I've read that it depends on the kind of turbine. Some rotate slowly enough that the birds can see the blades and avoid them. I don't know if that's the whole story however... Bird (and bat) conservation groups need to work with the Wind industry to figure this out. No need to be adversaries. We should be able to have both birds and turbines, as demonstrated by the email below.
--- On Sat, 11/15/08, Larry and Mona Rogers <[log in to unmask]> wrote:
> From: Larry and Mona Rogers <[log in to unmask]>
> Subject: [VTBIRD] Birds (and Bats) and Wind Farms
> To: [log in to unmask]
> Date: Saturday, November 15, 2008, 8:28 AM
> Perhaps a purely anecdotal story, but...
> We did a Elder Hostel birding tour to catch the western
> European migration through Andalucía and Gibraltar in the
> fall of 2006. Great birding - we bagged 117 life birds and
> saw hundreds of eagles. Andalucía is very big in
> commercial wind farming. We saw several sites with over a
> 1,000 turbines each. We asked our guides, who were
> obviously very experienced birders, about turbine kill
> damage to birds of all types - raptors, passerines, storks,
> etc. Our guides said that it didn't seem to be much of
> a problem. Lots of amateur and government types are
> censusing the wind farms for kills and very few deaths are
> being reported.
> The Andalucian wind farms were in mountainous but open
> country; typically cattle grazed between the turbines. We
> didn't ask about bats and don't know much about how
> they faired.
> For what it's worth.
> Larry and Mona Rogers