This one is an immature Striated Heron. The lack of a large yellow stripe down the side of the neck, combined with more streaking on the chest (with more white) and the motley pattern on the wings (rather than scalloping on an immature Bittern), indicates this. The posture is also much smaller and less hunch-necked than a Bittern, and the yellow-green legs are more suited to Striated Heron.