If you raise chickens, they will almost certainly attract rats. It's not the chickens themselves but their feed that attracts the rodents. Once they are in the coop, however, they can be a major nuisance. Many chicken owners deal with the problem by setting out rat poison where the chickens cannot get to it, but there are natural ways to kill rats, too.
Choose a Strategy
Decide if you really want to kill the rats or if you simply need to ensure that they stay out of the chicken coop. Killing the rats will not solve the problem forever; your coop is likely to continue attracting more rats after you kill the ones you see.
If you choose prevention over killing, you can protect your chicken coop by enclosing it in chicken wire or other small-mesh fencing. Do not forget to protect the ground, as rats burrow. Bury mesh in the ground to a depth of 1 foot all around the coop.
If you decide you do want to kill the rats, you will have several options. Consider whether you are hoping to avoid the use of poisons that might harm your chickens, whether it is artificial chemicals you are worried about or whether you are simply interested in simple, inexpensive solutions to your rat problem.
One easy, traditional method of controlling rats is keeping cats and allowing them to do it for you. Not all cats are expert rat-catchers, however, and you will have to ensure the chickens are protected from the cats. Some breeds of dogs are also known as rat catchers, particularly Jack Russell terriers.
Another traditional home remedy for rats is to mix equal parts of cornmeal with plaster of Paris and leave it where it is accessible to the rats (but not to the chickens). Supposedly, the rats will eat the mixture, and the plaster of Paris will harden in their stomachs, killing them.
Natural Rat Poison
Depending on your reasons for wanting to use an all natural way to kill rats in your chicken coop, you may want to look into natural rat poison. Natrocell manufactures an all natural product called "Eradibait" which is made entirely from food-grade products and claims to eradicate rats effectively. Another natural product traditionally used as rat poison is red squill, a Mediterranean plant. The powdered bulbs of this plant are toxic to rats, but generally safe for other animals to ingest.