Squirrel migration

Today I saw a fascinating squirrel. Its fur was a mixture of reddish underfur with longer black fur growing over it. Sort of like dogs that have shorter and longer hairs of different colors.

This is significant because in California, when I was growing up a few decades ago, there were no red squirrels (only black and gray). At college in the East, I saw grey and red squirrels, but not black. Are the red ones (or more precisely, their genes) finally getting here? And could I see black (-ish) squirrels on the East coast now?

It would be fun to see one of those maps showing migrations of animals, with a timetable for different types of squirrels spreading across the US.