How is your duck doing this morning?
In most cases, a good lavage of the wound using saline, or antiseptic wash is recommended, but that is contradicted in cases such as your bird where the fluids may end up getting inside her body. Instead, try to make a clear zone around the wound, either via trimming, or plucking the feathers to do so.
Once a small featherless zone around the wound has been established, you can gently wipe the wound with a soft damp towel to help lavage out debris or bacteria. A syringe equipped with a needle can be used to accurately flush the wound diverting the stream "away" from the major hole.
If you are able to pinch the skin together, you could either suture/staple it together, or use superglue (preferably derma bond) to seal the hole. If you are unable to do either, consider just applying a decent amount of antibacterial ointment such as Preparation H/ Silver Silvadene, and repeat as needed to keep the wound moist.
Bandages have endless benefits for wound healing, but given the location of the wound, it will be likely too hard to keep it in its correct place. Considering bacteria may have entered her bloodstream, you may contemplate getting some antibiotics fish/bird form for her. Those can be obtained online, without a prescription. In addition to what's suggested, attempt to keep her in a warm, stress-free area, away from flies. Pay close attention to how her feces look, and how much feed, and water she drinks.
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