It looks like a Black-crowned Night Heron. I'm not quite sure on the age from these photos, but I think it's a juvenile (maybe second summer, but certainly not adult, which are grey).
For me, the most useful feature here for identification is structure - the bird is quite bulky with a large head and heavy bill. Yellow Bittern would have a longer, slimmer bill and usually look quite slender in flight.
The second photo seems to show some dark on the rear side of the wing, which is a feature of Yellow Bittern, but I think that this is due to shadow - on the first photo it is clear that the underwing is brown without any black.
