If birds arn't on soil, they cant get worms. Alot of folks raise birds on wire, so they dont really have to worry about their birds picking up worm eggs off the ground. That's not to say that an infected insect might wander in, but not likely having ingested worm eggs, only time would tell after it had been eaten by a chicken.
One thing to take in consideration is when building a new coop and pen, there could be worm eggs already in the soil. Worm eggs can survive in the soil from a year up to 4-5 years, depending on the type of worm.
Sure, I agree that Ivermectin may not work where we live and may work elsewhere. However it's best to use a wormer without any guesswork whether it will work or not.
Like you, my brother lives on the west coast. He's had his miniature Aussie's for years and has never wormed them. One of them excreted roundworms and the vet gave the dog Pyrantel Pamoate. One dose every other day for a total of 2 days, and the wormer was ineffective.
He had to take the dog back to the vet and was given Panacur to give the his dog 5 days straight.
Here, Pyrantel Pamoate is effective for dogs and chickens, as well as Panacur.
You already know I worm my birds monthly, my birds dont free range. They are kept penned up. By free range, that means they are let out of their pens and go foraging on the property or wherever they decide to wander.
I've been using the same wormers for years and I rotate them occasionally.