The bird's lungs are not very “compliant”. That is, they don't expand and contract much, like ours. That is the job of the elegant set of airsacs. They act like bellows. As a bird flaps it's wings, this causes the airsacs to expand and contract, forcing air through the system.
The keel is the anchor bone for the two sets of muscles that pull the wings up and flap them down. As these muscles work, the keel pulls the breast plate up and down. This causes the abdominal cavity (actually the “ceolomic (pronounced seelomic) cavity”) to have negative, then positive air pressure, causing the airsacs to expand and contract.
Remember, the system is a one-way flow, so there is a constant flow of air through the lungs. Oxygen is exchanging with CO2 100% of the time, rather that part time. The faster the wings flap, the more air is driven through the lungs and the more exchange can take place.
There are other adaptations that allow the bird to respirate more efficiently, as well. One (which allows the extreme tolerance to high altitudes) is that avian hemoglobin has a higher “affinity” for oxygen. O2 “sticks” to it better, allowing a higher percentage of that hemoglobin present to be carrying O2.
Also, birds have a higher percentage of red blood cells than mammals by body weight (10 – 12% of a bird's body weight is blood, compared to 7 – 8 % in the mammal, and a higher percentage of whole blood is composed of RBC's), again allowing for more O2 to be carried.
Avian RBC's have a nucleus, unlike mammals. This allows them to last much longer and be able to use other chemicals than just glucose for their energy needs (free fatty acids). This makes them more efficient and allows other tissues to utilize the available glucose.
I agree, birds are VERY C@@L. Every single difference between them and mammals is in place to allow for flight. I could go on and on…..
Who thinks that its way cool that hitch-hiking hummingbirds have been seen jumping out of the feathers on the backs of geese in South America when they land from their journey over the Gulf on their southern migration.