Windmills

I like windmills, but there are many problems with them. They kill birds. The most efficient production area are far from consumption areas. Alternative energy plants (nuclear, coal or gas) have to be built to supply the equivalent energy when the wind isn't blowing, etc.

But I wonder what would be the impact of large scale farms. I haven't been able to find out any research so this is just speculation.

The windmill converts the energy in the wind into electricity. But doing this reduces the amount of energy is the remaining wind. How many windmills can be built before it impacts the environment? Will it effect where the rain falls? Can it turn a area to a dessert or cause droughts? Or cause floods as rain falls in more concentrated area? Will it effect the migration of birds?