There are a couple different types of yeast used as nutritional supplements ... Brewer's Yeast, Nutritional Yeast, and then there are livestock feed-grade yeasts.
Here are links to the two "human grade" yeast supplements ...
Brewer's Yeast is often recommended as a supplement for ducks for the niacin ...
http://nutritiondata.self.com/facts/custom/1323569/2 You can see it is 55% protein. There are even some companies selling Brewer's Yeast with garlic (I think) for poultry.
Nutritional Yeast is often recommended to vegetarians for the B12 they add to it. It has more niacin than Brewer's Yeast, yet I have never received an answer when I question why it isn't recommended for ducks instead of Brewer's Yeast.
http://nutritiondata.self.com/facts/custom/1323565/2 You can see it is 71% protein. Before I started using the custom feed, I mixed Nutritional Yeast into our Fermented Feed just before it went into the troughs. I had the local big discount grocery store order it for me in bulk. I also use it pretty often as popcorn or pasta topper. It's kinda cheesy. Yum.
And here is a link to the Diamond V page that talks about the application of their feed grade yeast products for poultry. The have different strengths of yeast, with different protein concentrations, and some with selenium, which is supposed to be good for male fertility in breeding birds ...
http://www.diamondv.com/species/poultry-nutrition-and-health/ (We use either the Green or the Organic version for our custom feed, depending on which the mill has on hand/ordered.)