The chickens are amazingly brave around the cats, happy to ignore them (or even have a go at them). Cats on the other hand, bolted at 100mph to go hide under the bed the first time they met the chickens
Naw - I think there's something about birds with attitude. Maybe it goes back to the days when birds were 12 foot tall and the top predator - so cats get some atavistic reaction.
I recall my very first Siamese boy meeting a canary that my Dad had captured on the cabbage patch. It had been stuffing raw greens all day and was truly pissed off at being caught. It took one look at the cat (on the stairs) and squawked at him in pure fury. The cat fled and wasn't seen for the rest of the day. This was a canary mind you.
(And my Mum's (large) rabbits used to scare off dogs when they were loose. When a big mature buck turns round and lets loose with those back feet, there aren't many animals that can stand it.)