By phrasing it "Of birds that migrate south in the fall," it becomes a superset/subset relationship rather than an intersecting set relationship. The word "Many" does not impose a limitation on the birds being considered to return in the spring. The word "Of" DOES impose a limitation, because you are restricting your conclusion to birds within the set of those that migrate south. "Many" yields no such restriction.

I honestly don't think this lesson sucks at all. As DAVEMARINO said below, you need to have a strong grasp of the lessons on sets. I too was having problems recognizing formal logic flaws in drill and preptest questions until I went back and reviewed both of the sections on sets. One thing that significantly helped me was to write down each of the 8 formal logic flaws on a separate piece of paper (with their examples and Lawgic forms) and have this paper next to me as I drilled. Having the flaws in open view as I drilled helped to facilitate my pattern-recognition of these flaws whenever they appeared in questions. Eventually, you begin to recognize them instinctively. Above all, practice practice practice.