Saline or salt water should be mixed 2 tsp (9 grams) of table salt to each liter or quart of water. That makes it isotonic, the same pH as the body, and it won’t damage tissue. At that same ratio, 1/2 tsp of salt to each cup (240ml of water) is equivalent. It can be squirted on the wound with a...
Some people use aspirin for chicken pain. It can be given 1/2 to 1 tablet low dose 81mg twice a day for up to 3 days. Meloxicam is by vet prescription only if one will prescribe it or see your duck. I have had some leftover from a dog surgery before.
My, that is a pretty deep wound. I am not familiar with ducks, but hopefully, @Miss Lydia or @Isaac 0 will have a look. I would get some Chlorhexidene 2% at your pharmacy to spray the wound twice a day. Vetericyn may also be used instead. Then apply plain Neosporin ointment afterward. It is...