Because birds have no teeth, they must break up hard food by grinding it in a muscular area of the throat called the crop or gizzard. To help that process along, birds swallow bits of sand and gravel to store there. These materials are called “grit”. Help your bird visitors by offering them grit such as crushed eggshells, builder’s or play sand, or bird gravel from a pet store, which is usually made of crushed oyster shells. Grit is important in winter because birds can’t scratch it out of snow-covered or frozen soil. Put it out separately on a cookie sheet or add it to the seed mix.
For more information about grit see the file Grit for Birds in Yardener’s Tool Shed.