Why Can Hummingbirds Fly Backwards

Why can hummingbirds fly backwards? Because a hummingbird's wing doesn't only beat up and down. It sweeps forward and backward, close to horizontally, and turns over between strokes so the same edge always meets the air first. That means the wing pushes air on the way back as well as on the way forward — and once a wing can push in both directions, the bird can aim that push almost anywhere, including forward, which sends the body backwards. An ordinary bird wing is a one-way pump. The downstroke does the work; the upstroke is recovery, with the wing folded and feathers opened so air spills through. A one-way pump can't push you backwards, because the only strong stroke points the wrong way. The hummingbird rebuilt the machine. Its shoulder joint is unusually loose and can rotate the wing nearly upside down between strokes. The wing itself is mostly hand — the fused wrist and finger bones make up roughly two thirds to three quarters of its length — so it works as one stiff blade driven from the shoulder, light enough at the tip to reverse direction dozens of times a second. And the muscle that powers the upstroke, tiny in most birds, is roughly half the size of the main flight muscle in a hummingbird. But the two strokes aren't equal. In 2005, researchers used lasers and seeded air to measure the flow around hovering rufous hummingbirds and found about 75 percent of weight support came from the downstroke and about 25 percent from the upstroke. In 2012, a wind-tunnel study of Anna's hummingbirds trained to feed while flying backwards found the birds held their bodies nearly upright — and that flying backwards cost about the same energy as flying forwards, both cheaper than hovering in place. This video also corrects the claim that hummingbirds are the only birds that fly backwards — plenty of birds shuffle back for a wingbeat or two, but only hummingbirds sustain controlled, steerable backward flight. Plus the leading-edge vortex that adds lift, why swifts share the wing bones but can't hover, why thin air limits them, the nightly torpor that keeps them alive, and why no hummingbird can be bigger than about twenty grams. CHAPTERS 00:00 A bird that looks like it's on rewind 00:35 The short answer 01:15 How a normal bird wing works 02:30 Why big wings need forward speed 03:00 A shoulder that rotates 03:50 Sculling instead of flapping 04:40 A wing that's almost all hand 05:40 A real muscle for the upstroke 06:40 The laser experiment: 75 and 25 07:50 The tornado on the leading edge 08:30 Training hummingbirds to fly backwards 09:40 Backwards costs no more than forwards 10:10 Are they really the only birds that fly backwards? 11:10 The price: a thousand heartbeats a minute 12:10 Why thin air sets a ceiling 12:50 Why haven't other birds copied this? 13:40 What the flower demands 15:00 Why hummingbirds can't be bigger 16:20 The final answer FOLLOW-UP QUESTIONS Q: Are hummingbirds the only birds that can fly backwards? A: Many small birds can shuffle backwards for a wingbeat or two, usually by stalling. Hummingbirds are the only birds capable of sustained, controlled, steerable backward flight over real distances. Q: How do hummingbirds hover? A: Their wings sweep forward and back in a roughly horizontal plane, flipping over between strokes so both strokes produce force. The wingtips trace a flattened figure eight, similar to sculling with your hands in water. Q: Does the upstroke produce as much lift as the downstroke? A: No. Flow measurements on hovering rufous hummingbirds found about 75 percent of weight support from the downstroke and about 25 percent from the upstroke. Insects split the load more evenly. Q: Is flying backwards harder for a hummingbird? A: No. Wind-tunnel measurements on Anna's hummingbirds found backward flight cost about the same as forward flight at similar speeds, and both were cheaper than hovering in place. Q: How fast do hummingbirds flap their wings? A: It depends on size. The smallest species beat on the order of eighty times a second, while the giant hummingbird manages roughly ten to fifteen. Q: Why aren't there bigger hummingbirds? A: The power needed to hover climbs faster with body size than the power muscle can produce. The family tops out at around twenty grams with the giant hummingbird, which hovers with visible effort. ACCURACY NOTE This video draws on published flow-visualisation studies of hovering hummingbirds, wind-tunnel measurements of backward flight energetics, documented hummingbird wing and flight muscle anatomy, and research on hovering performance at reduced air density. #hummingbird #animalscience #biology