I've found that there are two things that make birds roost where they do. air temp and sequrity. Weather has a lot to do with it. Early in the seasons, at least here in the northern states, birds tend to roost near the top of the ridges, because the air temps are warmer than it is in the hollows, as the days start to warm up the birds roost lower down the ridges, and if there is a creek or river close by they roost as close to that as they can because the air temp around water will tend to be a bit cooler as the days warm up. Preasured birds a lot of times will roost above water for safty reasons, but even on warm nights they may be closest to the thickest cover they can find away from the water, again for a safty reason.