Hi Tom,
If you are feeding whole grain food or weeds with seeds, then it could certainly add to the seed bank. While I used chickens to prep one of my gardens (and will hopefully again), it is not perfect and there are some things to keep in mind.
Justin Rhodes used to use his chickens to do that, and has switched over to just tarping because he has better success for the amount of work. I personally don't like the idea of tarping, but you can't argue with the results. When I put the chickens in for a short while they knocked things down and scratched it up... except the big weeds. Those were still there. If I moved the chickens at that point, I would still need to go in and remove all the large weeds. I left them long
enough to even up
root the big weeds and get all the green stuff out (scorched earth style). I was left with a bug free, weed free, well fertilized... and yet extremely hard packed soil. I had to go through and chop it up with a hoe in order to plant in it. I think it was also a bit hot because the manure only sat for a couple weeks. I think if I could have covered it over with
wood chips and let it sit for 2 months, it probably would have been great loose soil underneath, but because of my time frame it just changed the work I was doing. Instead of weeding, I was chopping dirt.
Would I do it again? Yes. Would I be more careful about my timing and expectations? Yes. Would a pig provide some of the same or better services? Probably. Would tilling it up, waiting 2 or 3 weeks and tilling it again also provide a prepped bed? Yes.
There are many different ways to do things, we just need to know what will best fit out timeframe, lifestyle, and beliefs.