Feeding birds will not stop them from migrating, usually -- though I suspect it might if a species has both resident and migratory members. It will cause birds to cluster around the food source. If that disappears, they will disperse to seek food elsewhere. This is a normal thing in the wild, where a flock will find a source such as a wild crabapple tree, spend a day or few stripping it bare, and then move on. However, it is recommended that if you start feeding birds, you continue for that whole season.