Estimates of up to a million or more birds a year are killed by turbines in the US but that is far exceeded by collisions with communications towers (6.5 million); power lines, (25 million); windows (up to 1 billion); and cats (1.3 to 4.0 billion) and those lost due to habitat loss, pollution and climate change (American Bird Conservancy, Nature). Even if there were twenty times more wind turbines, enough to supply the US with electricity, the number of birds killed, assuming no improvement in wind turbine design, would be about 10 million--still far less than most other causes of bird deaths.
While the toll from turbines is far from a major cause of bird mortality, it's not negligible and will increase as more wind turbines are operating, so the American Bird Conservancy and wind energy experts are working to reduce the rate (e.g. see No-blade wind turbines). For information on reducing bird mortality visit the American Bird Conservancy site.
Wind turbines can be removed when something better comes along. The habitat destruction and pollution from coal, oil and gas extraction and burning, building of pipelines and other infrastructure plus negligence and accidents is very difficult, even impossible in many cases, to undo.
For the sake of birds, the environment and nature's beauty wind and solar power are a vastly better choice than coal, oil and nuclear energy.

Mountain top removal (MTR) mining for coal lays waste to Appalachia. Wind turbines could prevent this.
More information:
GENERAL
- Sierra Club Wind Siting Advisory
- No-blade wind turbines, Sierra Magazine
- Michigan's Clean Energy Future
- OHVEC.org
- ILoveMountains.org
BATS
- Bat Fatalities at Wind Turbines—Investigating the Causes and Consequences, USGS
- Bats & Wind Energy, Bat Conservation International
BIRDS
- Wind Energy and Birds FAQ - American Bird Conservancy
- 8 ways wind power companies are trying to prevent deadly collisions, Grist