Can we have a better answer that won't effect all our sceneries?
The complexity slider is there for precisely that reason, to selectively control the complexity of a scenery and exclude some objects.
It's just wrong expecting to set it once and never move it again because the overall complexity depends on the scenery so, for example, "Extremely Dense" in the middle of nowhere can be used without any fps impact, but "Dense" in New York city might be too much. Which is why, the slider should be actively used depending on the scenery.
At ENBR, the Slider can be used exactly as it supposed to be used: to turn off unwanted items, in this case birds (but not only).