We have lost countless chickens to dogs. Literally, countless. I don't care how nice your neighbor's dog is, even if they coexist happily with the chickens while separated by a fence, the moment that barrier is gone, they will snap the neck of every chicken in your flock and fling feathers across the whole yard. We have had our chickens fly yards with dogs, and we have had dogs dig into our yard, but the outcome is always the same. Feathers everywhere and dead chickens. Sometimes there is no good solution. For us, we couldn't rest easy until one neighbor moved away! If you want to have backyard chickens, it's just a risk you have to take.
2. Chicken Hawks
Chicken hawks are mean. They are large birds that will circle over your yard, studying your beautiful hens, and then nose dive down and snatch them right from in front of you! I have read a lot of diy ways to keep away chicken hawks (from plastic owls to shiny metallic things hanging all over) and tried most of them. The only way to really stop them is to make sure your chickens have a place to hide where they can relax- a coop, some bushes, tree cover, etc. The smart chickens will learn to take cover when the hawks are around, but like I said, smart chickens are few and far between. Once chickens get full-grown, especially if you have the heavier dual purpose breeds, they will be too big for the chicken hawks. If you have chicks or bantams, I would suggest keeping them in a coop or run that has an enclosed top until they are big enough to hold their own.
Chickens can become egg bound, which means that an egg that they should be laying instead gets stuck inside of them. It cannot stay there long without the chicken dying. If you notice a chicken trying to lay but unable to, give it a warm bath as soon as possible. I know it sounds funny, but it will almost always relax the muscles enough for the egg to dislodge. If that doesn't work, you may have to go in there after it, but that gets tricky because you don't want the egg to break inside of the hen in the process.
Raccoons will be attracted by the chicken food, but will then go after the chickens. Even chickens inside of a coop will not be safe from these destructive beasts. Raccoons will reach their tiny hands through the holes in the cage and literally tear your flock limb from limb. Chickens are VERY heavy sleepers, and probably won't even wake up through this process, believe it or not! To keep the raccoons from getting to the chickens, make sure they sleep in an enclosed area that has solid walls. If your chickens like to sleep on a roosting stick next to the chicken wire, put a board over that part of the wall so that the raccoons can't reach in.
Chickens can get worms, and particularly gape worms, which could kill a chicken if left unchecked. Learn the signs and treatments HERE to avoid losing a chicken due to this very treatable problem.
You probably think I'm talking about dehydration, which would kill a chicken, but that isn't actually what I meant. When you have small chicks, they will often fall into their water bowl and drown, even if it is only an inch deep. Make sure to fill the water bowl with rocks so that they can sip the water from between them without falling in.
Even if you specifically order all hens, every now and then you may end up with a rooster or two...or eight, like we did. Roosters don't generally mix well with suburban neighbors, so this means they will not make it to adulthood. Instead of waiting until our neighbor to murder it for sanity's sake at 3 in the morning, we decided to take the initiative and turn the roosters into gumbo. If you end up with some males, it will change the number of chickens in your flock.
8. Crossing the Road
No I'm just kidding, although if my chickens got out of the back yard, they probably would cross the road and get hit by a car. However, there are many, many other random things that can kill a chicken. For example, if a chicken looks up when it rains, it will drown. Also, they tend to try to eat things that are too big for them to swallow and choke to death. Or if one chicken is injured, the other chickens will try to peck at the wound until the chicken dies. Why, chickens? Why?