I haven't had to cull a sick bird now in a very long time. There's a fox den in the forest next door and they prowl around here occasionally. Last year I had a total of 5 chickens go missing, which is anywhere from 5-10% of my chicken population on average
Natural selection seems to be keeping...
I never vaccinate or medically treat my chickens and I haven't seen a sick chicken in years. In nature predators cull the sickly. When a farmer removes predation, they must then assume the role of predator for the flock