The Green Heron is a very small, stocky heron (the size of a crow), with comparatively short legs. This heron most often is seen with its neck coiled up (as in your photo), giving the impression of having no neck. Because of its short legs, the Green Heron generally hunts from a perch (log or branch). The Tricolored Heron is a medium-sized, slender heron with very long legs, a much longer bill, and a much more slender neck than the Green Heron. A Tricolored Heron's neck is so long it could never coil it so as to appear neckless (as with the Green Heron in your photo). The long-legged Tricolored Heron generally hunts by wading into the water. The immature/juvenile plumage of both species is different from the adult plumage, and while an immature Tricolored Heron's coloring superficially resembles a Green Heron's, the two species are not often confused with each other because their sizes, shapes and behaviors are so different.
Here's a Green Heron in classic "no neck" pose:
Here's a Green Heron hunting, first with neck coiled and then fully extended:
And here's a Green Heron in flight:
For comparison, here's an adult Tricolored Heron:
A Tricolored Heron sometimes holds its wings partly open while hunting:
