The Wildlife Society has a great post on the benefits of feeding birds, not only benefits the birds (especially in the lean winter months) but also has a mesurable beneficial effect on the psychology of those people who love feeding our feathered friends.
However, during outbreaks of avian influenza, some organisations were cautioning against feeding wild birds in order to stop disease transmission. The interplay of disease, wellbeing and policy are explored in a paper in “People and Nature“.