Birds, Tires and Gravel

I've noticed an interesting thing - maybe you have too. The parking lot at the barn where my horses live is gravel, so my car and truck come home with bits of gravel wedged in their tires. And the small birds, particularly the sparrows, will get under the cars while they are parked in my driveway, or sit on top of the tires, and pick gravel pieces out of the treads. I presume they are doing this to ingest gravel for their crops.