I've been keeping birds for about 15 years, chicken and quail for almost ten. I have made many mistakes. My first mistake when I first got chickens was thinking chicken wire was going to keep things away from my chickens, lost 4 birds to opossums in one night. We wrapped everything in hardware cloth and these days I opt for 1/4" heavy gage wire for all my projects.
This year I failed on maintenance, I didn't inspect the wooden fencing around our back yard where about half of my quail we're kept, and a pack of stray dogs chewed through the boards in one corner, they chewed through the wood frames and ripped back the hardware cloth and killed about 30 of my birds, didn't even eat them, just killed. I have reinforced the entire wooden fence with pig fence panels, and put up privacy screen on all the chain link. I also haven't moved any quail to the back yet, I've been nervous, but I have 33 new quail chicks and I have the front yard pens full so now it's time to move some to the back. I'm just grateful I didn't lose all of my birds, I've been breeding for temperament for several years and I'm starting fresh with 4 new bloodlines, so not starting over completely from scratch, but close.
The take away for me it an ounce of prevention is worth it's weight in gold. Check your fence lines, inspect the pens and coops regularly for any weak points. Build stronger than you think you need.