The way I think about it, smaller birds have less weight and less wind resistance. Shouldn’t that mean they would be faster?

In: 7

Weight and volume go down with the cube of size. Surface area, wind resistance and strength go down with the square of size. It’s basic geometry. Because different properties scale at different rates as size changes, you can’t just scale creatures up or down and expect them to work the same way. Larger creatures tend to be faster than smaller ones but, eventually, large size makes them too fragile to survive, so there’s a limit.

Air resistance is less of a factor for bigger objects.

Explanation through analogy:

Imagine a small pistol, and a German WW2 artillery piece. Which one do you think can fire the longest distance? If you answered Dicke Bertha, you’re correct. That’s because Dicke Bertha’s shells 1. leave the cannon at a higher speed 2. cuts through air like it’s not even there ?

So? Which is it?

2! Exactly. In fact, a 9mm Berreta has a much higher muzzle velocity than Dicke Bertha (+50%). The bigger the shell, the longer the range, all other things being equal (muzzle velocity, projectile density…)

Works with planes too. The fastest planes to ever fly are also some of the biggest. But I like Dicke Bertha’s example better because an artillery shell is such a dumb object.

Sciency explanation: see other answers with keywords such as “cube, square, surface, volume”.

It’s probably a bit easier to visualize if you look at cars rather than birds. Let’s pretend you could make a bigger car by gluing a number of small cars together.

Say you have a car that has 100 horsepower and it weighs 1 ton, and you glue nine of them together like so:

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Acceleration depends on the power-to-weight ratio, and since your big car weighs 9 times as much and has 9 times as much power, it’s still going to accelerate the same as the small car.

However, the top speed depends on the power and the wind resistance. Your glued-together big car has 9 times as much power, but it’s only 3 times as wide, so the wind resistance is only going to be about 3 times as large. That means the top speed is higher.

As you increase the size of your vehicle, the frontal area grows far slower than the combined power of the engines.

The same happens when you increase the size of a bird. It may have 8 times as much weight and muscle power, but the frontal area of the bird will only grow by a smaller factor.

Is that true? Hummingbirds are crazy fast and peregrine falcons are fairly small birds

Like, what do you mean as fast? Average travel speed, diving speed, long vs short distance travel?

Really comes down to power. Bigger birds have larger and more powerful wings and the size to weight ratio is in their favor.

You can also think about it with cars. There are very small very light cars but they are not faster than big heavy super powerful engines

A Bugatti is a big and heavy car but it’s much faster than a ford fiesta