Wanted to post a follow up regarding the loss of my flock.
My DH and I have spent most of our spare time recently renovating the coop and run with an eye toward discouraging predators. When we rolled back the railroad cross ties that surround the outside, we discovered a veritable maze of tunnels under both the run and the coop. Some were small (2") and others were easily 4" in diameter. We have found 3 different types of scat. To sum up our thoughts, we think the tunnels were dug by voles/moles or something else just wanting to stay warm in winter (we use heat lamps in the coop) but are now being opportunistically used by a weasel or raccoon to kill the chickens.
We poured concrete into the tunnels and around the base of the run, covered with hardware cloth and weighted with railroad cross ties. Inside the coop, we laid hardware cloth and toenailed 2X4's around the edge. There isn't much we can do about the tunneling but they shouldn't be able to get into the coop or run now even if they can tunnel under them.
With the exception of two bags of quick concrete and one roll of hardware cloth, all items were scavenged from the farm.
Oh, and DH relented - I have a Blue Heeler puppy on the way!
I would welcome further suggestions from this awesome community.