Many of the Hebrew words for unclean birds listed in Leviticus 11 are of uncertain meaning. Tradition and translators have made some pretty decent educated guesses, though. In general, you can bet that long-legged wading birds, large flightless birds, and any birds that subsist primarily on prey or carrion are unclean. Some people say that all web-footed birds are unclean, but I don't think the text supports that.