First off, let me say that one of my biggest mistakes in my early hunting career was thinking that the bottoms were the place to be at flydown. I had heard turkeys needed to find water, so I always set up in the bottom next to a creek. I would spend the rest of the morning chasing the birds up the hill.
I finally decided to start at the top. We're in the Trans-Bluegrass of Kentucky. We have about 100-200 feet of elevation difference between the creek bottom and the top of the ridge. The turkeys roost about half-way down the sides of the ridges. They pitch down to the uphill side most days and then work their way up to feed. I fill my tags most years either getting them coming to me off the roost or waiting until 9-11 for them to appear in the pastures to feed.
When its cold, especially like it can be in the early season, the coldest air is at the bottom of the ridge. That pocket of cold air can stay until mid-morning. The turkeys want warmer drier air first thing, so they tend to gravitate to the top of the hill and catch the sun. Down in the bottom it can be 8 F colder than the top. Believe me, I sat freezing my butt off enough mornings to know. What's more, it'll tend to fog up in the bottoms and the fog will stay longer. All this is not to a turkey's liking.