A stone placed in the center will give the birds a place to rest and drink, and a good visual perspective. To avoid disease breeding, wash the birdbath with a diluted bleach solution perhaps once weekly. Of course, birds need a place in disagreeable weather to stay comfortable. They also need it for protection, nesting and relaxing. Trees that are compact, and shrubs, and brush piled in a heap are good protection and refuge from the elements.
Birds love evergreens especially in winter when other trees have lost their leaves.
The boxes should be cleaned when the season for nesting is completed. Purple Martin houses are most certainly popular, and should be placed in an accessible area un-obstructed and with in approximately 100 ft. of a house. Once setting up residence on a particular birdhouse, the martins will return year after year. You may have to shoo out starlings and sparrows until the purple martins are settled in. Clean the house after and before each season.