I'm surprised that nobody has mentioned geese. They are the rollerbladers of the animal kingdom. They will size you up and calculate whether or not they really need to get out of your way. Your only recourse is to maintain speed and come at them head on, until they reach that panicked point where they say to each other, "Oh quack, this cat's serious!" and skeedaddle. You are then left to weave through 50-100 yards of their turds, like Han Solo maneuvering through the asteroid field.