The best study of this issue that I have read is "Lead poisoning as a Mortality factor in Waterfowl populations" F.C. Bellrose, 1961. It is available on line but is quite lengthy as it involved nationwide input and control experiments involving feeding raised Mallards lead shot in controlled doses. It addresses many of the questions expressed in this thread and is, in my opinion most informative. If someone would post a direct link to it that would be beneficial.
All the best!

Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745