With many of the chicken tractor designs, including my own (Highly Vented Chicken Tractor). a gap is formed between the bottom board and the ground due to the rise of the wheels. If the yard is flat and smooth then this can be kept to a minimum by the wheel placement. However, if your yard isn't smooth or has exposed roots, rocks, or other low obstacles then the wheels must be set as to raise the board high enough not to hang up as the tractor is maneuvered. Then we are back to the gap issue. Young birds may slip through, or other critters slip in - either is bad news.
My solution to this gap is cheap and simple. Go to your local Home Depot or Lowe's and pick up three four-foot lengths of the cheapest rubber cove molding (approx. $1.55 per 4ft. section), a number of fender washers with the small center hole, and some 1 inch screws. You want the cheapest cove molding because of its thin, flexible nature. With a set of scissors make cuts half-way through the cove mold about 1 inch apart. I made these from the curled edge. Then mount these strips low on the bottom boards of your tractor where there is a gap using the screws & fender washers, (see photos below). Now your gasp is sealed with a flexible skirt that allows for any obstacle in the yard!