By using the official Angry Birds walkthrough videos on YouTube with Tracker, Allain was able to determine several important factors -- that the birds exist in a drag-free world (no air resistance), that the sling shot is huge (5 meters -- 16 feet) tall, and the birds are good-sized as well. Y'know that little red bird? He's about 70 cm (27.6 inches) tall. That's a big bird!
To see how Allain used Tracker to plot the horizontal and vertical motion of the birds in flight, follow his analysis, and review his assumptions, be sure check out the original post.