Here is a Canada forum ask here where to get poultry feed. I also asked a member who lives in Canada to help.
https://www.backyardchickens.com/threads/canadians-check-in-here.144/page-563
https://www.backyardchickens.com/articles/tips-tricks-for-raising-ducks-the-complete-essential-guide.76260/
Do you know the breed of duckling you have? Age? Pictures?
Bird seed isn’t going to give your duckling all the vitamins and nutrients it needs. Can you buy chick feed or waterfowl feed? Are you in the USA telling us what country can help us help you better. They also do better if they have a friend of the same species.
What you are seeing is what's called preening after they swim or get wet they use their bills to dry their feathers and spread oil over them to make themselves waterproof. It can look like they are hurting themselves but they arent.
I hope you are also giving your duckling feed that is made for...
I am so sorry your duck was injured. What breed and are you sure you saw all the bite wounds. When there is an injury it's best to clip the feathers away from the bite wounds so you can clean them real good and keep the feathers from getting into the wound when it's healing up.
Most likely when...