Some birds will do anything for a little grit.
Birds have no teeth for crushing food items, so they utilize small, hard pieces of stone, eggshell, or sand as an abrasive digestive helpers. This is known as grit. Grit is consumed and housed in the bird's crop where it helps to break up food. Grit is a commonly consumed staple especially for birds that eat lots of seeds or insects with hard exoskeletons.
We see lots of examples of birds eating grit.
Our barn swallows spend most summer afternoons sitting on the front yard powerline, waiting, like vultures at an abattoir, for us to throw some eggshells onto the sidewalk and garage roof. I've seen evening grosbeaks hit along winter roadsides where they were ingesting small bits of rock salt and cinders. I've seen purple martins, aerial insect eaters, scarfing up eggshells from feeding platforms during the breeding season. We've even published articles in BWD about paint-eating blue jays! All of these birds are after grit to aid their food processing.
As a kid, in the early 1970s, I often noticed house sparrows clinging to the brick foundation of the neighbors' house and I wondered what was up. It was years later that I figured out that they were getting dietary grit from the sand in the mortar between the bricks. The old mortar was just soft enough for the hefty-billed sparrows to nibble away at it. If this went on forever, in theory, all the mortar would be consumed and the house would tumble to the ground. But the neighborhood cats and the resident sharp-shinned hawk probably kept the house sparrows in check, so the house is still standing all these years later.
And how cool is that?
If you look closely you can see a male house sparrow getting his grit, slowly demolishing this house one nibble at a time.