Division of Birds, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA On 18 November 1894, Frank B. Armstrong col- lected an unusual woodcreeper near Altamira, Ta- maulipas, Mexico. This specimen was sent with others for identification to the United States National Mu- seum during the winter of 1894-1895 (Richmond 1896), and was eventually described as a new species, Den- drornis [=Xiphorhynchus] striatigularis, based on its dis- tinct plumage (Richmond 1899; USNM 135157; 18 No- vember 1894, Alta Mira, Tamaulipas; female). Despite the fact that this region can now be con- sidered rather well collected, this taxon continues to be represented only by the unique type, suggesting that it is: (A) a very rare and now probably extinct (but valid) species; (B) the hybrid product of two valid species; or (C) an aberrant individual of a valid species (cf. Graves 1990). Xiphorhynchus striatigularis has re- ceived little attention since its description. Some au- thors simply list the taxon with little or no discussion (e.g. Ridgway 1911, Cory and Hellmayr 1925, Peters 1951, Blake 1953, Miller et al. 1957), while others either exclude it altogether (Edwards 1972, 1989, Pe- terson and Chalif 1973), or consider it an aberrant X. fiavigaster (AOU 1983, Sibley and Monroe 1990). No evidence has been presented to favor any of the above hypotheses or treatments. My examination attempts to resolve the nature of the unique type. Methods.--It is assumed that woodcreeper species occupying the region around Altamira (lower ele- vations of northeastern Mexico) will be morpholog- ically distinct and that, if valid, X. striatigularis would exhibit a size and shape different from that of same- sex individuals of other species in this area. This as- sumption would not be robust for all taxonomic groups or geographic areas. It seems robust here, however, because the center of species diversity in Xiphorhyn- chus occurs in South America (Sibley and Monroe 1990), and no cryptic species of Dendrocolaptidae are known in northern Middle America. Armstrong's handwriting is apparent on the original label of X. striatigularis, suggesting that the data there are correct (see Oberholser 1974:8). The hypothesis of hybrid origin does not make morphological predictions. Although it would seem likely, intermediacy in body form might not occur, even if the individual is an F hybrid (G. Graves pets. comm.). If X. striatigularis is an aberrant individual of a valid species and the aberrancy is restricted to pig- ment deposition in the plumage (the latter is the basis for this taxon; Richmond 1899), its external morpho- metrics should match one of the species occurring in the region. Diagnosis of this unique type is aided by the fact that southern Tamaulipas is near the northerly limits of the Dendrocolaptidae, and only two species (of two genera) currently occur in the region where the unique type was taken: X. fiavigaster and Lepidocolaptes affinis. A third species, L. souleyetii, reaches central Veracruz, and is ,ncluded in this study. Three other dendro- colaptids occur in northern Mexico, but were not in- cluded in this analysis. Xiphocolaptes promeropirhyn- chus and Sittasomus griseicapillus were excluded be- cause their sizes, ranges, and plumage characteristics preclude their involvement in the latter two hypoth- eses presented above. I excluded Xiphorhynchus ery- thropygius because of its range, its montane elevational preference in Mexico, and a lack of evidence in the plumage of X. striatigularis of a contribution from er- ythropygius. The unique type of X. striatigularis is in basic plum- age, and its distinct plumage characteristics are not TABIE 1. Univariate morphometric characteristics of female Xiphorhynchus striatigularis, X. fiavigaster, Lepi- docolaptes affinis, and L. souleyetti (œ ñ SD, with range in parentheses; measurements in mm). X. stria- X. fiavigaster L. affinis L. souleyetii tigularis (n = 46) (n = 26) (n = 30) Wing chord 104.5 107.6 ñ 3.49 (98.9-114.9) 106.0 ñ 2.59 (101.7-111.0) 92.5 ñ 2.66 (86.9-97.5) Tail length 80.5 85.9 ñ 5.95 (73.0-98.0) 91.0 ñ 3.86 (82.1-97.4) 78.5 ñ 3.66 (69.7-85.0) Tarsometatarsus length 22.6 21.6 ñ 0.55 (20.6-22.8) 19.5 ñ 0.58 (18.3-20.8) 18.3 ñ 0.64 (16.9-19.6) Bill length 30.1 30.8 ñ 1.42 (27.2-34.0) 22.7 ñ 1.00 (20.2-24.5) 22.2 ñ 0.89 (19.8-23.8) Bill height 8.0 7.7 ñ 0.54 (6.7-9.1) 5.3 ñ 0.33 (4.8-6.1) 5.4 ñ 0.28 (4.9-6.0) Bill width 6.4 6.2 ñ 0.81 (4.1-7.1) 4.4 ñ 0.60 (3.6-5.7) 4.9 ñ 0.74 (3.8-6.2) Length of primary 8 80.0 83.8 ñ 2.95 (77.0-92.0) 84.1 ñ 2.30 (79.0-88.0) 72.3 ñ 2.61 (67.0-77.5) Length of primary 9 75.0 79.5 ñ 2.98 (72.0-87.5) 80.5 ñ 2.44 (76.0-85.0) 69.6 ñ 2.38 (63.5-73.5) Length of primary 10 59.5 62.5 ñ 3.18 (50.5-69.0) 62.8 ñ 2.87 (55.0-67.5) 54.2 + 1.74 (50.0-58.5) caused by the retention of juvenal plumage charac- teristics; the juvenal plumage of woodcreepers tends to closely match the plumage of the adult (pers. obs.). For morphometric study, I examined a broad sam- pie of female specimens of X. fiavigaster, L. affinis, and L. souleyetii from the Atlantic slope of northern Mid- dle America. Postulations of sedentariness in Neo- tropical species can be invalid (see Winker et al. 1992: 714, Winker 1995). Also, in studies like this, it is useful to have a good sample of the morphological variation occurring within the taxa examined, and large spec- imen samples from restricted geographic areas in the Neotropics are rare. Measurements to the nearest 0.1 mm were taken of the lengths of wing chord, tail, tarsometatarsus, and bill, as well as of bill height and bill width, using vernier calipers (following Baldwin et al. 1931). Also, the lengths of primaries 8, 9, and 10 were measured to the nearest 0.5 mm (following Jenni and Winkler 1989). Specimens with missing values were not in- cluded. Patterns among the measured characters were examined using principal component analysis. The first two principal components were extracted from the correlation matrix of log-transformed mensural data (Tables 1 and 2). Unstandardized principal com- ponent scores were generated for each individual. Results and discussion.--Except for X. striatigularis, the taxa examined occupy discrete regions of mor- phological space (Fig. 1). Xiphorhynchus striatigularis is morphometrically inseparable from the common X. fiavigaster (Table 1, Fig. 1). Given the propensity for the species of Dendrocolaptidae in the lowlands of northeastern Mexico to be morphologically distinct, and under the assumptions upon which the study was based, X. striatigularis is not a valid species. Its taxo- nomic status, therefore, is either nomen hybridurn or nomen monstrositatum (Lincoln et al. 1982). It is more difficult to distinguish between the two remaining hypotheses of origin: hybridization or ab- errancy. The position of X. striatigularis in morpho- logical space (Fig. 1) does not approach either of the Lepidocolaptes species. It does, however, closely match other female X. fiavigaster. Morphometrically, the hy- pothesis of aberrancy is supported. The morphomet- ric data are neutral with respect to the hypothesis of hybrid origin (see Methods). However, hybrid origin seems highly unlikely from several perspectives. First, hybrids among the woodcreepers and their nearest suboscine relatives are decidedly rare; only one has been described (Graves 1992). Also, in this case, hy- bridization would most likely be intergeneric, a less common source of hybrids than congeneric crossings (Gray 1958). The geographically most probable second parent taxon for a hybrid woodcreeper at Altamira would be L. affinis, which occupies higher elevations than the other two species considered here (> 550 m; Miller et al. 1957). Armstrong did not obtain L. affinis at Alta- mira (Richmond 1896, Phillips 1911; extant specimen record), which is not surprising, given that it does not usually occur in the lowlands. Only one species TABLE 2. Eigenvectors and eigenvalues for first two principal components based on correlation matrix of log-transformed measurements of female wood- creepers of four taxa. Character PC 1 PC2 Wing chord 0.422 - 0.093 Tail length 0.282 -0.325 Tarsometatarsus length 0.384 0.225 Bill length 0.352 0.318 Bill height 0.324 0.371 Bill width 0.170 0.528 Length of primary 8 0.365 -0.319 Length of primary 9 0.230 -0.437 Length of primary 10 0.389 -0.156 Eigenvalue 5.097 2.744 Percent of total variance explained 56.6 30.5 Fig. 1. creepers. 4 Xiphorhynchus striatigularis .. Xiphorhynchus fiavigaster ' n .++ + + Lepidocolaptes souleyetii ++ . ;' Lepidocolaptes affinis -+ + + + + + -4 [ ] [ [ ' -6 -4 -2 0 2 4 6 PC1 Bivariate plot of individual scores on first two principal components of 101 adult female wood- of woodcreeper, X. fiavigaster, is known to occur at Altamira. Topographic maps show that the nearest locality with an elevation suitable for L. affinis occurs about 60 km from Altamira (in the Sierra de Tamau- lipas, where the species is known to occur; Miller et al. 1957). It is approximately 430 km from Altamira to the range of L. souleyetii (using ranges from Miller et al. 1957). Banding studies in southern Veracruz suggest that X. fiavigaster is sedentary there (unpubl. data). Morphometric studies of L. affinis and long-term observations in southern Veracruz (unpubl. data) sug- gest that this species is also sedentary in eastern Mex- ico. Given the inadequacy of morphometrics to resolve the two remaining hypotheses of origin, plumage ex- amination is useful. Unlike any other member of the family in northern Middle America, the throat of X. striatigularis shows heavy streaking (Fig. 2). These streaks are caused by feathers having a black margin around a buff center, and the nature of these feathers is also unusual, in that the black/buff interface is ragged, rather than smooth. This ragged interface, which characterizes streaked ventral feathers to the abdomen, is not found in other members of this ge- nus, nor in the two Lepidocolaptes species examined. It appears that genetic control of black pigment de- position on the venter went awry, causing aberrancy in how black pigment was deposited in ventral feath- ers, as well as an exaggeration in the occurrence of these feathers anteriorly. The density of pigment also appears greater than in other woodcreepers (X. fia- vigaster) taken at the same time and locality. Except for these characteristics, restricted largely to the an- terior venter, all aspects of the plumage of X. striati- gularis can be found represented in female X. fiavi- gaster from eastern Mexico (Fig. 2). Although the hypothesis of hybrid origin cannot be rejected, it is very improbable for several reasons: (1) rarity of hybrids in woodcreepers and their rela- tives (Graves 1992); (2) only one species of wood- creeper occurs at the type locality; and (3) the type, clearly different from other woodcreepers, neverthe- less shows only localized plumage differences that lack intermediacy between two possible parent taxa. The data are more consistent with origin through aberrancy. Unlike hybridization, genetic mutations occur regularly and, although odd melanistic plum- ages seem uncommon, aberrancies in feather pigment deposition are relatively frequent (e.g. partial albi- nism or leucism). Based upon this evidence, I con- clude that X. striatigularis is an aberrant X. fiavigaster. Markedly aberrant individuals are monstrosities, and a name based upon such an individual has the taxo- nomic standing of nomen monstrositatum (Lincoln et al. 1982). The day after Armstrong collected the unique X. striatigularis, he collected another bird in the same area that Wetmore (1942) later designated as the type of X. fiavigaster saltuarius (USNM 135161; Alta Mira, Ta- maulipas, 19 November 1894; male). Xiphorhynchus Fig. 2. Type of Xiphorhynchus striatigularis. striatigularis should henceforth be considered a syn- onym of X. fiavigaster saltuarius. 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