Ducks are less handled than most poultry, so a health check needs to be efficient and calm, and it starts, as with chickens, by watching. A healthy duck holds herself upright and buoyant, bathes enthusiastically, shakes water off in beads, and hurries to forage with the flock. The duck sitting apart on the bank, feathers rough, letting the others go to the water without her, has already given you your first finding before you lay a hand on her.
Watching swim, walk and waterproofing
Watch the duck on the water and on land. On water she should float level and high, with feathers that shed droplets; a duck sitting low, tail down, or coming out with soaked, stringy plumage has lost her waterproofing, which points back to preening, feather condition or general illness. Her preen gland, at the base of the tail, should be checked if feathers look poor. On land, watch the walk for limping, favouring one foot, or reluctance to keep up, since ducks hide leg trouble by sitting down and being left behind quietly. Look at her breathing while she is undisturbed: open-bill breathing or tail bobbing at rest is not normal for a duck and earns her a prompt, closer examination.
The gentle hands-on check
Handle the duck calmly, supporting her body and keeping wings secured to avoid injury. Check the eyes for brightness and any bubbling or discharge, and look at the nostrils on the bill, which should be clear and dry rather than crusted or running. Feel along the keel for condition and over the body for lumps, and part the feathers at the vent to confirm the area is clean and the skin unbroken. Run your eyes over the feathering for parasites, bare patches and broken feathers. Then turn your attention to the feet, the duck's most vulnerable point: lift each foot and inspect the underside for swelling, scabs, cuts or a hot, puffy pad, and check between the toes and along the legs for injury from wire, sharp shells or rough ground where she climbs out of the water.
While the duck is in your hands, also weigh her if you can do it without a struggle, using a simple hanging or kitchen scale and a soft bag or crate she can sit in calmly. Ducks change weight quietly, and a bird that feels lighter across the keel while her flockmates hold steady is giving you an early, objective warning that appetite at the feeder may be masking how little she is actually taking in. A short series of weights, even taken only at check time, turns that suspicion into a trend you can show a veterinarian instead of a worry you cannot prove.
Keeping track bird by bird
A duck that is off her food, breathing with effort, unable to walk, or sitting hunched and unresponsive should be brought into a clean, quiet pen with shallow water and given veterinary attention, the same day where breathing or mobility is involved. For the rest, quick checks pay off as records, because one duck in a lookalike flock is easy to confuse with another. Dated notes per bird, kept in KinsLe in the Pet Passport, let you prove that the small duck by the gate is the same one whose foot was puffy last month.