Maybe its just me, but it seems the higher the density and activity of the birds the bigger the hatch. This is especially true for the hatches of really big bugs. I swear when the Salmon Flies are hatching you can decide where to stop and fish by the number of birds flying in the air.

Sometimes I'm a bit blind. I'll be just relaxing on the river and then I notice the birds a swooping and diving. And then I really take a close look at the water, because something is probably hatching.

Sometimes, I really feel a bit sorry for those poor bugs. I've seen hatches were the fish were attacking from below and the birds were attacking from above. The birds were not even waiting for the bugs to get airbourne. They were just grabbing them off the surface.