Take care feeding the birds
Supporting birds in the garden is to be encouraged, but supplementary feeding creates risks. 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).
Another study from the USA has also been looking at the impact of constant food sources on migratory birds - it appears they become less likely to migrate. These more sedentary populations can become prone to infection because migration is a mechanism that weeds out diseased individuals. It also stops healthy individuals from moving far from sites where the diseases are present.
BTO's conclusion is not that gardeners should stop feeding the birds, but that we should heed the following advice:
“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; and rotation of feeding sites to avoid accumulation of waste food or bird droppings."