When was the last time he ate a full meal on his own? Did the vet think he was underweight and how severely? Do his droppings look small and dark or neon green like this one below to the right?
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One of my ganders made that after refusing to eat for three days, “I didn’t have a feeding tube at the time.”
That color also can be caused by liver problems or lead poisoning but happens during starvation because the liver is starting to convert its own cells for energy or so I’ve heard.
I’m just trying to gage wether your duck needs to be tube fed now or if he can wait until later tonight/early tomorrow if he’s not in a completely dire situation from not eating.
You can always try making him a soupy mash from his feed, it might coax him to eat a little on his own.