We analyzed patterns of species richness and endemicity in the avifauna of western Mexico from northern Sonora to southeastern Chiapas. The region was subdivided in 24 transects as study units, and analyses were performed using point data obtained from scientific collections and bibliography, helped by the use of a GIS. For the recognition of general biogeographic patterns and regionalization we used two analyses: Parsimony Analysis of Endemicity (PAE), and faunal congruence curves, along with an alternative species level taxonomy. A total of 783 landbird species was recorded, 157 of which are endemic. Results suggest an increase of species richness from north to south and higher concentration of endemism in the middle of the region. Faunal congruence and PAE analyses suggest the presence of three main avifaunal groupings: Oaxaca and Chiapas east of the Isthmus of Tehuantepec, Sonora south to Jalisco, and central Jalisco south to Oaxaca west of the Isthmus, as well as subgroups within these three main groupings.
