Pine shavings work for many, but this duckherd is allergic! Just an fyi, I am sure I am in the minority. I really like Wifezilla's small brooder setup. I just haven't figured out how to scale it for eleven six week olds who hatched way ahead of schedule . . . .
Anyway, I love the concept of a little e-booklet of normal duckling feces. Wow. There's a project!
When mine are nervous, theirs is more watery. If they have had water without food for a while, it is also more watery.
Often their waste is the consistency of damp play dough, sometimes more a thick, viscous liquid. The color is usually similar to what they recently ate - dark green for salad, golden brown for pellets, black for dirt, gray-green for peas. The odor is richer after eating fresh vegetables.
If it is very dark, and smells like iron, that would indicate to me internal bleeding, and something to work on right away. One duckling had viscous, dark brown feces the first couple of weeks. I was concerned, but everything else seemed fine, it did not spread to the others. So far, we still have everyone and they are all healthy.