wait I have a University login now which means I have access to science and academic journals????
I can read about how soaring birds evolved an air sack connected to their respiratory system that plays a role in the mechanics of flight ???
The air sacks (subpectoral diverticula) inflate to reduce the amount of energy the birds need to spend to keep their wings stretched while gliding. They need to keep their pectoral muscles flexed at a specific length, to pull on the wing bones and generate lift. The air sacks inflate under the pectoral muscles and provide extra leverage, so they can keep the wings stable while using less muscle force. This is such a useful feature it evolved in soaring birds at least seven different times!