The Paraguay River divides the Republic of Paraguay into two distinct regions: the Chaco to the west and the Orient to the east. The relatively flat Chaco is characterized by semiarid thorn scrub forest in the west, subhumid forests in the northeast and palm savannas in the southeast. The topographically diverse Orient is characterized by campos cerrados in the northeast, savannas in the west and southwest, and humid forest elsewhere that increases in height toward the east. None of the hills exceeds 800 m in height. Paraguay's climate is subtropical. Rainfall in Paraguay increases roughly fourfold from west to east.
Previous studies of the birds of Paraguay have focused primarily on the distribution and taxonomy of resident species. There have been no thorough reviews of the avifauna of Paraguay since 1940. This monograph summarizes the status and distribution of the birds of Paraguay, documents patterns of bird distribution, and attempts to analyze the factors affecting bird distribution. It is based on a review of the available literature, examination of thousands of Paraguayan specimens in major American museums, and extensive field work in Paraguay by myself (1987-1989, briefly in 1993 and 1994) and others.
The avifauna of Paraguay includes 642 species that have been reliably recorded thus far. An annotated checklist summarizes the status, habitats and relative abundance of each species in seven geographical regions of Paraguay. Distributional notes are provided for the more unusual bird records, taxonomic notes are provided for species whose taxonomic status requires clarification, and additional information is provided for species whose occurrence in Paraguay is regarded as hypothetical.
Species richness in Paraguay increases from west to east. Faunal similarity between regions is best explained by habitat similarity, with the avifauna of the Chaco-like southwestern Orient being more similar to that of Chaco regions than to other Orient regions. The avifauna of each region has high affinities with adjacent areas of South America, except that few Andean birds occur in Paraguay. No species is endemic to Paraguay. Tests of a suite of predictions suggest that the forest-savanna transition, which roughly coincides with the upper Paraguay River but shifts eastward in the southern Orient, appears to be the most effective barrier to bird dispersal in Paraguay. There is little evidence that the relatively broad Paraguay River is an effective dispersal barrier. The Pilcomayo River may have formed a forest bridge between Yungas and Paranense forests, and subdivided the Chaco avifauna, during more humid interglacial periods.
Key words: avifauna; biogeography; conservation; dispersal barriers; distribution; gazetteer; Neotropics; ornithological history; Paraguay; seasonality; status; South America; taxonomy