Researchers Finally Understand Why Hummingbirds Hum

Native to central and South America, with their long beak and beautiful, vibrant, colorings, hummingbirds are a delight to watch. However, what separates them from most other species of bird is their hum. Scientists wanted to learn more about where the hum comes from, and how the birds create it.

What is a hummingbird?

There are over 300 species of hummingbird, and they are the smallest breed of bird in the world. The smallest of the species is the bee hummingbird, measuring only two inches in size, and the largest, the giant hummingbird, is still small at around eight inches long. What is so distinctive about a hummingbird is its ‘hum’. A hummingbird’s wingbeat is the fastest of all birds in the world, flapping at around seventy strokes per second, more than four thousand strokes a minute. Scientists know that the sound comes from their wings, but they wanted to find out more about how they made the sound.

The science

Using a specially constructed flight enclosure at Stanford University, scientists captured six Anna’s hummingbirds, and by using some high-tech equipment they created the world’s first-ever 3D acoustic model of flying hummingbirds. To create this, they used the information they captured from pressure plates, microphones, and high-speed cameras, and they were able to determine that the humming sound comes from the upstroke of their wingbeat, which provides an extra push of lift. In other bird species, this isn’t the case. What is unknown is if the humming is how they communicate to one another, or if their humming is heard by other creatures, including the fruit flies which the humming birds often eat.

Make the sound visible

Whilst the six captured birds were exploring their temporary enclosure, and sipping nectar from fake flowers, the scientists used high-speed cameras linked to microphone arrays to create a heat map which would make the sound visible in a similar way to how a thermal image camera may take a thermal image. This meant that the scientists could create a frame-by-frame map that would sync audio data to wing motion.

What else did they find?

The scientist’s also measured the aerodynamic wing force in the upstrokes and downstrokes by using pressure plates. The findings showed that even though gravity is pulling the hummingbirds down constantly, they created enough force from their wing flaps to offset the pull of gravity a small amount with every beat. The downstroke is creating more lift, and the upstroke is creating a little less lift. This is different from most other birds, where the whooshing sound is generated from the downstroke, the only wingbeat to generate a lift, but in a hummingbird, both the downstroke and upstroke generate the lift due to the shape of their wings brushing through the air. With the speed that they flap their wings, this along with the air pressure accounts for the humming sound.

Hummingbirds are wonderful to look at, and the sound they make is enjoyable to many. The science behind the sound also makes them interesting, and unique from other species of bird.

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