Winter is coming. In a hard freeze, your local wild birds can still find food. What they can't find is water that isn't frozen - and that can mean the difference between making it through winter, or not.
When temperatures drop, the puddles, creeks and birdbaths birds rely on lock up. Birds can eat snow, but melting it inside their bodies burns energy they can't spare in the cold. They still need to drink, and they need clean feathers to stay warm.
A birdbath heater fixes it, and now's the time to set one up, before you're chipping ice out of a frozen basin. Get a heated bath with a thermostat or a drop-in de-icer, and plug it into a GFCI outlet.
Never add antifreeze, glycerin or salt to keep the water open. Antifreeze is poison, and glycerin mats feathers so they can't hold heat.
One catch: on the coldest days, birds will soak themselves in a heated bath and fly off with their feathers freezing. People have found cardinals too iced up to fly. When it's bitterly cold, lay a few sticks or a grate across the top so birds can drink but can't bathe.
Keep it clean, and keep it full.