Thursday night’s “American Idol” ratings fell from 3.9 to 3.8 from last Thursday. It’s not a lot but it’s worth noting that on Wednesday, “AI” had 13.3 million viewers, and the next night they were down to 12.47 million. Why aren’t people coming back the second night? Do they need to tie Nicki Minaj to a treadmill heading toward a buzz saw? “AI” still takes the night by miles, but that doesn’t matter. They’re “Chasing Ice,” to borrow a phrase from the award winning documentary. Time lapse photography may reveal the erosion. “AI” actually benefited last night from “Big Bang Theory” and “Two and a Half Men” being reruns. Otherwise, the “AI” number would have been lower.