The best thing you can do to protect your new pets (and investment) is predator proof their coop and run. Chickens are notoriously difficult to keep safe, and thoughtfully securing their living space is the best place to start.
Securing the Coop
Your coop walls should be made of solid, sturdy material, and enclosed on all four sides. Make sure any cracks in the walls are sealed. This accomplishes two purposes; It prevents drafts, and keeps rodents out. Mice can fit through impressively small cracks, as can rats. Both rodents will primarily go after the chickens’ food and water, but rats can eat young chicks. It is important to be diligent about keeping rodents away and out of the coop.
The floor should ideally be solid, rather than dirt. Traditionally made of wood, the floor should be elevated at least a few inches to keep it off the ground and out of the moisture. Over time, any wood that touches the ground will rot and create an entryway for mice, rats, snakes, and other opportunists.
The coop should have a sturdy, locking door, and all windows should have locks on them as well. The lock can be as simple as a hook and eye latch, but it is best to use a latch that can at least be secured with a snap clip. Raccoons are quite adept at breaking into simple closures, and this adds an extra measure of protection. Windows should be able to shut fully to keep drafts down in the winter. If it is necessary to leave them open throughout the night, add hardware cloth to the inside to keep the critters out.
A solid, leak free roof is important in a coop, not just for keeping the chickens dry, but also for making it hard for predators to get in. If you have eaves under the roof, make sure they are covered with hardware cloth so nothing can find its way in that way.
Fortifying the Run
If you plan to reliably and consistently lock your chickens in the coop during the night, you can be less cautious when fortifying your run. Otherwise, this is usually the weak point of defense. If you plan on leaving town for a few days with no one to watch your birds, be sure to ramp up the defenses.
The sides of the run are usually the weakest point. Hardware cloth is much sturdier than chicken wire and more efficient at keeping predators out, however it can be cost prohibitive. If it is possible for you to enclose the entire run in hardware cloth, that is the safest option. Otherwise, you can use chicken wire on your run and fortify the bottom with hardware cloth. Lay hardware cloth in an L shape on the bottom edge of the coop and the ground. Make sure to come up the sides at least 18” if not more. Similarly you should have at least an 18” extension on the bottom. This prevents animals from digging under, which is the most common way varmints make their way into coops. The line can also be buried underground in a trench along the sides of the run. The hardware cloth also adds a second line of defense and keeps animals like weasels from being able to chew right through the wire. Adding the hardware cloth alone will keep predation down a great deal. If you choose not to, be aware that the chickens will often scratch at the perimeter of the coop, and create small holes underneath the wire. The wire that was once touching the ground will no longer make contact. This lets predators in and also allows smaller chickens to escape. When the birds are outside of the run, they are not protected.
The most effective method of keeping predators out is an electric fence. The insulators should hold the wire or tape about 4” from the side of the run and be placed the same distance from the ground. This one wire close to the ground is enough to keep the majority of predators out of your coop. If you have still problems even with this setup, you can run another line around the top of the coop.
The roof for the run does not have to be as solid as the roof on the coop. It helps, and is often more aesthetically pleasing, but for most chicken keepers a solid roof over the run is overkill and cost-prohibitive. Bird netting is usually enough to stop anything coming in from the top during the day, and is especially effective when coupled with an electric fence.