Looks amazing Ken. Yes, impressive work yet again.
Ever wondered why birds can fly at such high altitudes and bats can't? Bats have a bellows type of lungs like we have (being mammals). There is always a dead space, so there is always some oxygen left in the stale air that we exhale. Birds' lungs, on the other hand are extremely efficient. They have a system of two sets of air sacs. Their lungs DO NOT expand and contract like ours. As they inhale, the air sacs expand with fresh air entering the posterior air sacs and stale air from the lungs entering the anterior air sacs. As they exhale the air sacs contract with fresh air from the posterior air sacs being pumped into the lungs and stale air extruded to the exterior by the anterior air sacs. Thus the inhaled fresh air only move in one direction through the lungs and allows almost 100% of the oxygen to be absorbed. This brilliant animated gif shows you how it's done: Brilliant GIF shows how Humans, Birds and Insects BreatheView attachment 36711
The bar-headed goose is one of the world's highest-flying birds, having been heard flying across Mount Makalu – the fifth highest mountain on earth at 8,481 m (27,825 ft) – and apparently seen (unverified) over Mount Everest – 8,848 m (29,029 ft). Bar-headed geese have a slightly larger wing area for their weight than other geese, which is believed to help them fly at high altitudes.