please don’t be to quick to judge me..
I'm sorry if I came off as judging, it wasn't my intention!
I've seen a lot of birds come in with debilitating injuries/conditions that otherwise we wouldn't have known about without internal examination. These birds are prey animals, so weakness and pain is something they can't show if they can help it.
We once had a broiler hen come in with a defective breastbone. It was likely that it was broken at some point as a chick, but it healed improperly and she grew a long calcified spike that poked directly toward her chest. It would have jabbed into her muscles every time she breathed. You'd have never known she had the injury because she never showed signs of pain other than the fact she never moved her left wing much, but there's no doubt that she was suffering from it
The only way to relieve the pain would be surgery to remove the spike, and the price of the surgery was far more than she would earn as a meat hen, so the decision was made to cull her.
Sometimes birds show no signs of illness because they're prey animals, and it's in their instinct to not become food. Often it's too late to treat them by the time they start showing symptoms. Should the duckling survive its first two weeks, an exam would be the easiest and most humane way to determine if the bird will have a good quality of life as it matures, or if it should be culled. (And again, many vets would do an exam for this duckling for free because it's such a rare occurrence!)