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
