested this as a potential subject of investigation. Until recently however, the relatively few archived audio recordings of unduligera were almost entirely from e Ecuador and ne Peru and pertained to the recognized subspecies fulva. There simply was not enough vocal material from elsewhere in the species’ range to provide the basis for a vocal analysis.

Recently, several workers have made a concerted effort to obtain recordings of unduligera vocalizations from throughout the known range of the complex, and these recordings have served as the basis for a recently published vocal analysis by Isler et al (2009).

Analysis & New Information: Isler et al (2009) published an analysis of geographic differences in vocalizations within the F. unduligera complex, based upon 85 recordings broken down as follows: fulva (54 recordings, 17 localities), diversa (14 recordings, 8 localities), pallida (15 recordings, 3 localities), and nominate unduligera (2 recordings, 2 localities). Their quantitative analysis was confined to loudsongs because the sample size of recorded calls was considered adequate for only one of the four named taxa (fulva), and because many of the recordings of calls lacked data regarding the context in which different call types were given. Calls were still examined, and potentially important differences between populations were noted. Three different types of calls were identified for fulva (descending whistle, snarl, and a call that combined elements of each of the other two call types), and similar homologous call types were found in other populations, although observed differences suggested that, with larger samples, calls of diversa, pallida and unduligera might prove different not only from fulva, but also from one another.

Loudsongs of all populations consisted of clear, evenly spaced whistles that initially grew louder but otherwise were relatively constant in intensity. Note shape differed between fulva and the other taxa (upslurred in fulva versus rounded into an inverted “U” in the other three populations), and this difference in note shape translates into differences that are readily audible to the human ear. In addition, the pace of fulva loudsongs was significantly slower than that of the other three populations combined, and they maintained near constant frequency between the first half of the song and the second half, whereas loudsongs of the other populations rose in frequency. Diagnostic loudsong distinctions in note shape, overall pace of notes, and change in frequency between fulva and the other three populations provided three independent vocal characters, which is consistent with the extent of vocal differentiation documented by the Islers and colleagues for closely related sympatric species of thamnophilid antbirds. Based on this, Isler et al (2009) recommended that fulva be treated as a distinct species separate from the other taxa in the unduligera complex. The status, relative to nominate unduligera, of the subspecies pallida and diversa remains ambiguous, and pending collection of more specimens and vocal samples, is probably not resolvable. The vocal analysis suggested some subtle differences in loudsongs between diversa and pallida/unduligera, but the differences did not meet the authors’ criteria for diagnosability. Similarly, the study provided indications that all three populations might differ diagnosably in certain calls, but again, sample sizes were judged too small to reach any conclusions. Given this, Isler et al (2009) recommended retention of pallida and diversa as subspecies of unduligera.

Recommendation: Based upon my own field experience with the taxa involved, and as a junior author of Isler et al (2009), I naturally agree with the conclusions advocated in that paper, and strongly recommend recognizing F. fulva as a species distinct from the other members of the F. unduligera complex. Sample sizes of loudsongs were reasonably large (54 individuals of fulva versus 31 of the other three taxa combined), and had a decent amount of geographic spread (17 sites for fulva versus 13 sites for the other three taxa combined), especially given the secretive nature, general scarcity, and patchy distributions of the taxa involved. The extent of the statistically significant vocal differences between fulva and the other taxa in the complex was consistent with species-level vocal differences in several pairs of sympatric thamnophilid antbird species whose status as distinct species is unquestioned.

Isler et al (2009) recommended the English name of “Fulvous Antshrike” for fulva, with “Undulated Antshrike” retained for the other three populations. “Fulvous” in this case refers to the female plumage of fulva, which is a more saturated, reddish-brown (fulvous) and more heavily barred than females of the other three taxa in the complex. Given that: a) the primary morphological differences in this complex involve plumage distinctions between females; b) geographic boundaries of the various subspecies are incompletely known; and c) that there are no known ecological distinctions between populations that would readily lend themselves to English names; naming the new species after the plumage of the female seems most logical (And, in this case, it squares with the Latin species epithet.). Given that heterogynism is characteristic of many species complexes within the thamnophilid antbirds, naming species after their distinctive female characters (as opposed to more subtly differing male characters) seems most appropriate.

References:

Isler, M. L., Isler, P. R., Whitney, B. M., Zimmer, K. J., and Whittaker, A. 2009. Species limits in antbirds (Aves: Passeriformes: Thamnophilidae): an evaluation of Frederickena unduligera (Undulated Antshrike) based on vocalizations. Zootaxa 2305: 61-68.

Parker, T.A. III, Castillo V., A., Gell-Mann, M. & Rocha O., O. (1991) Records of new and unusual birds from northern Bolivia. Bulletin of the British Ornithologists’ Club 111: 120–138.

Zimmer, J.T. (1944) Studies of Peruvian birds. XLIX. Notes on Frederickena and Octhoeca. American Museum Novitates, 1263, 1–5.

Kevin J. Zimmer, April 2010

Comments from Nores: “YES, las diferencias en vocalizaciones son lo suficientemente importante como para ser consideradas dos especies distintas. También hay algunas diferencias en coloración. A pesar de esto, resulta notable que en el HBW la única subespecie que no fue incluida en los dibujos fue fulva.”

Comments from Robbins: “YES. Primary vocalizations coupled with distinct female plumage differentiation supports treating fulva as a species.”

Comments from Stotz: “YES. Seems straightforward based on vocal differences.”

Comments from Jaramillo: “YES. Vocal differences well established in paper, and female plumage (taxonomically more informative than male plumage in many species of this group) differences support the division into two species.”

Comments from Pacheco: “YES. O bom número e a cobertura geográfica das amostras vocais e os resultados após criteriosa análise de Isler et al. (incluindo notórios expertises da avifauna amazônica) conferem um bom suporte ao tratamento proposto.”