I have not seen this since I have been at Wilkes but I have heard rumors of mayfly hatches so large that they shut down the Market Street bridge with their dying bodies.
This year the river-facing side of Cohen Science Center is covered in mayflies and there are a few birds eating them. I've only seen starlings out but I bet the tree swallows are having a feast.
The idea behind such massive numbers is that predators can't possibly eat all of them so at least some survive. I also think such massive emergences are also based on maximizing reproductive events or maybe development is so constrained that optimal conditions are the same for all.