The British Trust for Ornithology's advice is as follows:
“We’re calling on everyone who feeds wild birds to be aware of their responsibilities for preventing disease. Simple steps we’d recommend include
- offering a variety of food from accredited sources
- feeding in moderation, so that feeders are typically emptied every 1-2 days
- the regular cleaning of bird feeders
- rotation of feeding sites to avoid accumulation of waste food or bird droppings."
Using feeders and bird tables for birds to congregate can transmit diseases. In recent years one victim has been the greenfinch, whose UK population has radically dropped after the species became host to the parasite trichomonas, previously only seen in pigeons and doves. The repeated gathering of many different bird species in one place, day after day, and exposure to each other's droppings and regurgitated food is thought to add to the risk.
See this paper from the British Trust for Ornithology (BTO).