Bitterns are specialized herons that live in marshes, hold territories, and sing. In all these respects they are more like rails than the other herons, who nest in colonies and rely more on lovely plumes than voice when it comes time to advertise. Its their habitat that has favored these adjustments.
The song of the American Bittern is often transliterated as oon-ka-choonk, but is also likened to the sound of a pump. After hearing it you may think of earthier similes. The recording above contains two songs, a faint one near the beginning, and a louder one around 0:20. To hear and see it you need to be ready for something really low, as in lower than 400 Hz. The moving sonograms from Ebird are one-size-fits-all, as far as I can tell. Otherwise I would set the maximum frequency to 1000 Hz (1 kHz). You will find the dark traces sliding along just above the blue bar at the bottom of the sonogram. They disappear behind the characters "kHZ" on the left margin.
This bird was recorded at Fern Ridge Lake, a fantastic birding spot just outside Eugene, Oregon. The soundscape is dominated by the many Marsh Wrens that nest in the marshes at the edge of the lake. Wintering geese also contribute. And a Pied-biled Grebe was singing. The other bittern recording (follow link to Ebird list) captured that bird singing at exactly the same time as the bittern, producing a really unusual effect.