I'm sure that if the bird didn't get instantly vaporised by the focusing mirrors (on a wind turbine? Hang on, what are we talking about again?) they would totally be able to measure these things.
But to answer your question, nowhere does the source say, or do I say, that wind turbines do not kill birds. "Not measurable" doesn't mean none killed. It merely means that the installations of wind turbines could not explain variances in local bird populations beyond natural fluctuations; whereas the installation of oil and gas production sites was observed to reduce local bird populations beyond what was expected from natural variance.
The point is, quite obviously, that if you do care about birds and bird habitats, you would likely want to transition away from oil and gas to wind turbines as much as possible. I believe that was clear from my quote. Unless you don't actually care about birds, but just like to hate on wind turbines? In which case, I would direct you away from Clemens and to Cervantes instead.
You do bring up a good point though, people like to use the "cats kill 4 billion birds each year, wind turbines only kill 1 million" statistic, in which case I would ask, "ok then, how many cats would you say a wind turbine is worth?"