After giving antibiotics to my ducks my avian veterinarian recommends giving bird probiotics grown without dairy. I type in avian probiotics on Amazon and found a few options. I sprinkle the powder on my ducks favorite wet snack, tomatoes, and they gobble it up...
My ducks often have poop about that color, but they forage and eat grass. If they don't their poop is the color of their pellets. Can you send a photo to their vet that gave you the meds?