Diagnosis of a Hybrid Antbird {Phlegopsis Nigromaculata X Phlegopsis Erythroptera) and the Rarity of Hybridization Among Suboscines
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18 December 1992 PROC. BIOL. SOC. WASH. 105(4), 1992, pp. 834-840 DIAGNOSIS OF A HYBRID ANTBIRD {PHLEGOPSIS NIGROMACULATA X PHLEGOPSIS ERYTHROPTERA) AND THE RARITY OF HYBRIDIZATION AMONG SUBOSCINES Gary R. Graves Abstract. —Phlegopsis barringeri Meyer de Schauensee, 1951, is suggested to be a hybrid between Phlegopsis nigromaculata and P. erythroptera. This rep- resents the first case of hybridity in the large monophyletic assemblage of New World suboscines comprising the woodcreepers, antbirds, ovenbirds, and ta- paculos (parvorders Thamnophilida and Furnariida). The rarity of hybridiza- tion among suboscines (Suborder Tyranni) may result from the absence of song learning in this assemblage, resulting in vocal stereotypy within populations and the reduction of mate recognition error by females. Hybridization in birds occurs most fre- in zoological nomenclature, the burden of quently in sexually dimorphic species with proof rests upon a taxonomist to reject hy- polygynous breeding systems (Sibley 1957). bridity before conferring species status More than half of all known cases of hy- (Graves 1990). Meyer de Schauensee com- bridization in nature occur in four avian pared the specimen of P. barringeri with families or tribes: Anatidae (ducks, geese, three morphologically similar taxa but failed and swans), Phasianidae (grouse and pheas- to address hybridity. That possibility was ants), Trochilidae (hummingbirds), and raised by Willis (1979), who suggested par- Paradisaeini (birds of paradise) (Gray 1958) enthetically that P. barringeri was a hybrid (taxonomy of Sibley & Monroe 1990). The between the Black-spotted Bare-eye (P. ni- apparent absence or low frequency of hy- gromaculata) and Red-winged Bare-eye (P. bridization in other lineages may reflect re- erythroptera), which overlap broadly in ality or the failure of biologists to detect it western Amazonia from eastern Colombia (Graves 1990). Only a few cases of hybrid- (Hilty & Brown 1986) south to northern ization have been detected among New Bolivia (Parker & Remsen 1987). World suboscines (e.g., Parkes 1961, Short My analysis of P. barringeri confirmed & Burleigh 1965, Tyler & Parkes 1992), Willis' hunch. The proposed hybrid origin which constitute a quarter (n = 1097) of all of the specimen (P. nigromaculata x p. er- Western Hemisphere species. ythroptera) cannot be rejected. Here I pre- Meyer de Schauensee (1951) described a sent a diagnosis of the hybrid antbird, the relatively large and spectacular species of first known example of hybridity in the antbird, the Argus Bare-eye (Phlegopsis bar- parvorders Thamnophilida and Furnariida ringeri), from a unique specimen collected (Sibley & Monroe 1990), a monophyletic on the Rio Rumiyaco, Department of Na- assemblage representing 131 genera and 560 rino, Colombia. Determining whether a species of woodcreepers, antbirds, oven- unique specimen represents a valid species, birds, and tapaculos. I then briefly discuss a hybrid, or a genetic variant of a previously a correlate of hybridization frequency in described species can be a Herculean task. suboscines (Suborder Tyranni), the absence However, because hybrids have no standing of song learning. VOLUME 105, NUMBER 4 835 Materials and Methods Hypocnemis, Myrmochanes, Sclateria, Percnostola, Myrmeciza, Pithys, Gymnopi- I compared the holotype of Phlegopsis thys, Myrmornis, Rhegmatorhina, and Hy- barringeri (adult male, Academy of Natural lophylax, that occur in Amazonian forest of Sciences Philadelphia (ANSP), No. 162,675) Colombia and Ecuador. directly with specimens of P. nigromaculata Soft part colors.—The hybrid had bare and P. erythroptera from Ecuador under reddish orbital skin (now faded), an apo- natural light. A complete description of P. morphic character of Phlegopsis among the barringeri, which will not be repeated here, Thamnophilida. The size of the orbital patch can be found in Meyer de Schauensee (1951). in the hybrid is intermediate between that Series of Phlegopsis and nearly all other spe- off. nigromaculata (larger) and P. erythrop- cies of antbirds were examined at the fol- tera (smaller). This character alone suggests lowing institutions: American Museum of that the two species of Phlegopsis were the Natural History; Carnegie Museum of hybrid's parents. Other "bare-eyed" ant- Natural History; Museu Paraense "Emi- birds have smaller patches of bluish-gray or lio Goeldi," Belem; Museum of Natu- bluish-white orbital skin in life, which fades ral Science, Louisiana State University to gray in specimens. Hybrids between a (LSUMNS); Museu de Zoologia, Univer- species of Phlegopsis and a species from an- sidade de Sao Paulo; and the National Mu- other genus would probably, although not seum of Natural History, Smithsonian In- certainly, have a small orbital patch with stitution. Measurements of wing chord, tail traces of melanization (gray in dried skin). length from point of insertion of central rec- Plumage characters. —Because the hybrid trices to tip of longest rectrix, tarsus length, was male, the plumage diagnosis focused on hallux (plus claw) length, bill length from aspects of male plumage. Meyer de anterior edge of nostril, and bill width at Schauensee (1951:3) noted "this new spe- anterior edge of nostril were made with dig- cies combines in some ways characters found ital calipers to the nearest 0.1 mm. in both P. nigro-maculata and P. erythrop- In order to visualize morphological vari- tera, and has others not found in either." A ation in two dimensions, I used principal careful examination of the hybrid confirms components analysis of log transformed his observation (Fig. 1). No combination of data. Unrotated principal components were potential parental species, except Phlegopsis extracted from covariance matrices (Wil- nigromaculata and P. erythroptera, could kinson 1989). f-values of <0.05 were re- have produced the distinctive characters of garded as significant for two-sample (-tests. the hybrid: (1) unmarked glossy black head, Diagnostic assumptions and methods of throat, and breast; (2) brown mantle with hybrid diagnosis based on plumage char- contrasting spots; and (3) black and rufous- acters and morphology follow Graves cinnamon remiges, rectrices, and upper- and (1990). For brevity, I refer to Phlegopsis undertail coverts. Significantly, the hybrid barringeri as a "hybrid" throughout the text. lacks any trace of pale markings on the head, throat, and breast, or an interscapular patch Results (found in one or more species of Myrmo- Potential parental species. —Because the borus, Hypocnemis, Myrmochanes, Myr- hybrid strongly resembles species of Phle- meciza, Myrmornis, and Hylophylax). Most gopsis in plumage pattern and color (Fig. 1) elements of the hybrid's plumage pattern and because its collection locality is known, and color are easily identified as interme- determination of the potential parental spe- diate character states or mosaics of char- cies seems straightforward: species of Phle- acters of Phlegopsis nigromaculata and P. gopsis, and related genera, Myrmoborus, erythroptera. However, the buff and rufous 836 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON Fig. 1. Lateral views of adult males (from left to right): Phlegopsis nigromaculata, "P. barringeri" (ANSP 162,675; -P. nigromaculata x P. erythroptera), and P. etythroptera. Note ocelli on wing coverts and back of hybrid. ocelli encircled by drop-shaped black spots brown with a terminal black spot (rounded on the feather tips of the mantle and wing triangle). The color pattern in P. erythrop- coverts of the hybrid are not found in either tera is markedly different—back feathers are parental species. By comparison, back entirely black with a narrow white subter- feathers in P. nigromaculata are olivaceous- minal band (back feathers of juvenile males VOLUME 105, NUMBER 4 837 Table 1. —Ranges and means of measurements (mm) of adult male Phlegopsis nigromaculata, P. erythroptera, and their hybrid (="/>. barringeri"). Phlegopsis specimens are from Ecuador and north of the Rio Amazonas in Peru. Significant f-values indicate that means of Phlegopsis species are different (two-tailed /-test). Phlegopsis Phlegopsis P. nigromaculata nigromaculata erythroptera x P. erythroptera /'-value («- 15) in = 8) (ANSP 162,675) Wing NS 87.1-94.0 88.6-93.6 90.9 91.4 93.7 Tail <0.001 58.5-64.5 59.6-68.0 61.6 65.2 61.9 Tarsus <0.001 29.2-32.4 30.1-33.3 30.6 32.2 31.2 Hallux <0.01 18.5-20.9 19.7-21.3 19.8 20.6 20.4 Bill length <0.05 12.0-14.6 11.9-13.7 13.2 12.6 12.8 Bill width <0.01 4.6-5.7 4.5-5.0 5.2 4.8 5.2 have broad rufous edges, e.g., LSUMNS ence or absence of subterminal band or ocel- 110,210). Pattern elements in the hybrid lus (expressed in P. erythroptera and hy- may be interpreted by identifying probable brid); and (5) color of subterminal band or homologues in the parental species. For ex- ocellus (expressed in P. erythroptera and hy- ample, the black spots of the hybrid and P. brid). Serial patterns among feather tracts nigromaculata are probably homologous, suggest that some genes affect several plum- while the buffy ocelli of the hybrid are ho- age tracts, for example, the terminal (or sub- mologous with the subterminal white bands terminal) black spots on the back, wings, in P. erythroptera. This suggests that during and tail of Phlegopsis nigromaculata. feather development in the hybrid, the sub- External morphology. —Variation of terminal band inherited from P. erythrop- morphological characters of avian hybrids tera invaginated to form an ocellus within falls within the ranges exhibited by their the black spot. Less complete ocellations are parental species (Buckley 1982, Graves found on the tertials and secondaries of the 1990), reflecting a polygenic mode of in- hybrid, where the terminal black spot failed heritance. As expected, the size of the hy- to enclose the irregularly shaped ocellus. brid antbird conformed to this pattern (Ta- This example offers rare insight into the ble 1). Phlegopsis nigromaculata and P. complexity of genetic control of plumage erythroptera are morphologically similar and patterns in antbirds.