Go take a look at this site. Do a search for both birds. Note that it doesn't reflect immature birds.
http://sibley.enature.com/Sibley writes a small carry in the field and a large leave it in the car and use for reference. I think I believe that maybe for certain possibly that I'll probably stick to the American Bittern probably. Except the size is Green Heron, so probably it is maybe for possibly certain a Green Heron, immature. Also known as the "KWOK" bird because when it flies it utters a call that sounds like Kwok. But then I look again and say AMBI!
A GRHE would also come closer to the house if he saw a water puddle, or a pond with koi or little gold fish. AMBI would probably only do that if it was sick or a baby.
Come on Tina give us the correct answer!