Apparently these YCNHs have been on the NIH campus for years. My dad, who works at NIH, showed me a few weeks ago the NIH Record, which had a tiny article about them. Dr. Eleanor Hoff has taken some good photos of them on the stream between MLP-10 and Wilson Drive, just about in the same area you described. She notes that last year she observed a juvenile.

Here is the article:

"NIDDK health science policy analyst Dr. Eleanor Hoff recently took photos of the yellow-crowned night herons that live along the NIH stream.
“They appear in mid-spring—I usually see them starting in April,” she reports. “Usually I see just one, rarely I see two at once. Last year, I also saw a juvenile. They are quiet and relatively shy, though they tolerate my presence. They have gray bodies, red eyes and a yellowish-white crown with a long skinny feather(s) that usually lies flat and back, but perks up like Alfalfa’s hair when they do something super active. Oh, and despite the name, they are active during the day–I see them at lunchtime.”"

I should also note that since these birds are on the NIH campus, I don't think the general public will be permitted to chase them.

Robert Ostrowski
Crofton, MD