Anna hummingbirds often winter over in the Pacific Northwest so we keep our feeders out throughout the winter. Keeping the nectar from being frozen can be difficult during cold snaps such as the one we're experiencing now. So we devised a heater - nothing more than an inexpensive cup heater (20 watts) in a cookie tin. It's not very pretty but it's functional. We've had as many as on and around the feeder at the same time.