To add to the conversation about finches and salt, I have been told that
Soda Springs in Tuolumne Meadows (California) is a great place to see Pine
Grosbeaks and Crossbills. They apparently come down to pick at the minerals
on the ground. Per a website: "Soda Springs, located on the north side of
the meadow, is a naturally carbonated mineral spring. Pools of mineral water
bubble up from beneath the ground." (I can't find information on what
minerals..)

I have also seen Western Bluebirds pick at gravel and parking lots-- our
local bluebird box expert told us that bluebirds need minerals as part of
their diet.

Kris Olson, California