Eli5 : why are bigger birds generally faster than smaller birds?

Question

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

Answer 1

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”.