Thank you all!
They are quite eye catching & I'm happy to say we have LOTS of them here! I looked them up & it seems their favorite host plant is the pawpaw:
http://garden.org/plants/searc...
and there is an entire field of it behind our house & down about 10 or 12 acres. The field is quite large; consisting of what I would guess must be around 15 acres so I assume that's where they are coming from.
I had never seen them before now & of course knew by the tail that they were swallowtails but had to look them up to determine which. That red underneath the wings flashes as they fly & we've really been enjoying watching them.