Do typical birds usually fly normally?

‘Birds fly, Tweety is a bird, does Tweety fly?’ has become the canonical example to introduce default reasoning. Since its introduction many variations on this example, stretching from ‘Birds normally fly…’ (Lifschitz, 1988) to ‘If something is a bird, then it flies…’, (Schurz, 2002) appeared in the literature. All these sentences are believed to express the same default rule. But do they? In order to do proper experimental research on default reasoning, this is an important question to be answered.