The Black-Capped Chickadee, Parus atricapillus, was studied by Margaret C. Brittingham and Stanley A. Temple of the University of Wisconsin. They determined that backyard feeding does not affect survivability or create dependency. (Have you ever felt guilty when your feeders were empty?)
They also found that Chickadees fed from feeders were still obtaining four fifths of their feeding from natural sources, such as insect eggs and larvae, arthropods, and seeds.
Note that this study applies only to the Chickadee, and in particular to its year round residency. The authors note that migratory birds may benefit more from supplemental feeding, being not familiar with the locality.
The usual gang of titmice and chickadees just blew through, picking up an afternoon snack. It's good to know that my feeding didn't hurt, might have helped, and certainly brought me closer to the local wildlife.
See here for the complete article from which I drew a few facts.