I hear a lot of people saying wind power affects migrating birds. Can anyone provide any definitive evidence or research on this? Are we talking about a couple of birds getting chopped up a year or entire flocks?

Also, there's got to be some simple technology that can be used to scare off the birds. Maybe a high-pitched sound alert when birds are detected by the windmill.

The bird problem must be solved!

Seems like windmills are the answer to H5N1.
Horns Rev has proved not to be a problem for migrating birds. They actually avoid the farm, and if they decide to go throw, they also avoid the blades.

Sina,

Go to google and put in this phrase in the search bar:

Wind turbines and birds

You will have reports to read for days and days!  The problem is, the results are somewhat inconclusive.  From my reading (and I have made my eyes hurt reading the subject, since I am involved in an experimental wind turbine firm)
the main consensus looks like this:

a.  Wind turbines do sometimes kill birds.
b.  Location means everything.  If the turbines are located in a flyway of large predatory birds, the damage can be severe.  If the birds are already endangered species or a population small in number, it becomes a serious issue to confront.
c.  In areas not frequented by larger predatory birds such as hawks, owls, eagles, the damage is less severe, and probably no worse than any other tall structure such as cell phone towers, guy cables to cell and broadcasting towers, taller glass buildings, and fossil fuel towers, etc.  Birds do also strike those.  Let us not forget the killing of birds by aircraft.
d.  It is the spinning motion of the blade that makes wind turbines more dangerous to birds.   They never know what hit them.  However, some studies do indicate behavior change by the birds over time.  The sight/smell of dead birds on the ground make the living ones leery to go there (this is not a joke, birds do "learn".  Experiments have also been done with colered ribbon, noise makers, etc., to dissuade birds from going near wind turbines.  The results are still inconclusive in most cases.
e.  The issue of perception is crucial.  Since wind is percieved by it's supporters as "environmentally friendly" power, any bird kill by wind turbines will be viewed much more seriously than the same kill would be viewed if it were caused by a fossil fuel plant.  Thus, the Audoban Society, Seirra Club, National Wildlife Fund, etc.,  have shown great interest and made intensive studies of the bird kill issue, and the wind industry is always working to address it.  
f.  Regarding the above, almost no such scrutiny concerning bird kill has ever been placed on the fossil fuel industry.  Open waste storage tanks are deadly to birds, and the "catch basin", "retention ponds" and lagoons for oil and coal "tailings" of many oil/coal drilling/processiong operations can contain hundreds, sometimes thousands of dead birds.
g.  Other industries and agriculture can be deadly to birds.  One of the great killers of predatory eagles and hawks is the fence, be it barbed wire or woven wire.  When a predatory bird spots prey, their eyes focus to that prey, and the fence between them and the prey become invisible to them.  I know that in Kentucky and Indiana, several eagles have been rescued from fences with broken wings.

In closing, wind power is expected to act to a higher standard than the fossil fuel industry, and rightfully so.  Any developing technology reducing danger to birds, should and I think will be implemented.

Roger Conner  known to you as ThatsItImout

A local ornithologist here at the west coast of Norway has complained that a new wind farm kills sea eagles in large numbers, particularly in the mating seasons. The park in question can kill over ten eagles per month then, with an average per year of about two eagles per months. Given the low population and slow reproductive rate of these eagles, it's significant. The park's owners have said that they are open to the idea of shutting off the windmills in the mating season.
Very expensive eagles they have down there.
In this thread that discusses the bird kill issue, I offer citations that find each wind turbine is responsible for 2 bird deaths per year on average. That's 0.003% of the total number of human-caused bird deaths -- not even considering biggest factor of all, loss of natural habitat.

In addition to ThatsItImOut's points, it's worth mentioning that the newer, bigger turbines with larger, slower-spinning blades are much easier for birds to avoid and pose less of a danger than the models of 20-30 years ago.

This won't do much to assuage the concerns for Raptors, but Cats and Cars are a couple of our best forms of bird-killing.

Cats are said to take upwards of 10 million birds a year, don't know what the auto estimates are.

http://www.ctaudubon.org/conserv/nature/cats.htm

also:

Cats kill hundreds of millions of birds annually in the United States, said Jimm Edgar of the Mount Diablo Audubon Society, citing an article in Birders World titled "Cats -- they're soft and cuddly. They're America's favorite pets. They're also natural-born killers."

"Domestic cat attacks ... (have) contributed catastrophically to the decimation of certain avian species, especially some of our most beloved songbirds in North America," the article asserts. The American Bird Conservancy concurs with the hundreds-of-millions figure.

Dairne Ryan of Fix Our Ferals, another cat rescue group, challenges it. "You can find studies that counter that," she said. "We would all like to see better statistics, but feral cats defy statistics."

Many birds are killed not by feral cats but by pet cats whose owners let them outside in the morning and bring them in at night, Ryan said.  

http://www.contracostatimes.com/mld/cctimes/living/science/14465894.htm?source=rss&channel=cctim es_science

Besides cats killing birds, our tall buildings kill them very efficiently.  They try to fly thru the glass windows that reflect the natural outside environment.  Our cars mow them down all the time on our roads.  Cars also kill lots of other animals that we just leave to rot by the side of the road.  We have hunters that think it is great sport to shoot them (including eagles).  We build subdivisions in their breeding areas so we can enjoy nature.  

Compared to all this, windmills are an absolute pleasure.  We worry about a couple of birds and ignore our wholsale destruction of them.