30 - 40 ducks sounds like way too many to be housed in that size of a space, especially if the geese, Rouens and Muscovies are in there also.
I guess I should clarify that we use two separate canopies. The Rouens and Muscovies have their own (although the khakis and baby geese sometimes sneak in there with them). Also, most of the birds are under 2 months old (a few are 3 months old) and they are only in there at night to sleep.
