Birds regularly reach altitudes higher than human mountain climbers can ascend without using ...

Birds regularly reach altitudes higher than human mountain climbers can ascend without using auxiliary breathing apparatus, both as residents and during migration.
 
  For example, radar tracking of migrating birds shows that they sometimes fly as high as 6500 meters, the alpine chough lives at altitudes of around 8200 meters on Mount Everest, and bar-headed geese pass directly over the summit of the Himalayas at altitudes of 9200 meters during their migrations. Explain the features of the sauropsid respiratory system that allow birds to breathe at such high altitudes.