Birds need water to drink and bathe in year 'round, just as do we humans. The first few nights below freezing remind me that it's time to dig out the bird bath de-icer and position it in the round plastic basin which is attached to the railing on our back deck. An extension cord plugs into an outlet on our screened porch, and the element turns on when temps dip below 32 degrees.
This simple apparatus cost me some $15, I suppose, and it's functioned well for years. In the off-season it lives in a kitchen drawer, near the cupboard where the thistle seed is kept.
I place a small rock on top to prevent dislodging in high winds, or by over-enthusiastic critters. Of course, the basin gets cleaned every few days to keep things safe for our avian friends.
Even after all these years, it's still a thrill to see finches, wrens, chickadees and other birds taking a drink of clear water in the depths of winter when most other sources are frozen.
Mother Nature appreciates every little bit of help.