Muffins that taste good but develop long tunnels

My blueberry muffins taste good and rise nicely, but the inside has long vertical tunnels rather than an even, tender crumb. I mix the wet and dry ingredients by hand until I can't see flour, then fold in the berries. I may be stirring more than I realize. Is tunneling mostly a sign of overmixing, or can oven temperature and an overly wet batter cause the same thing?