Presented a wide view above of this spectrogram to illustrate how I find American Bittern, cuckoo and other low frequency calling birds. The call detector software programs provided by Old Bird do not search the lowest frequencies and thus will miss these calls.  I use Ravenlite to search a soundfile minute by minute looking for little dark slivers that stand out from the background noise.

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 undentified 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.