Hi All,
This is an interesting case of one of the those species we see all of the time in an odd plumage. It is not, strictly speaking, a juvenile, so it has lost a lot of the features used to differentiate the two species at that age. It is also not yet old enough to really exhibit any strong adult-like characteristics. One thing that really strikes me about this bird is its structure, particularly that of the bill. Night herons all have rather robust bills, but these two differ subtly in the shape of the bill. To my eye, the mandible of the mystery bird is not swollen and recurved enough to be a Yellow-crowned NH. It has more of an even tapered look, and the bill overall seems to have a slight droop near the tip, as it typical of Black-crowned. Everyone should google a bunch of night-heron images looking at the differences in shape. FWIW, the coloration of the bill is tricky at this age. I can see what looks like pale on the mandible and along the cutting edge of the maxilla. While not as bright as on juveniles, the soft part colors of herons change in the same manner they do in gulls. While not in itself conclusive, the wing looks rather uniform to me. I don't see any suggestion of darker flight feathers (a feature of Yellow-crowned), and none of the greater secondary coverts seem to have any suggestion of paler edges (another feature of Yellow-crowned). To my eyes, the bird just has the shape of an alert Black-crowned Night-Heron.
Chris