This is a quote from a Florida Extension Service article.
The chickens pick up the parasite eggs directly by ingesting contaminated feed, water, or litter or by eating snails, earthworms, or other insects (intermediate hosts) which can carry the eggs. Further down it mentions earthworms and grasshoppers as intermediate hosts for roundworms.
Here's a link to the article. The recommendations look like they are more for commercial growers than for me so read them and apply to your situation. I'm personally not going to use insecticides to try to kill insects that might come into contact with chickens.
http://edis.ifas.ufl.edu/VM015
Unless you keep your chickens in solitary confinement, away from earthworms, insects, and wild birds, there is a great chance they have worms. The contaminated feed, water, and litter means that it has bird droppings in it, like when an infected bird perches above a feeder or a wild bird leaves a deposit in the run.
Chickens can get worms from just being chickens!!! They scratch the ground and can pick up the parasite eggs directly by ingesting contaminated feed, water, or litter or by eating snails, earthworms, or other insects (intermediate hosts) which can carry the eggs. I clean up their coop and run daily picking up any and all pooh I find...... My girls do perfer to eat their treats directly off the ground even though I have chicken bowls for their treats....... Thinking about it, they like to eat off the ground the pooh oh....... it is really natural that they would get worms eventually.......... How many times have you given your girls veggies, fruits or scratch and their "table" was the ground??
Parasites literally scar the digestive tract as they burrow into it. Each scar in the digestive tract is one more place where nutrients can't absorb. I highly disagree that this is the way to go. I also disagree with constant worming (unless there are constant infestations). Additionally, worms decrease the immune system of birds, steal the nutrients, irritate the digestive tract, make the bird more susceptible to other digestive tract illnesses by stressing the good bacteria of the gut, increase incidences of coccidiosis (even in adults), and spread to healthy birds.
I totally believe in ecological balance, but in moderation. If you wait til you see worms, you're waiting too long.
The ideal way of doing things would be to test three or four random birds in a flock using a "fecal egg count" from a vet to see if you need to worm two or three times a year. That way it's less invasive and lets you know when to treat. For someone trained in that, it would be the best way. Or if you have a good vet who will let you just bring in a few fecal samples and charge you for that, not the visit.
