they are pretty vicious little mini dinosaurs when they wanna be, whereas a cow will run in terror from a mouse.. basically the cow is hoping to get lucky and punt birds, which it has a decent chance of doing while it jumps around..
I feel like a cow just reacting to that, not even trying to fight back, just reacting to the pain, would just lead to chickens getting kicked and trampled to death.
Elephants are herbivores as well. It doesn't make them subject to significantly smaller predators. An elephant simply existing and moving around would be enough to stop/injure/kill a great deal of predators, most of which don't even try because they know they'd lose.
I don't believe a cow would remain stationary if pack of Chickens started picking at its legs, and the act of moving/walking/running away in a crowd of attacking chickens would likely injure and kill chickens in the process. The difference in scale is just too great.
Plus a cow under attack would likely just run away. And cows are a lot faster than chickens, look it up.