Yernagates said:
I can only think of two possibilities.
Just thought of another...
I've been looking at the dung of my own poultry, and having vague recollections of bird biology.
As I remember (some?) birds can produce two kinds of droppings. One is from the caecum, a wide, dead-end part of the gut. You notice these particularly in domestic hens when a sitting bird comes off the nest for a dropping -- she's been saving it up, and often produces an enormous roundish pile of stuff, smelling even viler than chicken dung usually does.
However, droppings can also come more directly from the main intestine, when they are more extruded in shape. My chickens' droppings do often seem to have a bigger round lump underneath, with thinner stuff on top (not nearly as thin as the mystery one though).
The white part is of course urine, produced in birds (as in most reptiles, spiders and some desert mammals) as a paste of uric acid crystals. The urinary tract has a common opening with the gut (the cloaca), and so the urine is passed with the dung. I think it tends to come out first, so perhaps ending up mainly on the larger, caecal dropping.
So I wonder if the mystery dropping is all one bird, of a caecal dropping with fine-gut stuff on the top. So we just have to find a bird with a largish caecum and lots of smaller gut. I think not a carnivore, which tends to have a wide-bore, straight-through "sports" gut.
Richard