Natural Gas being deployed instead of solar or wind? Bad.
Natural Gas being deployed along side solar and wind? Best case.
One day the price of natural gas is going to rise again and people are going to be stuck with combined cycle natgas generators that are too expensive to run.
Once you get a certain distance you out of a birds normal range. Migration patterns are easy to track and plan for.
I agree that the effectiveness of the argument "cats kill more birds, so windmills can kill up to that amount" is quite limited - I wasn't the one who made that argument in the first place. It's effective only to the degree that it means if what we care about is reducing bird deaths, we might be able to make a bigger difference by looking at cats.
As it happens, though, I'm not convinced wind turbines do that much more harm than other methods of power generation. Mining for coal destroys habitat and generates pollution, &c... And wind turbines do not seem to kill all that many birds. The high end of estimates seems to be 600k (http://www.smithsonianmag.com/smart-news/how-many-birds-do-w...) in the US. Per https://www.awea.org/Resources/Content.aspx?ItemNumber=5059 the "total number of operating utility-scale wind turbines at the end of 2012" was more than 45,100. If we assume the (conservative) 600k is actually 1 million (even more conservative), that's just a little over 22 birds per turbine per year - most of which aren't the large birds you're concerned about. I've certainly no objection to considering flight paths when placing turbines, though.
Maybe switching to a somewhat cleaner source would slow down movement towards green sources, but it seems like using CH4 is an overall win given what it's a substitute for.