If you use the right feed and don't diminish with low nutrient treats their SHOULDN'T be an issue with vitamin deficit. Well turkeys might could use more protein to start.
Too many people use the wrong feed for ducklings NEED more niacin than offered in chick starter. Purina flock raiser IS an acceptable choice for ALL the birds you mention. It has 55mg/kg of niacin to meet duckling needs... verses most chick starters have only around 35mg/kg.
I NEVER had to supplement my birds and also NEVER had issues YET. Only raised chickens and ducks so far. I know I am not immune from possibilities though! My birds are in good health and condition and I have fantastic hatches.
My birds are also always at different ages and one reason why I use flock raiser. I proved oyster shell on the side for layers free choice. It may not be PERFECT for every single bird. But it IS a very happy medium and SIMPLE to feed everyone the same thing.
I personally think people OVER treating their birds and diminishing nutrients IS the COMMON problem. Also MANY will have birds that succumb to Marek's and are blamed on vitamins. Denial is strong. Additionally... some birds may have GENETIC absorption issues. No weak links welcome here... since I breed for vigor... assuming I am NOT the one who caused the issue... say like overcrowding and causing cocci.
One other thing I have seen as a root cause for vitamin deficit is people having bad information about what to feed their birds. Some people have used ONLY scratch or wild bird seed. This is what their friend or neighbor told them. And feed store employees who usually have little to no animal experience of their own have been KNOWN to give out some of the most God awful advice... ALWAYS get a second opinion, since my understanding is that the road to Hell was paved with GOOD intentions.
9 years now... no vitamin deficit (yet). Over treating birds is killing them and diminishing their health... NOT spoiling them PEOPLE!
Keeping treats to NOT more than 10% of the total daily ration should be fine. Not all treats are created equal. Cracked corn is about 7% protein verses sprouted barley is about 14% for example. These birds are omnivores and the
amino acids in the protein is what's important. It cannot be completely gotten from veggie sources so is
added into our
formulated veggie based feeds that most of us use. So even though our feeds may be corn based it the additions that make it complete.
Anyways, animals nutrition is something I'm passionate about... If you do choose to supplement vitamins and minerals.. NOTHING should be supplemented more than 10 days in a row.
Regarding the gosling you lost. First, sorry.

But second... I hatch up a storm, and NOT all birds will make it regardless of the nutrients we offer. Even the ones who look perfect and seem fine until their yolk is used up. Sometimes there are internal issues we simply cannot detect. So I would NOT assume it was a deficit that caused the problem.
I hope this is helpful.
