Ok, so yesterday's question was a set-up for the well know joke, "Why do hummingbirds hum? — because they don't know the words." Really though, what makes that humming sound? You can hear it if you're close to a hummingbird and it's especially noticeable if you're sitting near your hummingbird feeder and hummingbirds are visiting.
The humming sound comes from the fast wingbeat of a hummingbird's wings as they hover, so fast their wings seem a blur to the human eye. Hummingbirds can hover because bones in their wings are permanently fixed and rigid, except at the shoulder joint where the wings can move freely in all directions. Other birds have wings with several moveable joints. When hovering, a hummingbird's wing moves forward, and then the leading edge rotates nearly 180 degrees and moves back again. During this movement the tips of the wings trace a figure eight in the air.
Hummingbirds can beat their wings 78 times per second during regular flight and up to 200 times per minute during display dives. Rapid speeds enable them to vist more flowers and some can visit 2o flowers per minute to get the nectar that fuels their high metabolism.
Next time you're outside, sit near your hummingbird feeders and listen to the tune.