Fig. 1.

Experimental set-up for in vivo recordings of muscle activation and wingbeat kinematics. (A) Hovering performance in normal air and in normoxic heliox was recorded while the birds fed from an artificial feeder. A high-speed camera filmed through a mirror held at 45 deg to attain horizontal wingbeat kinematics. A bipolar electrode was inserted into the pectoralis major on one or both sides of the animal. Replacing normal air with normoxic heliox gradually lowered air density within the airtight chamber. (B) Maximum load lifting performance in normal air was recorded immediately following density reduction trials. A harness was placed around the neck of the bird, which was attached to a string with color-coded beads. As the hummingbird ascended while attempting to escape, it lifted progressively more weight until reaching its maximum burst performance at which point the animal hovered briefly.