The only type of bird that relies solely on its own strength to hover in the air, a hummingbird flapping its wings requires more mass-based mechanical power output than any other form of locomotion. Now, scientists have discovered that the tiny bird’s efficiency comes from the ratio of the wing’s length to its width. Researchers from Stanford University and Wageningen University tested the hover performance of 26 hummingbird wings from 12 different species in a machine that measured the torque and lift the wings produced at various angles. The study, published online today in the Journal of the Royal Society Interface, shows that the power needed to sustain a hummingbird midhover is highly dependent on the bird’s wing aspect ratio. During the down stroke, wings with a larger aspect ratio (3.5 to 4.0 for hummingbirds) use significantly less power than wings with smaller aspect ratios. The study also found that the aerodynamic performance of hummingbird wings is “remarkably similar” to that of an advanced microhelicopter rotor. But the wings were up to 27% more efficient.