You worm your dog monthly?The environment and soil conditions dictate how often birds need to be wormed. If you live in the northern states or Canada, you may or may not have to worm your birds as often as folks elsewhere.
No one can convince me that nematodes do not exist in northern soils. They exist EVERYWHERE. If their feet touch the ground, they'll get worms.
Any area of the country that has had a prolonged rainy season can expect their birds to have worms. Also, birds kept on the same soil need to be wormed more often than birds that free range.
You worm dogs monthly, why? The same reason I worm my dog. Chickens peck the soil constantly. In doing so they pick up worm eggs and swallow them. They are easily more susceptible picking up worm eggs than dogs or cats.
For the non believers in worming, move down south with your birds and your birds will be infected with worms within one month and your birds will eventually die from starvation without worming. Chicknmania is correct; environmental/soil conditions.