Birds need water, both for bathing and drinking. If there is no pond or other natural water source on your property, a bird bath or fountain can provide a necessary element for birds living in or near your property.
When it comes to providing a water source, shallow is preferable. Birds may attempt to drink from--or bathe in--deeper sources of water such as open rain barrels. As they can sometimes drown in those, I try to provide a safer alternative.
My birdbath recently sprang a leak, so I have to place a large plant saucer on top of it. Because birds are attracted to the sound of dripping, I then suspend a one-gallon plastic jug of water over that saucer. It hangs by its handle from a long S-hook that is attached to an overhead branch, actually the same S-hook I use for hanging the bird feeder during the winter. (In the summer, I expect my feathered freeloaders to earn their keep by eating bugs instead!)
After I poke a hole in the bottom corner of that jug with a large needle, it plinks drops into the saucer beneath it, letting the birds know that water is available there. The plastic sides of the jug will sometimes suck in as the H2O runs out, but you can prevent that emaciated look by poking another hole near the top.
It's best to cap the jug to keep out debris which can clog the drip-hole. The flow can get obstructed anyway, and you may need to tap on the jug or even reopen the hole with your needle.
Positioning the birdbath in the shade keeps the water cool and prevents it from evaporating quickly. Since birds fly somewhat drunkenly when wet, the tree directly overhead also provides them with a convenient place to dry out.
If you don't have a birdbath, you can just place a plant saucer on the ground, but that will make the birds more vulnerable to cats. Also, the dripper shouldn't be too far above the saucer or a breeze may blow the drops into the grass instead! If you don't have a convenient branch, try hanging the jug from a shepherd's crook instead.
Since the birds get the water dirty, I usually empty the birdbath or saucer before refilling the jug with fresh water each day. This also prevents mosquitoes from hatching in my birdbath, though the constant dripping would probably discourage the female blood suckers from laying eggs there anyway, as they prefer still water.
Since I have to clean droppings off the birdbath frequently, I should also be more conscientious about disinfecting it. A solution of 9 parts water to 1 part bleach, thoroughly rinsed off afterward, would accomplish that.
The plastic water jug does, admittedly, look a bit tacky. You can, of course, use more decorative containers for your water instead--as long as it's possible to poke a very small hole in whatever whimsical container you choose!
Photos: Thumbnail photo is by anyjazz65 and bird reflection photo by Elsa Blaine, both courtesy of Flickr Creative Commons. The other photo is my own.