Systematics and evolution in Diglossa (Aves, Coerebidae). American Museum novitates ; no. 2381

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New York, N.Y. : American Museum of Natural History
Diglossa, a genus of nine-primaried oscines that feed on nectar, can be diagnosed from more or less related genera of the families Coerebidae and Thraupidae by its peculiar, compressed, and characteristically hooked bill. It occurs on the mountains of Central and South America, except those of southeastern Brazil. Adaptations to nectar-feeding in Diglossa include a U-shaped tongue, a hooked bill, serrations along the anterior edge of the upper mandibular tomia (in some species), regurgitation behavior, an esophagal sac in nestlings, and strong interspecific aggressiveness. Diglossa might be a tanager derivative, but the evidence in favor of such relationship is not conclusive, and more work remains to be done before the relationships of the genus are ascertained. Diglossa consists of six unequivocal species and an additional 10 taxa on the borderline between species and subspecies. The taxonomic solution adopted in this paper is to treat these 10 taxa nomenclaturally as species, but to point out their evolutionary interest by considering them as semispecies, members of superspecies. The six species and four superspecies of Diglossa are arranged in four species-groups, which can be diagnosed by a combination of morphological characters. The caerulescens species-group comprises four species: D. caerulescens, D. cyanea, D. glauca, and D. indigotica. The major species-group contains a single species, the morphologically very distinct D. major from the south Venezuelan highlands. The lafresnayii species-group contains two superspecies, the lafresnayii and carbonaria superspecies, and the isolated species D. duidae, from the venezuelan highlands. The albilatera species-group includes the albilatera and baritula superspecies. An analysis of geographical variation, species limits, and interspecific relationships in Diglossa revealed active species formation in the lafresnayii and albilatera species-groups. The four superspecies included in these two groups total 10 component semispecies, each of which can be considered as (at least) an incipient species. Secondary contacts have taken place in two instances, both in the carbonaria superspecies of the lafresnayii species-group. In only one of these contacts, in northwestern peru, is there a suggestion that differentiation in color pattern is positively correlated with reproductive isolation. In the other case, in Bolivia, the two differentiated populations hybridize in the zone of contact. In three superspecies (lafresnayii, carbonaria, and baritula) the pattern of color differentiation and geographical isolation present intriguing similarities. The parallelisms are most remarkable in the lafresnayii and carbonaria superspecies and include the distribution of the superspecies as a whole, some of the barriers that have permitted morphological differentiation of isolates, and especially color and pattern exhibited by members of the two superspecies when sympatric. Thus black members of the lafresnayii superspecies occur with black members of the carbonaria superspecies and mustached birds of one are sympatric with mustached birds of the other. These parallelisms appear to be the result of complex social interactions between members of the two superspecies in areas of sympatry, whereby selection has favored morphological convergence rather than divergence (social mimicry). Extensive sympatry between species within each species-group, permitting up to seven species and semispecies to be sympatric in the northern Andes, suggests that Diglossa has been present there for some time and that repeated opportunities for speciation have existed in the past. Although it is difficult to draw inferences about early speciation patterns, present speciation trends suggest that Diglossa is in an evolutionarily active phase and that its adaptive radiation is by no means ended. Most of the geographical isolates that have the earmarks of incipient species in Diglossa are peripheral in distribution and occur over relatively small areas. It seems thus likely that the establishment of founder populatons is an important mode of species multiplication in this genus. The barriers separating differentiated isolates are usually dry intermontane valleys (most rarely wet lowlands) providing clear-cut interruptions in the distribution of upper montane forests and brushlands, in which iglossa lives. The temperature depression accompanying Pleistocene glacials would have brought closer together montane types of vegetation, thus narrowing the width of these barriers and favoring dispersal across them.
44 p. : ill., maps ; 24 cm.
Includes bibliographical references (p. 41-44).