Try putting water in the washer tank and cleaning your windshield in the middle of winter. The water could freeze instantly on contact with the glass, or it may just become a block of ice in the reservoir and damage the pump system. The recommended windshield wiper fluid has cleaning solvents that are designed to help dissolve contaminants on the windshield so they can be easily washed away.
Things like bird droppings or an abundance of dead bugs are no match for the better windshield wiper fluid. The reason that the windshield wiper fluid is recommended in winter is because it contains a small amount of antifreeze that allows it to stay in liquid form even as the temperatures outside drop below the freezing mark. The washer fluid can moisturize the rubber of the wiper blades so they don't crack in the cold. Come to Yarmon Ford Inc. right here in Paynesville, MN so we can replace the water in the tank with windshield wiper fluid.