This is a good illutration of how their coloration combined with this posture helps them blend with their enviornment."Thin as a rail" refers to Rails (family Rallidae), not herons (family Ardeidae) such as this bittern.Clapper Rail:
What a beautiful bird, I like the plumage and the soft contrast light. This might be considered sacrilege, but you might try a version that clones out some of the intersecting reeds to emphasize the vertical posture of the bird.