Why is This Barrel Attached to a Tree?

I took a kayak trip at Lake Martin today and came across a barrel attached to a tree.

Not just one; we saw at least 3 barrels attached to different trees across the lake. I got a close look at two of them, and they were both fairly well rusted. I couldn't get close enough to the third to determine its condition.

My first thought was that the barrels were installed on the trees to serve as some sort of nesting area for local birds; but my second thought was a 180 from my first: hunting blinds.

Can anyone tell me what these barrels are (or, by the looks of them, were) used for?