Ok, you finally got me to sign up for ulogs.org.

I think for me the biggest distinction is eyesight. Birds have vision that is extremely similar to humans', but better. They see movement and color in essentially the same way we do, although often with more acuity.

Flies, on the other hand, have compound eyes, which operate on an entirely different system. What we perceive as a single eye structure on a fly is actually several thousand individual eyes that the fly's brain integrates the data from in real time. Flies can't move or focus their eyes, and have no pupils to control the amount of light coming in, so all of their perception is done through processing in the brain. They primarily perceive motion rather than shape, and have very limited color vision, but can distinguish between polarized and unpolarized light, something no mammal can do.

Both birds and flies have vision as their primary sense, but they approach it completely differently.