Squirrels with white tails

These odd looking creatures have been visiting our yard alot this summer.  There are two large red squirrels and two small ground squirrels with these fancy white tails and I haven't been able to find out exactly why they all appeared at the same time.  From what I read on the internet white tails are due to a mutation ~ it just seems odd that these four squirrels would show up at the same time.  Must be global warming..... {grin}