The catchment area of the Balsas river delimits this Mexican EBA and for this purpose extends from Colima state in the west to interior Oaxaca in the east, and to Puebla and Morelos states in the north. The trans-Mexican volcanic range (EBA 006 in part) lies adjacent to the northern boundary of this EBA, and the interior slopes of the Sierra Madre del Sur of Guerrero and the Sierras de Yucuyacua and Miahuatlán in Oaxaca (EBA 009) abut its southern boundary. The altitude varies from sea-level (though most of the EBA is above c.300 m) up to 2,500 m (especially on some of the mountain slopes in the interior of Oaxaca state).

There is a marked dry season of five to eight months in this region, and low annual average rainfall (Rzedowski 1978). The vegetation is consequently dominated by tropical dry deciduous forest, especially at lower altitudes. These dry forests have been noted for their high levels of regional and local endemism in a wide range of taxa, especially cacti (Dinerstein et al. 1995). Other habitats include arid scrub and, at higher altitudes, especially in the interior of Oaxaca, arid oak scrub.

All of the restricted-range bird species occur in the dry deciduous and arid scrub habitats, but there are some clear differences in their distributions: Philortyx fasciatus and Aimophila humeralis are confined to the western and central parts of the Balsas drainage and do not occur in Oaxaca; conversely A. notosticta and A. mysticalis are found mainly in interior Oaxaca.

Another possible addition to this EBA is Balsas Screech-owl Otus seductus, confined to the western and central Balsas region. It was listed as a species by Sibley and Monroe (1990), but was later treated as a subspecies of the widespread Western Screech-owl O. kennicottii (Sibley and Monroe 1993). Most of the ranges of Slaty Vireo Vireo brevipennis and Dwarf Vireo V. nelsoni are confined to this EBA, but neither is treated as a restricted-range species because each is judged to breed over an area greater than 50,000 km².

The largest remaining dry forests north of the equator, of which this EBA holds a significant amount, are found in western Mexico (Ceballos and García 1995). Most have been affected by human activity; this EBA is no exception, with agricultural expansion, intensive cultivation for export crops, conversion for cattle-ranching, coffee and citrus plantations posing threats to wildlife (Dinerstein et al. 1995). There are very few protected areas within the EBA, but none of the restricted-range species is currently considered threatened as all of them are fairly common in their appropriate habitats (Howell and Webb 1995a).

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Balsas region and interior Oaxaca. Downloaded from https://datazone.birdlife.org/eba/factsheet/27 on 23/11/2024.