I was jogging in Long Grove, IL yesterday (4/6/13) and saw two large birds standing in a field. They were "thin" but around 3 - 4' tall. They looked like a skinny version of an Ostrich (they were really tall). As I walked toward them they did take off in flight and shrieked a very loud "warbling" sound.
I have been Googling them but so far have not been able to identify. And they are obviously not Turkey Vultures, Bald Eagles, Herons or Egrets.