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

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.