I keep a hummingbird feeder outside my dining room window all winter long. To make nectar I put 1/4 cup sugar into 1 cup of water and warm in the microwave to dissolve the sugar. This mix lasts about a week before the hummingbirds drink it all up. This winter I made a shelter for my feeder to keep it from freezing in colder weather. I put a single red 7 watt Christmas bulb inside a 2 liter bottle and covered it with clear bubble wrap. It helps to keep the nectar from freezing down to about 20 degrees at night.
...once nearly had a Portland TV crew out to photograph our hummingbirds.. Reading all I could find, I definitely stopped feeding them in time to encourage their return migration South. They do that.
Now, even I have relatives feeding them all winter… Having lost ‘that argument,’ I’ve watched their methods to keep the ‘nectar’ from freezing. One, takes the feeder in at night, then places it out around daybreak, often with the HB’s waiting.. The other, attaches a ‘clamp lamp’ with the light aimed at the feeder, everything under a roof overhang, during the coldest days.
I’ve now suggested to both (actually linking the insulated feeder photo here) to increase the sugar ratio (as Jafar suggested above). In SW Virginia now, my neighbor was ‘stealing’ all my birds last summer -- as I learned, instead of the customary ¼ sugar to water mix - the neighbor had been using a ¾ to one sugar mix! I thought that would kill them ... but researching online, they can apparently handle it...
So, both my winter-feeding relatives have received the suggestion they increase their mix to half sugar (½ to one) during the winter - and monitor the point at which it freezes… And, I’ll assume the additional increase in calories will benefit the Hummers -- though I still refuse to feed them through winter 
Anna's hummingbirds are year-round residents here in Washington. The rufous hummingbird doesn't stay around all winter.
Washington Department of Wildlife website gives a lot of info about species in Washington state.
http://wdfw.wa.gov/living/hummingbirds/
Sugar/water ratio is discussed too: "Hummingbirds can get fatal hardening of the liver from eating a heavy sugar syrup. For that reason a solution that is no more than 1 part sugar to 4 parts water is recommended"
Won’t be changing my ¼ mix… but hadn’t noticed any deaths near my neighbors feeder with his ¾ mix.. I'll talk to him about it, though. I’d assume a ½ mix during colder periods would work … but then I’d not be feeding them through winter, but encouraging their return migration South.
Used to follow an online site that tracked their return up the West Coast every Spring… I know we had Rufous HB’s in the foothills of Oregon’s Coast Range. Maybe I can’t tell the difference between them and an Anna, but the ones I’ve watched at the coast looked identical, to me, to Rufus ...now they’re Ruby-throated, for me 
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