Are you talking about framing in walls that connect those front support posts to the back wall? Will these walls only be containing poultry? And, what will the wall covering be - chicken wire, hardware cloth, welded wire fence, etc?
Operating on the assumption that the answer to the first two questions is "yes", what I would do is not worry about concrete at all. If the rest of the building is anchored to the ground already, this is just a partition wall that will only support its own weight. What I would do is dig a narrow, shallow trench and fill it with gravel a couple inches deep, to help with drainage. Compact the gravel a bit if you can - tamping a cinder block on it is probably all you need to do. Lay pressure treated 4x4 posts down on top of the gravel for a bottom plate. Make sure the posts are rated for ground contact. Do not try to use pressure treated 2x4s for the bottom as they use a different treatment process that is not ground contact rated - they'll rot right out in quick order.
Drive 12" galvanized spikes through each post down into the ground - one at each end, and one in the middle - then frame your wall on top of it. Spikes are typically 3/8" so pre-drill for them if you can, to make the job easier. With those in there, it's not going anywhere. Alternatively, you could buy some 3/8" rebar and cut it into lenths of 12-18" and use those instead of spikes (if you have a metal cutting tool).
Finally, any time you're using pressure treated lumber, make sure you use galvanized nails - regular nails will corrode. Alternatively, you can use deck screws as they're designed to work with the PT wood.