If you have chickens living outside in coops, your chickens probably have worms. There are several types of worms chickens can get. The best way for preventing worms would be to just routinely add natural wormers to their water.
The first natural wormer is garlic. Not only is it a natural wormer, it also enhances the immune systems and respiratory health within your flock. Some think parasites like mites, lice and ticks are not attracted to the blood of animals that regulatory eat garlic. Another good side affect, more for you and not the chickens, is that the smell of chicken manure is reduced. I'm sure some of you are wondering if the garlic will affect the flavor of the eggs. The answer to this is no. To add the garlic to your chicken's water, just break open a clove and toss a several pieces in the water dish. Remove and replace it once or twice a week. If the chickens eat it, no worries. Just add more.
Raw apple cider vinegar is also an excellent wormer. It too is good for many other things. Before you say, "yuck", know that chickens prefer water with vinegar to plain tap water. The vinegar balances the pH of the water making it a place where microbes and bacteria do not want to live. Keeping the bacteria out helps to keep the water and dish cleaner also. The antiseptic
properties of the vinegar kills the germs that cause respiratory issues. Another common condition found in chickens, caused by a parasite, is coccidia. Vinegar helps to fight this condition along with regulating your chicken's digestive systems, boosting immune system, and increasing the absorption of calcium and other minerals. Just add a table spoon of raw apple cider vinegar per gallon of water. Don't use a metal water dish as the vinegar will cause it to rust.