The rivers in the The Arid Chaco ecoregion lies west of the Córdoba montane savannas ecoregion, and south of the Chaco ecoregion. These ecoregions form the Gran Chaco area described by Davis (1997). The northern, southern, western and eastern boundaries of this ecoregion terminate approximately at the 280 and 330 west latitudes and 680 and 660 south longitudes, respectively. Most scientists agree that the Chaco formed during Pleistocene post-glacial fluctuations, from an arid to humid to semi arid environment. The climate is dry on the west, with an annual rainfall of 650-350 mm, and an average temperature of 28°C-12°C. The Gran Chaco is shared with Bolivia and Paraguay, and is home to a big habitat diversity that includes savannas with palm groves and marshlands on the East and thick woods of the genus Prosopis and Aspidosperma on the West (Dry Chaco). Some bird species of this region are the black legged seriema, the quebracho-crested tinamou, the Chaco owl, the strange-tailed tyrant and ultramarine grosbeak.