Chickens take up the hostile mob cap

I built a HUGE mob spawner near my house, and NO mobs were spawning whatsoever, not a single one. It was frustrating because I thought I had to take the entire thing down( after HOURS of building it). However after a little process of elimination, I killed the chickens in my chicken farm, and to my relief, Hostile mobs were spawning again. But what??! Chickens are PASSIVE mobs, don’t passive and hostile mobs have separate mobcaps?