Well heres my take on the subject from the other side of the pond,supplementary feeding in harsh winters helps birds they are able to come into the breeding season fitter & are better able to cope with the demands of the breeding season.In the UK a succession of poor springs has led to a under average breeding season,the birds I feed in the spring & summer are mostly African migrants ie European Redstart.For the past 3 yrs i`ve used mealworms at exactly the same spot & the birds have afforded me with some great photographic moments.The thing to bear in mind is I use the right food in the right seasons for the right species.When i`m not there the birds go back to their natural feeding routine.The point i`m making is that all creatures are opportunistic they have to be for the continuation of the species.Birds feed on rubbish dumps road kills etc.
Steve.