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The most important habitat for birds of the Neotropical Region, in terms of both area and diversity, is Amazonia. This includes not only the basin of the Amazon River, but also that of the Orinoco River and the forests of the Guiana lowland. Amazonia is considered to extend up the eastern slopes of the Andes and the Tepuis to 1000 m or higher. An important change in habitat and in avifauna occurs at about 500 m and c. 50% of the species that reach the mountain base do not go higher than this elevation. Approximately 800 species of birds have been recorded in Amazonia, of which about 265 are endemic. The most important families are Psittacidae, Trochilidae, Ramphastidae, Cracidae, Formicariidae, Tyrannidac, Furnariidae, Cotingidae, Pipridae, and Dendrocolaptidae. Notably, none of the Amazonian endemic species belongs to the widespread families Columbidae and Strigidae. Seven areas of endemism are recognised within Amazonia. Several hypotheses have been proposed to explain the biodiversity of this area, ranging from the long-term stability of the rain forest to its fragmentation during arid periods of the Cenozoic. Of several factors during the Cenozoic which contributed to the present high biodiversity; the most important may have been sea-level rises of about 100 m which fragmented the rain forest into many true islands and archipelagos.