Not sure why your picture didn't show,so I posted it again.
Anna's hummingbirds (and others) have a cold-weather survival mechanism called "torpor."
"Anna's Hummingbirds normally have a body temperature of around 107 degrees Fahrenheitthat's a scorching temperature for a human. When outside temperatures fall, Anna's and many other species of hummingbirds enter torpor. Their breathing and heart rate slow, and their body temperature can fall as low as 48 degrees Fahrenheit When the temperature warms, the hummingbirds can become active again in a few minutes."
"Anna's hummingbirds are the only hummingbirds to spend the winter in northern climates; they are able to do this as there are enough winter flowers and food to support them. During cold temperatures, Anna's Hummingbirds gradually gain weight during the day as they convert sugar to fat. In addition, hummingbirds with inadequate stores of body fat or insufficient plumage are able to survive periods of sub-freezing weather by lowering their metabolic rate and entering a state of torpor."
As far as I know there are no specific guidelines for over-Wintering a Anna's Hummingbird. I can tell you from experience that it is very satisfying to over-Winter a hummingbird in sub-freezing weather conditions;it's also a lot of work. Here is a picture of the feeder setup.The light was attached to a Sheppard's hook and the feeder was attached to the window. The whole thing was covered with plastic. The coldest temperature during that Winter was minus 4 Fahrenheit.
Besides that feeding setup,we changed the nectar solution to a 3 to 1, sugar to water, ratio. During the morning hours we would feed her a commercial mixture designed for hummingbirds in captivity,then we would switch to the 3 to 1 sugar water nectar mixture in the afternoon.