I compared this call to American Bittern call found on Stokes Field Guide to Bird Songs: Eastern Region (CD). Sounds like a pretty good match. Also, when looking at a spectrograph of an American Bittern flight call they "pleateau" at a low frequency whereas Great Blue Heron and Black-crowned Night-Heron calls peak at higher frequencies. Sometimes (if bird was near enough to microphone) Great Blue and Black-crowned calls will peak high enough to get picked up by the Thrush call detector program from Old Bird. However, a lot of heron calls I have recorded remain unidentified because they were too distant and do not show the high frequency peak. The call above is strong enough that had it been another type of heron it should have gone above 2 kHz.