The tip speed is what counts, the key governing factor is called the 'tip-speed-ratio', the apparent wind speed (as seen by the tips of the rotor) versus the real wind speed (as seen by an observer relative to the ground).
Almost all windmills that want to be efficient have design TSRs somewhere between 6 and 8, and consequently, in a given wind they all travel about as fast at the tips.
The design constraints are the speed of sound and the forces on the blade root as well as the tendency of the blade to start fluttering.
It has absolutely nothing to do with birds. Birds will fly in to a stationary building just as easily as they'll fly in to windmills, it's rare but it does happen every now and then.
The larger a windmill, the slower the blades will rotate.
The reason why they are so large is because the power harvested by a windmill goes up with the square of the rotor diameter, many small windmills are more costly to maintain and operate than a single larger one. Right now the sweet spot is somewhere around 2MW and 80 meter rotors for best $/W.
