By size, shape, neck length, and habitat, it keys out to be a green heron. But it is the wrong color and has a darker beak than green herons usually have. After much searching, the most likely ID is a dark form of green, striated, or green-backed heron. Some sources lump all those 3 into one species, Butorides striatus. I hope to see and photograph this individual again during different seasons. Any insights into the ID will be greatly appreciated!