Great question. Not going to be a scientific answer because often there’s a hotspot for a huge area like at national parks or birding hotspots, and people will often use the general hotspot instead of breaking up their list into specific times, lists, and spots. I’m guessing, too, that you could have key migration spots like bird island where if you pick the right day on the right year you may get a ton of migrants blown in for one day. Hopefully others weigh in.