As dozens of juncos, plus finches, jays, and other species mob the bird feeders and make serious dents in my black-oil sunflower seed supply, here are some articles on artificial feeding and bird populations:
• A German study suggests that feeding birds in winter is causing one species to divide. (Note: "bird table" is British for bird feeder.)
• Winter feeding may make for better breeding.
• Birdfeeders, populations, and the domestic cat.
Surprisingly, the research team also found evidence in several studies which indicated that the flurry of activity caused by bird feeding does not increase the birds' risk of predation. Counter-intuitively, the presence of feeders has been associated with lower levels of predation by domestic cats.
Yes, I would say that is "counter-intuitive," considering that I have had a couple of cats who would imitate a rock while crouched under the feeder and then shoot straight up, paw extended, to snag a bird.