Wazine only hits round worms, and only paralyzes them, so they fall off the gastrointestinal mucosa and get passed. Ivermectin kills worms, and all kinds of them. If your birds have a large worm load the death of the worms (rotting in the bowel) combined with the volume of worms being passed can cause severe GI distress, possibly killing the bird. If you have birds older than a year that are unwormed or birds with an unknown worming history, you're better off going with the "gentle" wormer, first.