It takes a lot of energy for a bird to migrate, and comes with numerous risks, so if it is possible for it to stay where it is, not migrating can give it an evolutionary advantage. An example would be Canadian geese who [live year round in Florida.][1]

I think that it depends a lot on the area and species, all of the humming birds in Wyoming would migrate with or without food, but the geese in Denver, CO stay all year round and are regarded as pests.

As to whether its right or wrong to feed the birds, I think that's up to the individual's judgement, and one feeder would be a drop in the bucket.


  [1]: http://njcooperator.com/article/send-those-canada-geese-packing