I think they excluded birds colliding with the base as that was suspected to be unrelated to the actual turbine blades. Removing confounding variables is part o

I think they excluded birds colliding with the base as that was suspected to be unrelated to the actual turbine blades. Removing confounding variables is part of statistics and always up for debate but the article/paper did not measure the effects on birds colliding with wind turbine bases, just birds killed by turbine blades. Then again, a lot of bad research results have come from ignoring the "right" data points.