Regardless of whether vermin can get into it or not, isn't having a full receptacle of food sitting around just attracting them by the smell? Also if your hens, ducks etc are leaving spilled food on the ground then they are being fed too much. The general rule of thumb for feeding poultry is only to give as much as they will clear up in 10 minutes or so, so there is no wastage and nothing to attract rats or pigeons etc.