Thanks to all that replied. I have been away from my PC.
My apologies to the list for my error. I was pulling from my memory and didn't have The Bluebird in front of me while writing.
Yes, it does state "Mississippi Lowlands", and I mis-remembered this fact, interjecting the state of Missouri. I also didn't realize that the Robins & Easterla resource was as old as it is, and that the Blue Grosbeak has expanded its range (presumably breeding range also).
I guess the hunt is on for me to locate and document nests and breeding activities. All right, now I have my summer project slated!