Simply put, the guy who made that asinine claim includes birds killed in the mining of uranium for nuclear power plant fuel. But that's not nuclear power generation; that's mining. And at the same time, the guy cherry-picks data to make it look like mining uranium kills a shitton of birds.
In order to make the comparison more apt, then, the guy should include the mining of materials used to build windmills. That means copper, rare-earth metals, and aluminum. How many birds die at those mines?
It's just more anti-nuclear propagandizing, and that's all it is.