That is an awesome amazing idea. Those birds will work that soil all winter, and lay down a nice nutrient rich fertilizer at the same time. In the spring you'll be able direct plant, you won't even need to turn the soil.
Do it...post pictures...and tell us about it.
I am definitely going to try it! And I will post pics this winter. I decided to make my nesting boxes like a shelf...where as I will attach them to the wall in the coop with sturdy shelf brackets, all in one piece so I can just unscrew them and move them to the back wall of the greenhouse! THANKS everyone for your input!!