Keeping Hens Safe, Fence height, clipping wings

This year I started my first flock. I began with six chicks. I lost one pullet, BR, when she flew our of her fenced in area when she was about 12 weeks. I just lost my second BR today. We started with a three foot fence and quickly changed to a four foot fence to match our perimeter fence and clipped one of their wings. I only have had trouble with the BR. None of the other hens fly out.

Can everyone tell me what has worked the best for them? What height of fence to keep them in even with clipped wings?

She was the heaviest of my hens and yet she could still fly over the four foot fence even with her wing clipped. I just recently read from one source that he likes to clip both wings to keep them from getting air. Other than that book, every other source has said to clip just one wing.

I am very distraught over losing this wonderful bird. Should I avoid that breed? Clip both wings? Obviously, I need to take the fence up to six feet and clip the wings. I am hopeful that will keep them in.

All suggestions are appreciated.