Winter. Drinking water is at a premium for birds in winter in cold areas of the country, where many natural water sources freeze over. Lay an inexpensive heating element or de-icer in your birdbath to keep the water accessible (you'll need to plug the de-icer into an outdoor outlet). Or, try the low-tech method of carrying out a saucer of warm water at regular times each day so the birds get accustomed to your schedule: Morning and noon are busy bathing and drinking times.
Drinking, not bathing, is the main activity at winter birdbaths in cold regions, although occasionally a starling will enjoy a splattering good time in the warm water. Though an icy shower makes our teeth chatter, birds don't mind bathing in cold water a bit. The layer of down feathers next to their skin stays dry and keeps them cozy while they clean their outer feathers. Birdbaths with built-in heating elements are the ultimate in deluxe bird bathing. They'll keep the water ice-free even on the coldest days, with no thought or care from you. You may want to invest in such a model if your winters are long and frigid.
Spring. Look for both resident birds and migrating birds that are passing through. Vireos, wood warblers, and other insect-eating birds that won't stop at your feeder will stop at your birdbath. Because birds move in and out of an area so rapidly during spring migration, delightful surprises are a daily pleasure. One gentle April morning I looked out the window to find a scarlet tanager and an indigo bunting drinking from adjoining saucers. Then a rose-breasted grosbeak fluttered down from the trees in a flurry of black-and-white wings and settled himself on the rim of another saucer.
Summer. When the weather gets hot, watch for resident songbirds like robins, orioles, catbirds, and tanagers seeking respite from the heat and dust with a cooling bath. Be sure to add a shallow dish of water near to the ground for birds like towhees and thrushes that like to stay low. Watch for family groups showing up at the bath as fledglings leave the nest. The young birds are fun to watch, especially when they line up on the rim of the basin like a bunch of kids waiting their turn in the tub. If summer brings drought, expect to see the unexpected. That's when birds that might not ordinarily visit your garden will come in for a drink. Meadowlarks, crows, blackbirds, and many of the shier sparrows, including grasshopper sparrows and field sparrows, will venture into unfamiliar territory to get a drink when a dry spell has its grip on the countryside.
Fall. Birdbath traffic often dies down in the fall as songbirds leave for warmer climes. House finches and house sparrows are still regular bathers, and titmice and chickadees visit for drinks though they don't usually stay to splash. In areas with abundant fall rains, natural water is plentiful, so birds can find natural puddles more easily.