<p>We evaluated alpha and beta diversity of bird species distributed in the Sierra Madre del Sur (SMS) biogeographical province, Mexico. Additionally, we tested the effects of climatic and topographical variables on alpha and beta diversity in the SMS across different cell sizes. We employed three different cell sizes as analysis units: 0.15°, 0.25°, and 0.50°. We calculated alpha (species richness) and beta (species turnover and nestedness) diversity values for each cell size across the SMS. We used species distribution models to estimate both alpha and beta diversity for each grid cell in all three analysis units. We applied mixed generalized linear models to analyze the effects of climatic and topographical variables on alpha and beta diversity in the SMS across the three cell sizes. The lower and mid-elevation zones of the central and eastern SMS exhibited the highest alpha diversity across all three spatial grainsizes. Species turnover and total beta diversity were greater in smaller cells (0.15°), particularly in the highlands, with the highest values of nestedness primarily at lower elevations in the western Sierra de Miahuatlán, Oaxaca. Precipitation and temperature during the dry season significantly affected alpha diversity. Species turnover correlated with temperature during the wet season. Variations in precipitation and temperature during both dry and wet seasons positively affected nestedness. Our findings underscore the importance of analyzing alpha and beta diversity patterns in relevant biotic regions to understand the role of environmental conditions and biogeographic history on the composition of bird species assemblages and their conservation.</p>