I don't find that so difficult to believe. Sure, big numbers are involved, but similarly are such large numbers involved in "how much damage can a road take bef

I don't find that so difficult to believe. Sure, big numbers are involved, but similarly are such large numbers involved in "how much damage can a road take before it begins to mechanically fail" As far as I can tell, a mouse does about 10^-19 times as much damage as a car. So a chicken does about 10^-9 times as much damage as a car. Considering weight alone and ignoring the difference between wheels and claws, a billion chickens doing the same damage as the weight of one car seems reasonable. In your example, the chicken, squirrel, and mouse all do such little damage to asphalt that it's all essentially zero. You can calculate the relative values as you did, but it's not really meaningful because it's still such a small number.