Good points, George.

And for the record, let's not forget the 5th & 6th "out-of-season,
out-of-range" hummingbirds that probably passed through MD at some point
during their visits:
   - the Black-chinned Hummingbird that visited the Mary Ripley Garden
along the National Mall in DC.
   - and the Green Violet-ear that was about 1/4 mi away from the MD
border in West Virginia.  

And you don't have to go too far afield to find others - Broad-billed
Hummingbird in New York, Violet-crowned Hummingbird in Virginia, and
Green-breasted Mango in North Carolina (and probably a few others that
I'm forgetting).

Good birding,
Paul Pisano
Arlington, VA