There is some work to maintaining a birdbath, but it is well worth any effort for all the hours of enjoyment you receive from watching the birds, as well as the benefits the birds derive from a fresh source of water.
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Cleaning
Change the water every few days (2-4 days are typical times), if it is not re-circulated through a filter system. A simple flush with a garden hose is all it takes. The amount of use by the birds will determine how often the water needs to be changed. Frequent changes minimize pollution from bathing birds, will prevent mosquito larvae from developing and will slow the growth of algae.
The bath should be scrubbed at least once a week with a brush to keep algae at bay. There are products (offered by companies like Carefree Enzymes Inc. and others) that provide bird-safe water treatment to help control the growth of algae.
If algae growth becomes a problem, use a 10% bleach and water solution to clean the bath. Be sure to rinse thoroughly after cleaning with a bleach solution. A 50-50 mix of vinegar and water can also help remove algae.
If the design of your birdbath supports it, a small pump or re-circulation system is a good idea. A filter arrangement helps keep the water fresh and moving water will control the development of mosquito larvae.
Cold weather use
Several days of freezing weather can limit the availability of free water. A heated birdbath can be just the ticket for a cold, thirsty bird. A variety of heaters are available for birdbaths.
Some birdbath designs include a built-in heater. Heaters require electricity. Be sure to use an electrical circuit with a ground fault interrupter.
If you do not use a heater, be sure to drain the water from your birdbath, hoses and pumps. Do not add glycerin or other additives to the water to prevent it from freezing.
West Nile Virus
West Nile Virus is spread by the bite of a mosquito. You can not contract the disease by standing next to a birdbath. Refreshing the water in the birdbath every few days will prevent any mosquito larvae from maturing.