13 birds should have this done in about a week to 2 weeks, especially if you pen them on the ground so they can focus their attentions....My current garden area is 10 feet by 25 feet and my 6 girls took piles of leaf litter and grass clippings and reduced it to a fine mulch in about 3 or 4 weeks. You have twice as many brids. They will scratch, peck and dig over this area all day everyday. No lawn can take that repeated punishment. Keep them penned over the area and they will turn it to a wasteland in short order. In early May we had deep litter Not so much anymore I'm ready to plant...I just gotta fence my birds OUT of the garden. That'll make 'em happy.