All nightshades, including the ones we grow and eat (tomatoes, potatoes, eggplants, tomatillos, or peppers) contain some toxic alkaloids such as atropine, nicotine, and solanine. That's probably why some somewhere claimed they are poisonous. The amount of solanine depends on the plant. From the HIGHLY toxic deadly nightshade, to the fairly toxic (potato stems and green tubers have the highest concentration of solanine amongst our common garden plants) to the pretty slight levels of toxins found in peppers. Unfortunately people write things based n partial and misinformation and so the fact that peppers are in the same family as much more toxic plants, they assume that they must also be "dangerous."
Birds love peppers and for some reason chickens really love the foliage. I had bhuts stripped bare by free ranging birds one year. Capsaicin, which to us tastes hot, doesn't effect birds and I've watched a flock of house finches devour every last one of the ripe Thai chilis I had in some pots. Chilis evolved this way so that mammals, which would chew and crush the seeds, wouldn't eat them but birds, who don't have teeth, would and disperse the intact seeds in their droppings.
