I have a redtail checking out my hens fairly often. They have alot of evergreens, white pines, eastern red cedars and lyland cypress to hide under..Thats one defense that seems too work pretty well...I also love the crows that hang out around my property...they go after a redtail in a flash when they come around here.
  I feed them chicken skin and other scraps, so there around here most of the time. My chickens have learned the crows warnings and cries of a present hawk and take cover...
  Nothing is going to work perfectly all the time, but so far,knock on wood, plenty of dense cover and the crows have worked well for me.
  I know some don't like crows, they steal chicks if given a chance, but i wouldn't let chicks range around, when they get big enough, then i let them loose...Kevin