You catch some good ones! Nice!
A pair to those visit us each year but they leave before it gets hot.
That's interesting. Cornell has an animated migration map which shows the movement of the indigo buntings in the US throughout the year.http://ebird.org/content/ebird/occurrence/indigo-bunting/?__hstc=75100365.7199823550d5fe9462e87888360d2a97.1439475825685.1463708358339.1463745356657.12&__hssc=75100365.2.1463745356657&__hsfp=3682891602#_ga=1.17808569.1654176499.1439475824The notes below the map note that the indigo buntings leave their wooded breeding areas to forage in fields from August through October (before then migrating south).
So it sounds as if you have a pair that breeds in your area then moves on to a more open area to forage when they finish breeding.