Niacin is the headline difference
Chickens and ducks look like close cousins at the feeder, but their vitamin needs part company over niacin, one of the B vitamins. Ducks need considerably more of it than chickens do, and chicken feeds — chick crumb, grower and layer alike — are blended to the chicken's lower requirement. A duck can eat its fill of chicken feed every day and still run short of this one nutrient. That is why mixed-flock keepers so often meet the problem without expecting it: the feed is good feed, honestly fed, and wrong for the species eating it in this single respect.
Ducklings feel the shortfall first
Growing birds build leg bone and tendon at speed, and niacin is part of that construction, so a deficiency declares itself in the legs of ducklings within their first weeks. The signs are a duckling that sits back, walks wobbling or bow-legged, grows more slowly than its hatch mates, or starts avoiding walking to the feeder at all. Caught early, correcting the diet can turn a young bird around, because the tissue is still growing; left alone, the deformity sets. Any duckling whose legs are clearly failing deserves a vet's eyes promptly — diet is the likely story, but joints and infections can look similar, and a young bird has little time to spare.
The practical ways to close the gap
The simplest answer is a purpose-made waterfowl feed, blended with duck levels of niacin from the start, for ducklings and adults alike. Where ducks share a flock and a feeder with hens, many keepers stay on chicken feed and add the niacin on top: brewer's yeast sprinkled over the daily feed is the long-standing method, since it is rich in niacin and ducks take to it readily. Whichever route you choose, make it the routine rather than an occasional gesture — a deficient bird needs the correction in every day's eating, and healthy ducks come to no harm on the duck-correct ration.
Ducks also eat differently: water is part of the meal
A duck washes food down and clears its nostrils and eyes by dunking, so feed without water beside it is only half a meal. Ducks need water deep enough to submerge the whole head, refreshed often, next to every feeder — not a chicken drinker they can only sip from. Ducklings need the same access with a safeguard: shallow enough at the edge, or with stones and a ramp, that a small bird can always climb out, since a soaked, chilled duckling fades fast. Expect the area around duck water to be permanently muddy, and site feeders on ground or matting you can cope with that on.
Watch the layer-feed trap with young ducks
One more chicken-feed mismatch catches people out: layer feed's high calcium is blended for hens in lay. Growing ducklings and drakes eating it long-term are taking far more calcium than their kidneys are built for. The tidy arrangement in a mixed flock is a grower or all-flock ration as the shared base — with the ducks' niacin added — and the laying hens' calcium offered separately as shell grit they can take free-choice, rather than baked into everyone's dinner.
Noting the flock's feed, the niacin top-up and the water routine as the ducks' duty in KinsLe's Care Teams means a relief feeder gives the ducks their duck feeding, not the hens' routine by mistake.