Well, maybe "elusive" is a strong word to describe green herons (Butorides virescens); however, they certainly aren't as obvious as some of their counterparts. While great blue herons (Ardea herodias), little blue herons (Egretta caerulea), and great egrets (Ardea alba) are readily seen as they wade in shallow, open waters in search of prey, green herons tend to perch near shore and remain still. An interesting fact is that green herons are one of very few tool-using bird species. They may drop feathers or insects on the surface of the water to act as a lure to attract small fish close enough for capture.
Green herons are smaller than some of the other herons and egrets, and their color pattern provides pretty good camouflage against vegetation at the edges of rivers, lakes, and ponds. If you're focused on observing or photographing the larger, obvious wading birds, it's easy not to notice, or walk past, a green heron.
I had this exact experience a few weeks ago on an afternoon trip to a nearby river. Great egrets, little blue herons, and Canada geese were scattered about the river, and ospreys were flying overhead. As I was slowly and quielty making my way down river toward a little blue heron, I caught the slightest motion out of the corner of my eye. And there it was, a juvenile green heron no more than twenty feet in front of me. It blended in so nicely with the root ball of a downed tree on the edge of the water. Fortunately for me, this young bird was using the "if I don't move, she won't see me" method of evading notice, and it almost worked! It actually didn't seem bothered by the sounds of my camera's shutter, and after shooting an abundance of images, I swung wide, and navigated around it without disturbing its afternoon. The green heron carried on with its motionless fishing tactics, and I carried on with what proved to be a glorious afternoon of bird photography.