Even in winter birds need water to drink and to keep their feathers clean. A birdbath with clean water will attract many birds if the water is not frozen. Commercial immersion heaters will keep the water in birdbaths from freezing. They are available from many nurseries or bird supply stores. Finally, cover includes shelter from harsh weather and enemies.
Some birds prefer to take cover in evergreen groves, while others prefer dense, twiggy shrubs.
One simple way to provide additional winter cover is to put your Christmas tree out in the back yard after the holidays.