The Auk 122(2):433^44, 2005 © The American Ornithologists' Union, 2005. Printed in USA.
CLINAL VARIATION IN VOCALIZATIONS OF AN ANTBIRD (THAMNOPHILIDAE) AND IMPLICATIONS FOR DEFINING SPECIES LIMITS
MORTON L. ISLER/'' PHYLLIS R. ISLER/ AND ROBB T. BRUMFIELD^'^ ^Department of Zoology•Birds, National Museum of Natural History, Smithsonian Institution, P.O. Box 37012, Washington, D.C. 20013, USA; ^Department of Biology, Box 351800, University of Washington, Seattle, Washington 98195, USA; and ^Museum of Natural Science, 119 Foster Hall, Louisiana State University, Baton Rouge, Louisiana 70803, USA
ABSTRACT. •In avian taxa in which vocalizations are considered innate, such as suboscine passerines, vocal characters are increasingly being used to help determine whether populations have achieved species status. In comparing vocal characteris- tics of distant populations, however, one must be concerned with the possibility of character gradation through intermediate populations. The first quantitative study of a species in a suboscine family to test for clinal vocal variation, our vocal study found clinal variation in the pace (number of notes per second) of male loudsongs, and revealed that the geographic pattern of the clines was consistent with genetic variation found in the companion molecular study (Brumfield 2005). The result underscores the necessity of searching for intermediacy when analyzing vocaliza- tions of geographically distant populations. Furthermore, given that male loudsong pace was the only vocal character that varied across the intergrading populations, the result also provides support to the guideline that one should expect thamnophi- lid species to differ in at least three vocal characters (Isler et al. 1998) and indicates that this degree of vocal character differences can be a valuable "yard stick" in deter- mining which thamnophilid populations have achieved biological species status. Received 1 September 2003, accepted 21 December 2004.
Key words: clinal variation, hybrid zone, speciation, systematics, Thamnophilidae, Thamnophilus caerulescens, Variable Antshrike, vocalizations.
Variación Clinal en las Vocalizaciones de un Hormiguero (Thamnophilidae) e Implicaciones para Definir los Límites entre Especies
RESUMEN. • En las aves en que las vocalizaciones se consideran innatas, como en los Passeriformes suboscinos, los caracteres vocales han sido empleados cada vez más frecuentemente para ayudar a determinar si distintas poblaciones han alcanzado el estatus de especies diferentes. Sin embargo, al comparar las caracterísiticas vocales de poblaciones distantes, es importante prestar atención a la posibilidad de que exista variación gradual en los caracteres a través de poblaciones intermedias. Este estudio es el primer trabajo cuantitativo que pone a prueba la posibilidad de que exista variación clinal en vocalizaciones en una familia de suboscinos. Nuestro análisis de vocalizaciones indicó que existe variación clinal en la cadencia (número de notas por segundo) de los cantos fuertes de los machos, y que el patrón geográfico de las dinas concuerda con la variación genética documentada en un estudio molecular conjunto (Brumfield 2005). Los resultados recalcan la necesidad de buscar rasgos intermedios al analizar las vocalizaciones de poblaciones geográficamente distantes.
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433 434 ISLER, ISLER, AND BRUMFIELD [Auk, Vol. 122
Más aún, considerando que la cadencia de los cantos fuertes de los machos fue el único carácter que varió a través de las poblaciones, el resultado también apoya el lincamiento de que debería esperarse que las especies de Thamnophilidae difieran en por lo menos tres caracteres vocales (Isler et al. 1998), e indica que este grado de diferenciación en vocalizaciones puede ser una "vara medidora" valiosa para determinar cuáles poblaciones de Thamnophilidae han alcanzado el estatus de especies biológicas.
INCREASINGLY, VOCAL CHARACTERS are con- subspecies recognized by Peters (1951). A com- sidered important parameters in species-level prehensive review of the status of those taxa taxonomic analysis of suboscines (Johnson et al. using modern analytic tools has yet to be done, 1999, Heibig et al. 2002) and are being used as but some taxa appear to intergrade morpho- such (e.g. Isler et al. 1999, 2001, 2002; Whitney logically. The plumage of the form found on the et al. 2000; Alvarez and Whitney 2001; Zimmer southern slope of the arm of the Andes in central 2002). Vocalizations of suboscines are consid- Bolivia that extends east to the city of Santa ered innate and reflective of genetic structure, Cruz, known as T. c. connectens, is intermediate though at least one species provides an excep- between those of T. c. aspersiventer of the north tion (Baptista and Kroodsma 2001). An earlier slope of that arm of the Andes and T. c. dinellii paper developed guidelines for employing vocal of southern Bolivia and northwestern Argentina characters in taxonomic studies of thamnophi- (Ridgely and Tudor 1994). To the east of the lid antbirds (Isler et al. 1998). However, when Andes in the Bolivian and Paraguayan chaco, a analyzing vocal data in that context, one must pale population is found that has been described ask whether the data were obtained from popu- as T. c. paraguai/ensis. lations known to be geographically isolated and Those populations have been the subject thus currently incapable of interbreeding and, of a study aimed at understanding patterns if not, whether vocal characters vary clinally of mitochondrial variation (Brumfield 2005). through intermediate populations. Clinal varia- Concurrently, a parallel analysis was undertaken tion has been found in thamnophilid plumage of Variable Antshrike vocalizations in the same coloration (e.g. Thamnophilus atrinucha; Isler et region, the results of which are reported here. al. 1997). Contemporary species concepts usu- In conjunction with the results of the molecular ally interpret broad geographic character clines study, the vocal study has sought answers to the as providing evidence of genetic integration following questions. To what extent do vocal- and regard populations along a dine unworthy izations of populations described as T. c. asper- of taxonomic recognition as separately evolving siventer, T. c. connectens, T. c. dinellii, and T. c. lineages (Haffer 1998). In contrast, clines that paraguayensis differ? Is there evidence of broad are narrow as compared with the distributions or narrow clines in vocal characters? How well of the interconnected populations may reflect do differences in vocal characters coincide with selection against hybridization (i.e. reproduc- genetic differences? What implications do those tive isolation), and the populations on either findings have for the use of vocal characters in side of the steep cline are often treated as bio- species-level systematic studies? logical species (Mayr 1996). The geographic range of the Variable METHODS Antshrike (T. caerulescens) extends in a wide arc from the Andes of north-central Peru south- We compared the vocalizations of five popu- ward through the Andes to central Bolivia and lations in central Bolivia and extreme northwest- northern Argentina, where it spreads eastward ern Argentina (Fig. 1) defined geographically on through the lowlands of Bolivia, Paraguay, the basis of current subspecies ranges, distribu- and Argentina to Uruguay and southeastern tion of available vocal recordings, and prelimi- Brazil and thence north to northeastern Brazil nary results of the molecular study (Brumfield (Zimmer and Isler 2003). The English common 2005). Those populations were designated as T. name reflects the substantial differences in c. aspersiventer (A), corresponding to population plumage coloration among many of the twelve sample Al in the genetic analysis; T. c. connectens April 2005] Clinal Vocal Variation in an Antbird 435
FIG. 1. Identification of populations, locations of vocal recordings, and references to locations employed in a parallel genetic study (Brumfield 2005). Location symbols represent small sectors (Isler 1997) to which individual sites are assigned. Solid circles = locations of recorded male loudsongs. Heavy lines connect locations included in the same population. Large letters identify populations as follows: T. c. aspersiventer (A), T. c. connectens (C), T. c. dinellii North (DN), T. c. dinellii South (DS), and T. c. paraguayensis (P). Open circles = locations of recordings of calls, but no loudsongs. Crosses = locations from which specimens and tissue were collected and analyzed but from which no vocal recordings were available. Smaller letter and number combinations identify locations of speci- mens examined in the genetic study. Locations between defined populations mentioned in text: Gutiérrez, Santa Cruz, Bolivia (D3), and Chuquisaca and northern Tarija, Bolivia (D4).
(C), corresponding to population samples Cl Two of those are included in the genetic analysis and C2 in the genetic analysis; T. c. dinellii North as population samples D3 and D4. (DN), corresponding to population samples The northernmost population, T. c. aspersiven- Dl and D2 in the genetic analysis; T. c. dinellii ter, occurs in La Paz and Cochabamba, Bolivia, South (DS) (no genetic samples were available on the northern flank of the Andean spur that from those populations); and T. c. paraguayensis extends eastward to the city of Santa Cruz (Santa (P), corresponding to population samples PI Cruz de la Sierra); males are mostly black with and P2 in the genetic analysis. Additional sites, white scalloped underparts; and its habitat is described as "between defined populations," the most humid of the populations studied. had an insufficient number of recordings for On the opposite, southern, side of this Andean analysis but will be referred to in the Discussion. spur, in the department of Santa Cruz, Bolivia, 436 ISLER, ISLER, AND BRUMFIELD [Auk, Vol. 122 recording locations for T. c. connectens are cen- of notes delivered in a consistent pattern and tered on Samaipata, Bolivia, the type locality, and were identified as to sex of the vocalizing indi- extend eastward toward the city of Santa Cruz; vidual whenever possible. We used CANARY plumages are most often intermediate between 1.2.4 (Bioacoustics Research Program, Cornell populations to the north and south, with males Laboratory of Ornithology, Ithaca, New York) showing patches of black feathers and whitish to make a spectrogram of every vocalization underparts; the habitat is drier than on the north type delivered by each individual of either sex slope, but not as dry as to the west and south. on every recording. All spectrograms were used Within the range of T. c. dinellii, two popula- initially in visual analyses. In a visual analysis, tions were defined because available record- we considered a character to be diagnostic only ings fell geographically into two clusters, with when examination of the character could distin- an inadequate number of recordings from the guish every spectrogram of one population from region between them: T. c. dinellii North and those of another with certainty. T. c. dinellii South. Both populations have been After examining spectrograms of all vocaliza- identified previously as T. c. dinellii on the basis tion types and recordings for diagnostic visual of plumage; males are gray with ochraceous differences, we selected a sample of male loud- posterior underparts. Note that the range of T. songs to obtain quantitative measures of other c. dinellii North extends to the south slope of the vocal characters (e.g. pace) that appeared, from Andean spur west of the range of T. c. connec- spectrograms, to vary geographically. The analy- tens in the vicinity of El Tambo (Tambo), Bolivia sis was limited to loudsongs of males to eliminate (location D2 on Fig. 1; habitat described in possible sexual differences. A parallel analysis of Schmitt et al. 1997). To the east, T. c. paraguayen- female loudsongs was impractical, because too sis occupies dry scrub (chaco; habitat described few loudsong recordings identifiable as repre- in Kratter et al. 1993); its plumage is the whitest senting females were available from most popula- of any of the populations studied. In most cases, tions. Spectrograms were projected on the screen tape recordings of vocalizations were obtained of a Macintosh G4 computer using the default at the sites of tissue collection. Recordings were settings in CANARY, except that the display was inadequate for vocal analysis at two tissue col- set to smooth, overlap was adjusted from 50% to lection sites (D3 and D4), and the precise loca- 93.75% depending on recording quality, and con- tions of two other sites (PI and P2) were slightly trast was adjusted according to recording inten- west of the recording locations. As noted above, sity, with care taken to retain all elements of the tissue was unavailable for T. c. dinellii South. vocalization. Cursor measurements were made We obtained vocalization recordings for each by P.R.I., typically at scales of 0.3 s per inch and of the populations from our own inventory, from 4.0 kHz per inch. We measured three loudsongs contributions from other individuals, and from on each recording for five to seven individuals of archives of the Macaulay Library of Natural each population. Frequency measurements refer Sounds (Cornell University, Ithaca, New York) to the highest frequencies of notes. and the National Sound Archive (The British As described in more detail in earlier papers Library, London). The assemblage included a (Isler et al. 1998,1999), to be considered diagnos- number of recordings made by R.T.B. of indi- tic, differences must be unambiguously distinct viduals that were collected subsequently to character states or, in the case of continuous vari- obtain tissue employed in the molecular analy- ables, ranges could not overlap and the means sis. We examined 154 recordings. The Appendix (x) and standard deviations (SD) of the popula- provides sample sizes and a list of recordings tion with the smaller set of measurements (a) and employed in the study with locality names, the population with the larger set of measure- recordists, and archival identification. ments (b) had to meet the requirement: We reviewed each recording to identify the number and sex of individuals vocalizing X +fSD A 1 1 1 1 0.5 1.0 1.5 2.0 2.5 3.0 iik.», Hiti W* ^Ä» ^» ^^ ^^ ^fc ^ii 0.5 1.0 1.5 2.0 2.5 3.0 3.0 1 D 3 1- ^is*.^ W 0 r III 0.5 1.0 1.5 2.0 2.5 3.0 0.5 1.0 1.5 2.0 2.5 3.0 Time (seconds) FIG. 2. Loudsongs of male T. caerulescens in Bolivia and northwestern Argentina. Spectrograms were selected to reflect means of diagnostic measures (Table 1) for populations. The following annotations include population, location, recordist, and archive reference. See Appendix for expla- nations of archive references. (A) T. c. aspersiventer; San Onofre, Cochabamba, Bolivia (Brumfield ISL-RTB.2:21). (B) T. c. connectens; Bermejo, Santa Cruz, Bolivia (Brumfield ISL-RTB.2:08). (C) T. c. dinellii North; El Tambo, Santa Cruz, Bolivia (Parker MLNS 33662). (D) T. c. dinellii South; Palomitas, Salta, Argentina (Pearman ISL-MISC.7:31). (E) T. c. paraguayensis; Perforación, Santa Cruz, Bolivia (Parker MLNS 82692). 438 ISLER, ISLER, AND BRUMFIELD [Auk, Vol. 122 4^ D 2- ^^mgÊÊÊummftm ñ ¡-K 0.5 1.0 1.5 2.0 2.5 3.0 Time (seconds) FIG. 3. Calls of T. caerulescens in Bolivia and northwestern Argentina. The following annotations include name of call, location, recordist, and archive reference. See Appendix for explanations of archive references. (A) Short caw; San Lorenzo, Santa Cruz, Bolivia (Brumfield ISL-RTB.4:03). (B) Long caw; Bermejo, Santa Cruz, Bolivia (Brumfield ISL-RTB.2:02). (C) Chuck calls of two males counter-calling; San Lorenzo, Santa Cruz, Bolivia (Brumfield ISL-RTB.3:09). (D) Whine; 10 km east Gutiérrez, Santa Cruz, Bolivia (Brumfield 1SL-RTB.5:02). (E) Growl; San Lorenzo, Santa Cruz, Bolivia (Brumfield ISL-RTB.3:21). Finally, we assessed the taxonomic status among populations. Individual notes typically of each consolidated population, employing descended slightly in frequency before drop- guidelines developed in previous papers. In ping more sharply at the end of the note; and summary, based on a study of vocalizations though there was variability in slope and in of closely related, syntopic antbirds and sub- details of note shape at the beginning of the sequent applications (Isler et al. 1998, 1999, note, those differences varied individually etc.), we would recommend species status for and not geographically. When compared with allopatric populations under the biological spe- one another within a loudsong, note shapes cies concept (BSC) if they differ diagnostically remained constant, frequencies of note peaks in both vocal and nonvocal (morphological or essentially remained at the same level, and a molecular) characters. Three independent vocal slight acceleration of pace was apparent. characters are employed as a point of reference Comparisons of 43 quantitative measure- for using vocalizations in this context, though ments of male loudsong samples supported not as a requirement; and for populations that the paucity of differences found in the visual are highly differentiated in nonvocal charac- analysis. Across five populations, ranges of val- ters, fewer vocal characters may be acceptable. ues overlapped in all but 39 of the 430 pairwise Morphological analysis was not undertaken in comparisons. The 39 comparisons in which the present study, and current subspecies defi- ranges did not overlap were confined to 12 mea- nitions were accepted as representing plumage sures: number of notes (1), duration (2), overall differences. Results of molecular analysis are pace (3), length of five selected notes (4-8), and described in a companion paper (Brumfield length of four selected intervals between notes 2005). We recommend that taxa that do not (9-12). When statistical tests of significance meet those requirements but that are distin- were applied subsequently, the number of pair- guished by at least one diagnostic vocal charac- wise comparisons found to be diagnostic was ter be maintained as subspecies under the BSC, reduced from 39 to 9 (Table 1), and the number though such taxa may be recognized as species of measures that expressed diagnosable differ- under alternate approaches, such as the phylo- ences was reduced from 12 to 6: overall pace genetic species concept. (1), four interval measures (2-5), and length of initial note (6). Although note length has been RESULTS considered an aspect of note shape in situations where note shapes differed visually, we consid- Loudsongs.•Male loudsongs of all five ered all those significant measures reflective of populations were extremely similar in all a single vocal character•overall pace of the visual aspects, except in length and pace (Fig. loudsong. 2). Note shapes did not differ diagnostically All diagnostic differences in measurements April 2005] Clinal Vocal Variation in an Antbird 439 were between the most remote populations: m 'P fi 0) (0 between T. c. aspersiventer and the southern- •T3 most and easternmost populations, T. c. dinellii ë fi íhn 'U ns South and T. c. paraguayensis (Fig. 1). Ranges 11 c .^ < o en .^ o .V of measurements for geographically interme- (0 o (N in ON fO tN CO -M CTs +1 ^sS CO diate populations fell between the extremes O in ^ Tí -'T' K (N and did not differ significantly from those of 1 X T-H ^ 1> neighboring populations. Measurements of the o CO fi C fi interval between the second and third notes « (1) c (interval 2; Table 1) exemplify those relation- 0) 5 ships. Measurements of interval 2 differed K .2 01 en O .^ 00 in t% in significantly between T. c. aspersiventer and fN ON T•I o " do =0 ¿- >^ r¡, both T. c. dinellii South and T. c. paraguayensis. +1 +1 3 +1 LO +1 ^ +1 g (N 1 I I in I 00 I However, although values in our sample for T. ?3 _j 'S in CTs 00 -* tv i-l in (N t% tv ON c. aspersiventer did not overlap those of either T. C fO OJ > U M c. connectens or T. c. dinellii North, and values of fi -'-' T. c. paraguayensis did not overlap those of T. c. •- C connectens, pairwise comparisons did not pass s 45 u TS ^ our statistical test. The value of the test was i^ fi in in K hT ON 00 00 ÜÜ U) borne out when additional vocal recordings, +1 +1 o +1 in +1 gj en +1 rn ON 1 rx m 1 n\ not falling into our measurement sample, were 2 g in C7N K ^ ¿ 00 ci. s measured for that variable; extreme examples were found whose pace overlapped slightly in both pairwise comparisons. Ranges of measure- ments for interval 2 from T. c. connectens and T. c. il dinellii North overlap completely, and ranges for •-^ 0) •^ ^ 1^ fc iî? both overlap slightly with T. c. dinellii South. M-, ^j O "3 Ç>fC s'-' oc^ i~^ .. (N•- ^ ,, ON•- _j +1 T-^ +1 As exemplified by interval 2, a cline in pace of •I ^' t-H fi -^ K I 00 I IX I fO in ON •I 00 fN K CTs IX CO male loudsongs was found in the region between populations identified as T. c. aspersiventer and T. c. dinellii South (Fig. 4A). Most (66%) of that sr en - S ffl c transition occurs between populations A and oj Ë ' C. The lack of samples between populations A > Z. and C prevented us from assessing whether the u=l •£ m K IX in M-, bo - .^ o IX ^H O fi ; T•1 T•t o ^H transition represents a gradual change from one en S T•1 +1 +1 CTs ^' IX I 1 IX 1 "* I 00 I vocal type to the other, or a narrow cline that is fi ^ 0) 0) fiN Oil IX IX CTs ^ in O SP 00 >£> in ^ centered somewhere in the unsampled region TS .5 m (Fig. 1). The remaining transition occurs gradu- -rj ally; 19% between C and DN, and 15% between ^ S r, Q Ji Pi 0-) DN and DS . A similar cline in pace was found ro T5 +1 fi ^ in in on a west-east axis (Fig. 4B), between T. c. dinellii fi fi ns fi ON 00 in fO ^ -rj ns CO,;^" 180 160 A A 140 137^^^^ (fl 120 •c • À ^^^A S 100 o 98Jp • U) "~~~••-á^I-^ A 2 80 u • ^ 60 • 40 20 0 North (A) Hybrid (C) South (DN) Argentina (DS) 180 160 B 140 « 120 100 80 60 40 20 0 South (DN) Gutierrez (D3) East (P) FIG. 4. Intervals (microseconds) between second and third notes of T. caerulescens male loudsongs for six populations. (A) Populations arranged on a north-south transect. (B) Populations arranged on a west-east transect. Line connects means (diamonds) of values for populations. Symbols above and below line reflect maximum (triangle) and minimum (square) values for each population. North (A) = T. c. aspersiventer (n = 6); hybrid (C) = T. c. connectens {n = 5); south (DN) = T. c. dinellii North (n = 7); Argentina (DS) = T. c. dinellii South (n = 7); Gutierrez (D3) = Gutiérrez, Bolivia (n = 1); east (P) = T. c. paraguayensis {n = 5). Cfl//s.•The commonly delivered call was a spectrograms produced no diagnostic differ- deep (mostly 1.5-2.0 kHz with overtones), mod- ences in the caw call among populations. erately long (typically 0.2-0.3 s), mostly flat note, Other calls included a longer (0.5-0.6 s) typically described as a caw. The length of this "whine," similar but usually flatter than the caw; call varies within populations (Fig. 3). Samples a frequency-modulated but more downslurred of the call were plentiful from all populations, version of the caw that might be described except for T. c. dinellii South where it was found as a "growl"; and a short (0.05-0.10 s) "chuck in only one recording. Visual comparison of call" that took the shape of an inverted U. A April 2005] Clinal Vocal Variation in an Antbird 441 "softsong," recorded only for T. c. dinellii populations. Eight percent of individuals from North and in Gutiérrez, consisted of a short population D3 and 23% from population D4 (Fig. downslurred note, shaped like the note of the 1) had paraguayensis mitochondrial haplotypes. loudsong, repeated a variable number of times Although the nature of the contact between dinel- at the rate of 2.0-2.5 notes per second (not illus- lii and paraguayensis is still unknown, both the trated). All those secondary calls were recorded vocal and genetic data suggest that introgression rarely, and sample sizes were too small to pro- is influencing the patterns of variation in those vide a basis, if any exists, for identifying diag- characters. Unfortunately, genetic data from the nostic differences among populations. type locality of dinellii in northern Argentina are lacking but, extrapolating from the vocal and DISCUSSION genetic patterns, it seems possible that the popu- lation found at the type locality of dinellii is an This is the first analysis to demonstrate clinal- intergrade between populations of dinellii to the ity of a vocal character in contiguous populations north and paraguayensis. of a thamnophilid antbird. Lindell (1998) studied Turning to questions of current taxonomic sta- vocalizations of a species in another suboscine tus of the populations of T. caerulescens in Bolivia family (Furnariidae) at three locations in north- and northwestern Argentina, even if there were ern Venezuela; and though she found differences no geographically intermediate populations, between populations (which do not appear to the extent of vocal differences between the out- meet our more severe tests for accepting vocal liers would fall well short of vocal distinctions differences as diagnostic), the geographic pat- found between closely related, syntopic antbird tern did not suggest clinality. In the present species. Reproductively isolated thamnophilid study, six quantitative vocal measures differed species are typically distinguished by three diagnostically between the population on the or more independent vocal characters (Isler northeastern slope of the Bolivian Andes (T. c. et al. 1998, 1999). Because significant variation aspersiventer) and noncontiguous populations between outlying populations was found in in northwestern Argentina (T. c. dinellii) and the only one character, vocal differences would be Bolivian-Paraguayan chaco (T. c. paraguayen- insufficient to support the hypothesis that the sis), but those measures reflected a single vocal outlying populations are specifically distinct character•loudsong pace. Furthermore, inter- even without a finding of clinality. mediate values were exhibited by intervening Clinality was found, however, and no vocal populations. Thus, the pace of notes, though an characters distinguished adjacent populations. important character in distinguishing loudsongs Thus, from a purely vocal perspective, the of syntopic species-pairs (Isler et al. 1998), can named taxa that occur in Bolivia and northwest vary clinally, even though antbird loudsongs Argentina (T. c. aspersiventer, T. c. connectens, T. c. are almost certainly innate. It remains to be seen paraguayensis, and T. c. dinellii) could be merged whether other antbird vocal characters are sub- into a single taxon. The name T. c. connectens ject to a similar pattern of geographic variation. should be eliminated, because individuals of The clinal variation in vocal characters is strik- this population are clearly intergrades (based on ingly concordant with the patterns of mitochon- genetic data and morphological features as well drial variation. Focusing on measures reflecting as vocal analysis), but to merge the remaining the change of pace from aspersiventer (A) through three populations into a single taxon obscures connectens (C) to south-slope dinellii foothill (DN) the fact that they may represent distinct evolu- populations, most (76-93%) of the transition in tionarily significant units, as suggested by mor- the six loudsong pace vocal characters (Table phology. The decision rests largely on the nature 1) occurs between populations A and C. That of the hybrid zones between them, and further is remarkably similar to the 83% transition in analysis of morphological and other characters mitochondrial haplotype frequency between is required. In the north, additional sampling those same populations. The gradual increase in is needed to characterize the transition from loudsong pace that explains the remaining clinal aspersiventer to dinellii, but the available genetic transition within the distribution of dinellii may and vocal evidence suggests that the transition be explained by a gradual increase in the amount may be restricted to a relatively narrow contact of introgression of paraguayensis into dinellii zone, with connectens representing one tail of 442 ISLER, ISLER, AND BRUMFIELD [Auk, Vol. 122 the zone. Thamnophilus c. aspersiventer should Sound Library of The British Library (R. Ranfft), continue to be maintained as a subspecies pend- for copies of vocal recordings in their care. D. ing further study of the contact zone when its Lane, N. Krabbe, S. Mayer, J. Mazar Barnett, status can be reassessed. To the south and east, M. Ñores, M. Pearman, T. Schulenberg, and intergradation between dinellii and paraguayen- B. Whitney responded quickly to our need for sis is also reflected in genetic as well as in vocal additional recordings from the region. Without data. Compared with that of aspersiventer-dinel- their assistance, the project's recording resources lii, however, the transition zone of dinellii and would have been much thinner. J. V. Remsen, Jr., paraguayensis appears to be broader geographi- and Z. Cheviron provided valuable comments cally, though its extent and configuration are on the manuscript. For logistical and permit poorly known. Given the absence of genetic assistance in Bolivia, R.T.B. thanks S. Davis, E. data from extreme southern Bolivia, northwest Guzman, O. Maillard, P. Rebolledo, and R. A. Argentina, and southwestern Paraguay, and the Vespa; for field-recording assistance, R.T.B. paucity of geographically fine-grained vocal thanks R. Faucett and C. Gordon. B. Schmidt and morphological information in that region, assisted in preparing Figure 4. We thank as well as the contrasts in plumage between D. A. Nelson, K. G. Smith, R. M. Zink, and two populations of dinellii and paraguayensis found anonymous reviewers for helpful comments in central Bolivia, the best treatment at this time on the manuscript. Fieldwork and recordings is to maintain dinellii as a subspecies. by R.T.B. were supported by National Science The companion analysis of genetic variation Foundation grant DBI-9974235. (Brumfield 2005) found geographic patterns of interrelationships among those populations simi- LITERATURE CITED lar to the patterns reported here and demonstrated that introgression is occurring or has occurred ALVAREZ, J. A., AND B. M. WHITNEY. 2001. A new recently among them despite substantial differ- Zimmerius tyrannulet (Aves: Tyrannidae) ences in plumage coloration. The concordance of from white sand forests of northern results between the two studies reinforces their Amazonian Peru. Wilson Bulletin 113:1-9. individual conclusions. Consistency of results BAPTISTA, L. F., AND D. E. KROODSMA. 2001. Avian also provides further evidence of the value of bioacoustics. Pages 11-52 in Handbook of vocal characters as an expression of genetic the Birds of the World, vol. 6: Mousebirds similarities and differences among thamnophilid and Hornbills (J. del Hoyo, A. Elliott, and antbird populations. More specifically, the earlier J. Sargatal, Eds.). Lynx Edicions, Barcelona, finding (Isler et al. 1998) that at least three inde- Spain. pendent vocal differences distinguished closely BRUMFIELD, R. T. 2005. Mitochondrial variation related, syntopic thamnophilid species is sup- in Bolivian populations of the Variable ported. We note that the critical assumption that Antshrike {Thamnophilus caerulescens). Auk those vocal differences evolved independently 122:414-432. remains untested. Although it is likely that vocal HAFFER, J. 1998. Species concepts and spe- differences serve as a mechanism for effecting cies limits in ornithology. Pages 11-24 in reproductive isolation of thamnophilid species Handbook of the Birds of the World, vol. (Seddon 2005), it remains to be seen whether 4: Sandgrouse to Cuckoos (J. del Hoyo, A. the observed gene flow between populations Elliott, and J. Sargatal, Eds.). Lynx Edicions, is a result of the lack of significant vocal differ- Barcelona, Spain. ences between them or, conversely, whether the HELBIG, A. J., A. G. KNOX, D. T. PARKIN, G. paucity of vocal differences is a reflection of a low SANGSTER, AND M. COLLINSON. 2002. Guide- level of genetic differentiation in the absence of lines for assigning species rank. Ibis 144: other isolating mechanisms. 518-525. ISLER, M. L. 1997. A sector-based ornithologi- ACKNOWLEDGMENTS cal geographic information system for the Neotropics. Pages 345-354 in Studies in We are grateful to the Macaulay Library Neotropical Ornithology Honoring Ted of Natural Sounds, Cornell Laboratory of Parker (J. V. Remsen, Jr., Ed.). Ornithological Ornithology (G. Budney), and the National Monographs, no. 48. April 2005] Clinal Vocal Variation in an Antbird 443 IsLER, M. L., J. ALVAREZ A., P. R. ISLER, T. VALQUI, furnariid, Synallaxis albescens. Wilson A. BEGAZO, AND B. M. WHITNEY. 2002. Bulletin 110:368-374. Rediscovery of a cryptic species and descrip- MAYR, E. 1996. What is a species, and what is tion of a new subspecies in ttie Myrmeciza not? Philosophy of Science 63:262-277. hemimelaena complex (Thamnophilidae) of PETERS, J. L. 1951. Check-list of Birds of the the Neotropics. Auk 119:362-378. World, vol. 7. Museum of Comparative IsLER, M. L., J. ALVAREZ A., R R. ISLER, AND B. M. Zoology, Cambridge, Massachusetts. WHITNEY. 2001. A new species of Percnostola RIDGELY, R. S., AND G. TUDOR. 1994. The Birds antbird (Passeriformes: Thamnophilidae) of South America, vol. 2: The Suboscine from Amazonian Peru, and an analysis of Passerines. University of Texas Press, Austin. species limits within Percnostola rufifrons. SCHMITT, C. C, D. C. SCHMITT, AND J. V. REMSEN, Wilson Bulletin 113:164-176. JR. 1997. Birds of the Tambo area, an arid IsLER, M. L., P. R. ISLER, AND B. M. WHITNEY. valley in the Bolivian Andes. Pages 701-716 1997. Biogeography and systematics of the in Studies in Neotropical Ornithology Thamnophilus punctatus (Thamnophilidae) Honoring Ted Parker (J. V. Remsen, Jr., Ed.). complex. Pages 355-381 in Studies in Ornithological Monographs, no. 48. Neotropical Ornithology Honoring Ted SEDDON, N. 2005. Ecological adaptation and Parker (J. V. Remsen, Jr., Ed.). Ornithological species recognition drives vocal evolution Monographs, no. 48. in Neotropical suboscine birds. Evolution ISLER, M. L., P. R. ISLER, AND B. M. WHITNEY. 59:200-215. 1998. Use of vocalizations to establish WHITNEY, B. M., J. F. PACHECO, D. R. C. BUZZETTI, species limits in antbirds (Passeriformes; AND R. PARRINI. 2000. Systematic revision and Thamnophilidae). Auk 115:577-590. biogeography of the Herpsilochmus pileatus ISLER, M. L., P. R. ISLER, AND B. M. WHITNEY. 1999. complex, with description of a new species Species limits in antbirds (Passeriformes: from northeastern Brazil. Auk 117:869-891. Thamnophilidae): The Myrmotherula surina- WILLIS, E. O. 1967. The Behavior of Bicolored mensis complex. Auk 116:83-96. Antbirds. University of California Press, JOHNSON, N. K., J. V. REMSEN, JR., AND C. CICERO. Berkeley. 1999. Resolution of the debate over species ZIMMER, K. J. 2002. Species limits in Olive-backed concepts in ornithology: A new comprehen- Foliage-Gleaners (Automolus: Furnariidae). sive biological species concept. Pages 1470- Wilson Bulletin 114:20-37. 1482 in Acta XXII Congressus Internationalis ZIMMER, K. J., AND M. L. ISLER. 2003. Family Ornithologici (N. J. Adams and R. H. Slotow, Thamnophilidae. Pages 448-681 in Handbook Eds.). BirdLife South Africa, Johannesburg. of the Birds of the World, vol. 8: Broadbills KRATTER, A. W., T. S. SILLETT, R. T. CHESSER, J. P. to Tapaculos (J. del Hoyo, A. Elliott, and D. O'NEILL, T. A. PARKER III, AND A. CASTILLO. Christie, Eds.). Lynx Editions, Barcelona, 1993. Avifauna of a chaco locality in Bolivia. Spain. Wilson Bulletin 105:114-141. LINDELL, C. 1998. Limited geographic varia- tion in the vocalizations of a Neotropical Associate Editor: R. M. Zink APPENDIX: INVENTORY OF VOCALIZATION Cornell Laboratory of Ornithology, Ithaca; RECORDINGS EXAMINED NSA = National Sound Archive, The British Library, London; ISL = recordings not yet For each population, recordings are listed archived in an institutional collection but that by country, state or department, and record- have been copied into the inventory maintained ing location. Recordist names aid in identifying by M. L. and P R. Isler. the recording as well as providing acknowl- Thamnophilus caerulescens aspersiventer. 19 edgments. Numbers following the recordist's recordings. BOLIVIA: La Paz: Caranavi 33-48 name identify the number of cuts per recordist km north of (Parker 3 MLNS); Sacramento per location. Acronym for recording archive: Alto (Rearman 3 NSA; Whitney 1 ISL); Rio MLNS = Macaulay Library of Natural Sounds, Tuichi (Whitney 2 ISL). Cochabamba: P N. 444 ISLER, ISLER, AND BRUMFIELD [Auk, Vol. 122 Carrasco (Whitney 1 ISL); Saila Pata (Krabbe 7 ISL); Pirirenda, Laguna (Brumfield 1 ISL). 2 ISL); San Onofre (Brumfield 5 ISL); Villa Chuquisaca: Bufete, Cerro (Schulenberg 2 Tunari (D. Willis 1 NSA); Villa Tunari- ISL); El Palmar (Mayer 1 ISL). Tarija: Entre Cochabamba Road (Whitney 1 ISL). Ríos (Brumfield 2 ISL; Parker 1 MLNS). Thamnophilus caerulescens connectens. 30 record- Thamnophilus caerulescens dinellii South. 17 ings. BOLIVIA: Santa Cruz: Bella Vista recordings. ARGENTINA: Chaco: Chaco, P (Brumfield 5 ISL); Bermejo (Brumfield 13 N. (Mazar Barnett 1 ISL). Jujuy: Aguas Negras ISL; Whitney 1 ISL); La Pajcha (Lane 4 ISL); (Krabbe 1 ISL); Calilegua, P N. (Mazar Barnett Samaipata (Lane 1 ISL, Mayer 5 ISL); Yungas 5 ISL; D. Willis 1 NSA). Precise location de Mairana (Parker 1 ISL). unknown: (Rearman 1 ISL); Salta: Joaquín V. Thamnophilus caerulescens dinellii North. 48 Gonzales (Rearman 1 ISL); Lipeo-Papachacra recordings. BOLIVIA: Santa Cruz: Comarapa, Trail (Rearman 1 ISL); Palomitas (Rearman 1 35 km west (Lane 2 ISL); El Tambo (Parker 1 ISL); Piquirenda (Rearman 1 ISL); Tartagal MLNS; Whitney 1 ISL); Masicuri (Mayer (Rearman 1 ISL). BOLIVIA: Tarija: Cambari 3 ISL); San Lorenzo (Brumfield 32 ISL); (Mayer 1 ISL); Pampa Grande (Mayer 2 ISL). Vallegrande (Mayer 4 ISL). Chuquisaca: Thamnophilus caerulescens paraguay ensis. 15 Sopachuy (Krabbe 5 ISL). recordings. BOLIVIA: Santa Cruz: Curuyuqui Between defined populations. 25 recordings. (Parker 2 ISL, 2 MLNS); Ibasiriri (Mayer 6 BOLIVIA: Santa Cruz: Gutiérrez, 10 km ISL); Perforación (Parker 5 ISL). east of: (Brumfield 11 ISL); Itai (Brumfield