Chickens are a delicious meal for foxes, hawks, bobcats, raccoons, and plenty of other predators. They are fat, defenseless, and slow. Most importantly, they need a shelter that protects them. In my town, any opening or weakness in your chicken coop means chickens that will be eaten quite soon. We sacrificed many, many chickens until we got our system down.
Every once in a while a predator may get through, so you may want to be cautious about naming your chickens or having kids get too attached until you've had them a while and all has been calm.
360-degree protection
The bottom line is that the chickens need to be protected on all sides. Their home needs a fence and a roof or net that a hungry fox can't get through. If you are just starting out and have a few chickens, a commercial coop should give you most of what you need.
Dissuading predators
The links below are provided as examples only. We have no affiliation with these brands or products.
Most of the feasting happens at night, so light is a big help. Solar powered flashing lights like these are very helpful for keeping predators at bay.
Solar powered motion sensor lights like these are also excellent.
A very cheap and effective help is to put down a 12 to 24 inch wide plastic mesh apron like this on the ground around the periphery of their fencing. This makes it too hard for raccoons or foxes to dig under.
If you have a run
If you have a run, you need fencing that goes down 6 inches into the ground, the mesh apron around the perimeter, and some kind of bird netting on top so that hawks can't dive bomb from above.