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[Summer_]

You can see it standing on one leg here

 

[Summer_]

Are you able to record a video of your duck showing the symptoms?
You can upload the video to YouTube (and some other platforms) and then embed it here.
Being unable to preen sounds more like a neck issue than a leg issue.

Got the vid

 

[Summer_]

The black stuff is dirt . It seems happy in the pool

 

[BelovedBirds]

The black stuff is dirt . It seems happy in the poolView attachment 3166550View attachment 3166551

have you stopped with the treats?

You can find good duck food online if not in store. How old are these ducks again?
Have a feel of both legs, check for heat and swelling. Then have a quick check of the undersides of their feet, check for lumps and cuts. Any leg issues, it's usually advisable to start supplementing niacin. I would get some B complex (durvet-cattle injectable) and start giving 1ml on a small treat every day.
@Miss Lydia @Quatie

 

[WannaBeHillBilly]

Reader discretion: I do not sugar-coat things, so continue to read without feeling offended.​

1. He (yes you have a drake) looks very overweight.
2. Deducting from his plumage, he is also malnourished.

For the weight problem: Swimming is the best exercise as it puts no weight on his legs and hips and the resistance of the water is a good training for his muscles.

Urgently change his diet to a commercially produced water-fowl feed. If you have a farm-store in your area, they usually have a house-brand where a bag of 50lbs will cost you between 10 and 15 benjamins. 50lbs will last a long time for two ducks. The house brand of layer-pellets or -crumbles will do. Don't feed anything that says meatbird... yuo already have a meatbird.
If you want to feed treats, use them as motivation: Sit down away from your ducks and shake the treats-container and have them run towards you. Then feed some treats, move away and repeat. This also works for swimming training.
I also would give him an otc multi-vitamin pill every day for at least two weeks in addition to the B-Vitamins. If you don't know how to insert a pill into a duck, read my instructions here:
Oh, if you have some left over antibiotics that are not too old, a single dose might be helpful too. Maybe he has an infection in his digestive-system too.

 

[Summer_]

have you stopped with the treats?

You can find good duck food online if not in store. How old are these ducks again?
Have a feel of both legs, check for heat and swelling. Then have a quick check of the undersides of their feet, check for lumps and cuts. Any leg issues, it's usually advisable to start supplementing niacin. I would get some B complex (durvet-cattle injectable) and start giving 1ml on a small treat every day.
@Miss Lydia @Quatie

I’ve stopped the treats, they are around 4 months old. The legs seem Normal, no extra heat or swelling, no cuts or lumps. I’ve been feeding niacin and b complex every now and then on a FEW (I mean few) peas.

 

[Miss Lydia]

https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/ in case you haven't seen this.

 

[Summer_]

Reader discretion: I do not sugar-coat things, so continue to read without feeling offended.​

1. He (yes you have a drake) looks very overweight.
2. Deducting from his plumage, he is also malnourished.

For the weight problem: Swimming is the best exercise as it puts no weight on his legs and hips and the resistance of the water is a good training for his muscles.

Urgently change his diet to a commercially produced water-fowl feed. If you have a farm-store in your area, they usually have a house-brand where a bag of 50lbs will cost you between 10 and 15 benjamins. 50lbs will last a long time for two ducks. The house brand of layer-pellets or -crumbles will do. Don't feed anything that says meatbird... yuo already have a meatbird.
If you want to feed treats, use them as motivation: Sit down away from your ducks and shake the treats-container and have them run towards you. Then feed some treats, move away and repeat. This also works for swimming training.
I also would give him an otc multi-vitamin pill every day for at least two weeks in addition to the B-Vitamins. If you don't know how to insert a pill into a duck, read my instructions here:
Oh, if you have some left over antibiotics that are not too old, a single dose might be helpful too. Maybe he has an infection in his digestive-system too.

They’ve been swimming for a bit, and it’s usually ok in the morning. They ran around their coop to go peck at grass, but then after a bit it started doing that again.

 

[Summer_]

We’re currently looking for a vet nearby that will attend to ducks, how much should I expect it to cost If I find one?

 

[Luv Ducks]

We’re currently looking for a vet nearby that will attend to ducks, how much should I expect it to cost If I find one?

I would call the vet and find out what their office visits and exams cost. In our part of the country, Ohio, location has a lot to do with it and the cost varies greatly. We have no one willing to look at ducks however as they consider them farm animals or exiotic pets. Good luck.