A zoogeographic analysis of the South American chaco avifauna. Bulletin of the AMNH ; v. 154, article 3

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New York : American Museum of Natural History
"The South American chaco is centrally situated on the border of the tropics and is xeric-adapted woodland with some open areas, wet places, and savanna. Driest in the center and wettest in the eastern pantanal savanna, the chaco experiences occasional frost throughout. About 409 avian species, including 22 that barely reach its borders, are resident or breed in the chaco. These number 218 nonpasserine birds, 100 suboscine passerines, including 52 tyrant flycatchers, 20 oscine passerines of Old World groups, and 71 New World nine-primaried oscines. Each species is treated taxonomically (based on a systematic reappraisal of each taxon), ecologically, and distributionally. There seem to be more superspecies in the chaco and in the tropics generally than in temperate North America. Most chaco species (252) are endemic in South America, but 28 percent reach Middle America, 12 percent attain North America, and 3 percent reach the Old World. A transatlantic distributional history is possible for as many as one-seventh the number of chaco species reaching North America. Only one species (Eudromia formosa) is endemic to the chaco; 11 others mainly occupy that area; five subspecies are endemic. Such low endemicity reflects the central location and accessibility of the chaco, and probably rather drastic historical changes in its extent and location. Major range disjunctions in chaco species and superspecies frequently involve northern South America, with Amazonia the apparent barrier. Isolates in the Andes and in the coastal Peru region are fewer, but well differentiated. The caatinga is another area of differentiation. Zones of avian interaction (70) and narrow range disjunctions (28) involving chaco birds show isolates and former isolates mainly in southeastern Brazil, in the campo-caatinga region, and in the Andes, indicating that barriers (e.g., grassland, water) exist or formerly existed between the paired forms. Significantly more nine-primaried oscines and fewer nonpasserine birds show interactions and narrow disjunctions compared with proportions of these species in the chaco avifauna. This perhaps reflects greater speciation and radiation in the nine-primaried oscines, a group still relatively new to southern South America. Primary intergradation in the 276 polytypic chaco species indicates the coincidence of racial borders with major environmental features such as the Amazon and Paraguay rivers, and with certain regions (campo cerrado, Andean base). Major size differences among races of 81 chaco species generally agree (69 cases) with Bergmann's Ecogeographic Rule, larger forms occurring south of smaller races. The chaco avifauna largely is derived from elsewhere, particularly from other xeric regions, savanna formations and edges or ecotones involving forested regions. The chaco nevertheless has been important in furthering range extension and the ultimate isolation of some xeric-adapted birds and species favoring edges and ecotones. There is a need for further in-depth taxonomic-zoogeographic investigations of particular Neotropical avian groups and regional South American zoogeographic analyses"--P. 167.
p. 165-352 : ill., maps ; 26 cm.
Includes bibliographical references (p. 349-352).