Currently there aren't that many wind turbines. I imagine the proposal is to have at least 100x the number to replace all of the fossil fuels. Imagine the planet covered with wind turbines.

If you look at the chart in the link, 100x-ing wind turbines, if it was linear, would still be a tiny number of bird deaths compared to other sources.

Well, we are trying to reduce the other numbers. 10 million in the US is still a big number. We are 4% of the world population. As per capita energy use increases throughout the world, we're going to generate a lot more clean energy.

http://world.bymap.org/ElectricityProduction.html

Is there a reason we need to kill tens of millions of additional birds every year so we can have "clean" energy?

10 million in the US is not a big number, relative to other sources of bird deaths. As long as it's insignificant, you'll have a hard time convincing people that it's significant, and should stand in the way of saving the planet.