Thats funny actually...
I had used some styro foam board in the past as a mat to lay the tarp on....and they found that and ate styrofoam till they crapped packing peanuts. I saw how much styrofoam they consumed and just knew that I had 2 dead ducks! I figured that much foam would plug them...
I recently discovered a reletively new but fairly expensive "feed" for the indoor ducks...
Carpet.
The little terrorist have been pulling the tarp back and exposing the carpet underneath and EATING it...in a couple spots. They are smart enough to cover their handy work and not make it too...