The rarest bird I’ve ever photographed? It’s most likely a Bittern. In fact, it’s very likely it was that same Bittern.
To be honest, I had no idea. There was a small crowd, but they were being unusually careful and quiet not to spook it. The Bittern was responding by calmly sitting right by the trail ignoring all the awed photographers.
Being a fairly novice birder, I asked what it was, and was told it was a Bittern and very unusual in this area. So I grabbed a couple shots then moved on so somebody else had a shot. Only one I’ve ever seen too! Since I only had a 200 mm at the time, I didn’t get the nice closeups you did. Since I had no clue it was there or even what it was, I can’t claim I worked for it.
The rarest bird I’ve actually worked to see is probably a Snowy Owl. My photos of it are clear enough to ID it, but not much more.