"This is the introduction to the world of birds," Aurora University biologist David Horn says of the six-session class he begins teaching this week at the Morton Arboretum. "We'll go from their ancestry from dinosaurs to extreme living examples," such as hummingbirds, whose wings beat 50 times a second, and penguins, who've adapted to "fly" in the water. Birds have been around for at least 150 million years, their closest living relatives are crocodiles, and their lineage goes back to primitive reptiles. Their image got a boost a couple of years ago when the discovery of feathered fossils related to velociraptors suggested that Tweety "is essentially a dinosaur." Horn's "Introduction to Ornithology" will cover the essence of bird: feathers, eggs, body heat, bill, and a one-way respiratory system that requires two breaths to get a gulp of air in and out. It begins June 4 and meets from 6:30 to 9:30 Tuesday nights (except June 18) through July 16 at the Aboretum's Research Center. Tuition is $168. If you just want to look, Helen Pierce's $25 one-day class, "Beginning to Watch Birds," is offered from 8 to 10:30 Saturday, June 1. Meet at the Visitor Center courtyard and bring binoculars if you have them. The arboretum is located just north of Lisle at the intersection of Route 53 and I-88. Call 630-719-2468.