So I was doing a little bit of cleaning up in my front walkway, when I found this:
It's some kind of structure made out of mud, with some pieces of straw sticking out of it. I use straw as mulch in my garden, so there is a lot of it kicking around for something to use if they were so inclined.
Under the mud cap, there is definitely an open space.. It sort of reminds me of crayfish chimney, but I have never heard of them sticking straw inside the opening like that. Also they tend to be more cylindrical.