How does a hummingbird hover in midair?

A hummingbird at a feeder can hang almost still in the air while it drinks. Its wings are a blur, sweeping forward and back dozens of times a second. The surprise is that the backward sweep helps hold it up too. The secret is in how each wing turns over between sweeps.

How can a wing push up while it moves backward?

Air pushes on a tilted wing roughly at right angles to its surface. What matters is which edge meets the air first. If that leading edge is raised, the wing turns air downward and gets pushed up. The tilt between a wing and the oncoming air is called the angle of attack.

On the downstroke, the wing sweeps forward with its leading edge raised, so the push points up. Between strokes, a hummingbird turns its wing nearly upside down. On the return, the edge leading the way backward is raised again. The push still points up.

Now take that flip away, as our model lets you do. The wingtip traces exactly the same path. But on the way back, the low edge meets the air first, and the push points down. Over a full beat, the no-flip return cancels the downstroke. The modeled push adds up to zero.

▶ Play with this now in Hummingbird Hover: turn off the flip and watch the return stroke push down.

What happens if you freeze one moment?

The interactive can pause the wing partway through its return stroke. The wingtip stays in exactly the same spot, at the same speed. With no flip, the model's push reads −0.50. With the flip, it reads +0.50. Only the tilt changed.

Those numbers are a force proxy, a stand-in scaled so the peak of the downstroke reads 1. They are not newtons. They show direction and relative size, nothing more.

What does the model leave out?

Our film and Hummingbird Hover use a teaching model of wing path, twist and a force proxy. It treats the wing as one rigid, flat slice. It does not measure lift or model the full airflow. It leaves out:

• Flexible wings. Feathers bend, and real wings curve and twist along their length.
• Swirling air. Air swirls around a real wing, including a vortex along its front edge.
• The wake. Each stroke flies through air stirred up by earlier strokes.
• Speed. The model's slow-motion wingbeat lasts about 4.2 seconds. A real one takes a few hundredths of a second.
• Variety. Real wing paths shift as a bird flies faster. Tobalske and colleagues (2007).

The model also gives both halves of the beat equal push. Real birds do not. In a wake study of rufous hummingbirds, about 75 percent of weight support came from the downstroke. The upstroke supplied the other 25 percent. Warrick, Tobalske and Powers (2005). The same team later measured the air swirling around the wing. In five birds, it was about 2.1 times stronger on the downstroke. Warrick, Tobalske and Powers (2009).

That later study found something else. The swirl does not die away as the wing turns over. The turning itself helps keep it going, so lift continues almost without a break. These are findings from specific studies, not a fixed rule for every bird.

Can you feel the flip with your own wing?

A flat plastic lid makes a good wing. Water makes the push easier to feel, so try it in a sink or a bathtub. A stiff paper plate swept through the air works too, but the push is gentler. These steps are a suggestion, so adapt them to the child beside you. Stay close to young children around water.

Predict. Hold the lid by one side, flat, below the surface. Tilt it so the edge facing forward is a little higher than the back edge. Ask your child: if you sweep it forward, will it pull up or down? What if you sweep it back without twisting your wrist?

Try. Sweep the lid forward and feel it rise. Sweep it back without twisting, and feel it dive. Then twist your wrist at each turn, so the edge leading the way is always the raised one. Now it should rise in both directions.

Explain. The lid gets pushed up when the edge meeting the water first is raised. Without the twist, the low edge leads on the way back, and the push points down. The twist is the hummingbird's flip. Water is far denser than air, so a slow hand can feel what a fast wing does.

If hummingbirds visit your yard, try your phone's slow-motion video at a feeder. Their wings beat from about 12 times a second in the largest species to nearly 100 in the smallest. Wikipedia: Hummingbird. At 240 frames a second, a wing beating 50 times a second shows up in about five frames per beat. Look for frames where the wing has turned over.

For the next question, explore Bartosz Ciechanowski's Airfoil and watch air flow around a tilted wing.

Hummingbird Hover is an ExplainerTools Original. Its real-bird figures come from the studies by Warrick, Tobalske and Powers cited here.