Had chickens for 1.5 years. Lost three in that time and each one as been a learning experience.
Loss #1:
I started with just a coop surrounded by plastic fencing 4 feet tall. Had our original two faverolles bantams inside and didn't realize that bantams can fly so well. They were only about 3 months old and one flew out and landed in the yard in the afternoon and a cat got her.
Improvement #1: Covered the run and coop with bird netting (my mindset still thinking "keep chickens in" not "keep predators out")Replaced the faverolles pullet with two faverolles (chicken math) .
Loss #2
Next disaster was a couple of months later while letting them free range. Normally we watch them pretty close but I went inside for probably a half hour and came back out to find feathers all over the yard (late morning early afternoon), a dead pullet in the middle with her head missing and the neck pecked down to the skin. Research told me it was likely a hawk. The other two pullets were hiding in the run, one in the coop and one had stuck it's head into the open void of a cement block under a coop leg and was laying on it's side. I spoke to her and there was absolutely no movement. I thought the worse and reached down to slide her out and as I did she picked up her head and looked at me! She was fine, just traumatized.
Improvement #2: We still free range as much as possible, but only under DIRECT supervision. If myself or daughters need to go in even just to use the bathroom, we have someone come out and watch them. Another improvemnt was to get a full sized faverolles to make our flock look less like easy pickings from the air. She also took over the flock watcher duties just like I had hoped.
Unrelated improvement #3: It became obvious that a tangle of bird netting and plastic fencing not only made it tough to care for them (especially for our neighbors while we were vacationing) but looking forward to Winter and snow, it was obvious this wasn't going to work. I found a 12'x12' chain link paneled dog run on Craigs List for $100 and placed it over the coop. So nice to have a secure pen with a human-sized door to make maintenance so much easier. The chickens did well through Winter and our one layer kept laying till about Christmas. After Winter we now have two chickens laying and they are enjoying not being cooped-up. They love having the coop door open after all the cold weather and are choosing to perch outside the coop in the covered run.
Loss #3:
Easter morning I awake early and look out the window to check on the chickens. There is something wrong out there because I see feathers blowing across the yard. I go out to see and am confronted by a carnage that took place in the left front corner of the run. There are tons of feathers and gore stuck all over that corner and for about a foot down each side. All that is recognizable are a couple wing pieces with feathers still intact. It looks like a coon came through the bird netting on top and fell/jumped onto the coop roof. It chased the bantam into the corner where another coon pulled her through the fencing. It was one of our two remaining faverolles bantams, but the remaining chickens are doing fine and show no signs of stress - except for the remaning bantam (the bantams were best friends) who was laying last Fall and also again this Spring has not layed an egg since that night to this day. We replaced her with a baby blue cochin bantam pullet and a baby standard size leghorn pullet (chicken math again!)
Improvements #4: OK now we have to get serious...
Electric fence added to the top of the run extending up about 5 inches above the top rail. Another electric run extending out from the top rail horizontally about 5 inches. I have added the heavy plastic 1/2" hardware cloth clear around the inside bottom 2 feet of our chain link run. I have also added the 1/2" hardware cloth inside the chain link at the ends of their perches (1x2" boards stuck through the corners of the chail link run walls) and lining the corners adjacent to the perches. Lastly I laid a chicken wire apron on the ground extending out 2 feet from the run and connected to the chain link and the bottom rail and covered with leaves and debris common to the area.
No more problems so far. I did shoot a coon on our back deck about a week after Easter, and my daughter mentioned that while sleeping out in a tent on the deck with a friend a couple weeks ago they heard something rattle the chain link of the run and then heard a blood curdling scream. I went out the next morning and notice the fencer was working fine but there was a sign that something struck the electric wire and bent the strand slightly right on the same corner where the chickens roost. I think we retrained something that night. So far, so good!