while most a lot of studies are estimates there has been some extensive studies with hard data.. A study done in Spain over a 3 year period in the Gibraltar area. (which is the location of a huge bird migration route..) They searched and counted for dead birds every day on a sample size of about 200 turbines, as well as observing and counting bird traffic at various times. They found that on average each turbine killed 1.33 birds per year..
Denmark also did a study on some turbines that where smack bang in the middle of a roosting to feeding ground route of pink footed geese and cranes where many thousands of birds flew through twice a day to feed then return home to roost. They found turbines killed less then 2 birds per annum..
Both these measured studies where conducted in extremely high bird traffic areas and there was stuff all kills.. Cars, cats and glass windows also dwarf all wind turbine kills..
And again i ask you this, why the concern with just birds? No concern of human deaths, let alone the millions and millions of fish, turtles, whales, dolphins and of course millions of birds killed from oil spills alone.
