In spite of low overnight temperatures here (17 degrees at 7), the Rufous Hummingbird in my yard was feeding by 7 am this morning (1/22). 
 
 There is a heat lamp trained on her feeder right now.  
 
The FONZ website suggests that this hardy species can survive to -20 degrees C, which my arithmetic translates as -4 degrees F.  However, Bruce Peterjohn, who banded her last month, tells me that the other Rufous Hummingbirds in the Midatlantic area did not appear to survive last weeks' low temperatures.  (There had been  birds in Pa and one in Fairfax).  The Calliope in Va did survive a night of -4 degrees F.  
 
sue hamilton