Grit is an important part of a bird's diet. Since they have no teeth, birds must ingest small, hard particles to grind seed and food in their gizzard (a sac-like organ lined with muscles that facilitate the grinding of food). These hard particles are known as grit, and can be supplied in the form of eggshells or oyster shells.
Grit is also a major source of calcium that is important to backyard birds during egg laying. Blue jays and crows often turn to nest robbing to get the needed calcium through eggshells. Though grit requirements differ for each species of bird, it is often in short supply – especially in winter.
Since eggshells on their own may go unrecognized and unused, be sure to mix them in the birdseed to ensure the birds consume it. Mix five to ten pounds of grit for every 100 pounds of birdseed. Purple martins in particular love eggshells and do not have any difficulty recognizing the shells. Simply place the eggshells at the base of the pole, and they will find them.