This morning I received an interesting email from Bird Watching HQ with the subject line “Do birds become dependent on feeders? Here’s what the research says!” The research is posted at DOES WINTER BIRD FEEDING PROMOTE DEPENDENCY?
The answer is No. They experimented with Black-capped Chickadees in a rural area of mostly woods in Wisconsin from Oct. 1984 to April 1985. There were no more mortalities in the experimental group than in the control group.
They do suggest that this might be different for migratory birds or in urban areas, or if feeders are removed in the middle of winter. They did not know about these situations.