Late summer is a beautiful time of year to enjoy herons. Sometimes, herons are seen flying over, at dusk, belting out their unique, throaty call as they pass overhead. Below I share sound clips of the flight calls of eastern NA herons. All audio clips are available via xenocanto. A written description, taken from those described in the Sibley Guide to the Birds of Eastern North America, of the call has been included.
Great Egret: http://www.xeno-canto.org/17012 and http://www.xeno-canto.org/131650. Very deep, low, gravelly kroow and grating, unmusical karrr. Other low croaks as well, all fading at end. Lower and coarser than Great Blue Heron, and without the trumpeting quality.
American Bittern: http://www.xeno-canto.org/338164. Loud, hoarse, nasal squark intermediate between night-herons and Mallard.
Least Bittern: http://www.xeno-canto.org/326521. Loud, harsh, nasally, quacking rick.
Sibley, D.A. 2014. The Sibley Guide to Birds. 2nd ed. New York: Alfred A. Knopf.