Peregrines don't rely on gravel like seed-eaters but its not unusual to see them eating small pebbles. And in most cases they aren't actually eating them, just picking them up and rolling them around - its like babies (as I understand it), they put things in their mouths to learn about them ...
As for food, they eat flesh and internal organs. Bits of bones, feathers and other undigestible bits are regurgitated as pellets. But they don't set out to eat bones, feathers etc, they just do, i.e., small downy feathers are easier to eat than to try to pluck. When they eat bugs like grasshoppers, their pellets are full of the carapaces, kinda cool actually.
