When you examine the duck does the leg obviously appear to be broken (rather than just strained)? Can you tell where the break is? Does the lower half of the leg seem like it still has blood circulating (e.g. is it somewhat warm, or does it seem totally cold and dead)? Is there any sign of infection (any swelling or a bad smell)? If the leg seems cold and dead or there is infection then it is a job for a vet.
But if you are sure your duck's leg is broken, rather than just strained, and there seems to be blood circulation and no infection and the break is recent, then perhaps it can be splinted and set.
Here's what Storey's Guide to Raising Ducks says about broken bones:
- treat broken bones within 24 hours after the accident (how long has your duck's leg been like this for?)
- gently pull apart the broken parts of the leg and align them in the normal position (sounds like this would hurt so I'd have someone hold the duck)
- Splint the leg using something straight and rigid - like two popsicle sticks. Put the supports on either side of the leg so they extend above and below the fracture. Hold them securely in place with strong tape (perhaps masking tape) wrapped around the popsicle sticks and the leg
- check the patient regularly to make sure the splint is staying in place and that blood is still circulating to the foot (feet the foot to see it's warm but not unusually hot compared to the other foot)
- splints can normally be removed in 14 to 28 days