Muscles. Pectoral muscles = 15-40 % of bird's weight (vs. 1-2 % in people—Superman must have been a really strange looking dude). Another difference is that the muscle that pulls the wing back (= supercoracoidius) is attached to the sternum rather than the backbone (= better balance for flying). There are no heavy jaw and facial muscles and no heavy back muscles.
EXCRETORY SYSTEM. There is no urinary bladder.
DIGESTIVE SYSTEM. Birds eat high energy foods--there are almost no strictly vegetarian birds. They have efficient digestive enzymes.