Gritty details about attracting winter birds

Goldfinches love nyger seed

I shovel off a space to allow birds access to the sand

Our friends tell us that we have more birds coming to our feeders than they do and wonder why.
One reason might be the attraction in front of our tractor shed. When we built the shed a few years ago we had a couple of dump truck loads of coarse sand delivered. The sand was used as the base for the shed and also for a drive that leads up to the door. It seems ideal as bird grit. Each morning all the birds flock to an area I keep cleared of snow and pick up grit before visiting the bird feeders. Interestingly the list of birds visiting the sand each day includes a couple of ruffed grouse.
I suppose this bird feeding tip only applies to areas like ours that have continuous snow cover in the winter.
There is sand at intersections of roads too, but the closest spot is about 300 yards away from our feeders.
Speaking of birds, our chocolate Lab, Cork, seems to have gotten the wrong idea about what it is to be a bird dog. Every time he goes outside he beetles over to beneath the feeders and cleans up any seeds or seed hulls that have fallen to the ground. The seeds seem to have a laxative effect on him which means every hour he needs to hurriedly go back outside. Then while he is out there it dawns on him there might be more seeds to gobble. And the cycle is repeated.

What else would you expect a bird dog to eat?