If you live in an area where the night-time temperatures dip below freezing regularly you will need to make sure your nectar feeder does not freeze. In areas where the nighttime temperatures only dip slightly below freezing your hummingbird nectar may not freeze as the sugar solution has a lower freezing point than plain water. However, it’s better not to have your hummingbirds drink very cold nectar; this can actually cold-stun them. For cold weather feeding, either bring the feeder indoors overnight when it gets cold and put it back outside first thing in the morning (hummingbirds need to feed as early as possible, especially when it’s cold, to keep their energy up) or you can hang an incandescent light bulb near the feeder. These bulbs give off enough heat to keep the feeder warm.
Full article: http://www.audubon.org/news/hummingbird-feeding-faqs