If you had a hamburger out in 10F weather with snow, but also had a hamburger waiting for you inside your house at a mild 65F, which would you choose?
Hummingbirds are simple, they will move to the shortest distance needed to find food, while also avoiding energy sapping cold (more calories are needed to stay warm). So if you have cold winter, they are going to stay as long as they possibly can before being forced to move in order to stay energy efficient.
Here in the PNW like Jodelak said, we tend to see Anna's over wintering because of our mild winter weather.