Big birds require larger shot and late seasons birds are wilder, smarter, bigger and more heavily feathered. It take larger shot to get through the feathers and into vitals.
Part of "bigger" simply = more mature. Birds of the year start to approach older ones in size. But part of the "bigger" for all late season birds--especially in cold country--is that they start putting on extra fat to make it through the winter. I've never seen wild rooster pheasants with as much fat as those we shot in mid-Dec in North Dakota. Under the right conditions (good cover and maybe some fresh snow), they will sometimes sit as tight as opening day birds. But if you're all bundled up, you're likely to be slower getting on them than you were back in October. Good reason to switch to somewhat heavier shot . . . although I've never killed a pheasant with anything larger than a #5.
