[We don't know how much birds kill the windmills so] "This makes it a weak argument against windmills"
"We don't know how much, so it may not happen, or is not so relevant as we think, but is repeated by ideology", is a nasty trick. Nice smoke curtain. Specially when is joined later with:
"it is true that all humanmade structures are technically bird killers, but..."
Either it happens, or it does not happen.
We aren't talking here about a sparrow crashing against a window. What we do know is that carcasses of big raptors, vultures, storks, other birds, and even bats can be found near the windmills basis often, in a distribution that is not aleatory.
Even if we never achieve to calculate an exact value (before the corpses are quietly removed by foxes or companies) we can identify that there is a problem here. As predators are scarce (by definition) and some are endangered, the impact on populations is not negligible.
Killing endangered species is illegal. It does not matter If is "just one" or "just a few" eagles.