One of wind power's oft-cited drawbacks is that it kills birds and bats - and man, isn't that a depressing thought? Poor little flying sparrow scissored down by the rotating blades of man's consumptive nature manifested in reality? Or something?
For both land and off-shore wind turbines - the big industrial ones - the major concern is with birds and bats. Generally, only flying species seem to be affected, because at the ground level wind turbines just don't take up a lot of space. Lots of people graze livestock like cattle on wind farm land (and apparently they like it - they move their grazing throughout the day to be in the shadow from the turbines!). Same story with off-shore; the construction might spook some fish, but the concrete base and metal pole are so inert they probably won't have a huge effect on local water animals.
So that leaves the ones that can fly into the whirling blades of death.
Generally-speaking, trying to measure a number of bird/bat kills at wind turbines seems to be heavily influenced by local geography, luck, weather, and whether or not you're for or against wind technology in general. For example:
The folks at windaction.org (an anti-wind power site) put together a report based on a paper that says that some European wind farms have had "mortality rates as high as 103 to 309 birds/turbine/year". Reading an abstract of the paper they're basing this off of, though, it turns out that the actual range of bird kills is not 103 to 309, but "respectively 2.8-103 and 8.5-309" for two different locations. Clearly, it's hard to estimate how turbines will affect birds.
Bats might be a greater problem - it seems that the acoustics of industrial wind turbines can mess with a bat's sense of echo-location. The American Wind Energy Association (AWEA) notes that bat deaths were found to be higher than previously understood in 2003, and that the industry is working to fix it - possibly through the use of sound alarms that alert bats to the danger. Whether or not that'll work, who knows?
The AWEA also notes that even if we take the upper numbers of bird deaths for wind turbines, the number of flying creatures killed is magnitudes lower than the number killed by tall buildings, airplanes, and human-owned cats (really). Their numbers - probably on the low end, since they're an advocacy site - are that less than 1 in 100,000 of human-caused bird deaths are caused by wind power sites. Anti-wind people counter that in-city bird kills are of common species while wind sites are sometimes situated in the path of more-in-danger migratory birds - though that's based off one dude's anti-establishment paper, and seems a little suspect.
In the end? Wildlife deaths have become a bigger part of the conception of wind power because it has such an image attached to it - and because wind power's other problems are so variable. Because wind power is so open-ended in terms of where we put it, we feel compelled to latch onto a generalized draw-back to be fair.
In the end, pollution and smog has probably killed plenty of flying creatures - as well as our cities, our windows, and our cats; never mind the larger, indirect costs of habitat destruction (especially of wetlands). The world isn't ideal; there's no reason to expect wind power to be either, in regards to wildlife.
Not to say we shouldn't try to prevent animal deaths, though - bats can be really cute sometimes.