I hate to see Iowa DNR putting money into stocking birds...  As has already been mentioned, the money should be put towards habitat.  It's best done right the first time, rather than playing with releasing pen-raised birds.
Predators can only be considered the culprit if there isn't sufficient habitat.  Sure, they still kill their fair share, but it's very minor with the correct habitat.  Birds need plenty of grassland space and dense thermal cover.  Just realize I'm not sticking up for birds of prey or varmints.