Offering fresh food to birds won't affect their migration schedule at all, since their comings and goings are governed entirely by the changing length of daylight, winds, and the sun's angles. In fact, if you have fresh food out, migrating birds may rest up (for a day or two) in your yard for the next leg of their trip south.
Migration presents the opportunity to spot a bird that's not normally seen here. In north Texas, there have been sightings of Tanagers, Baltimore Orioles, Nashville Warblers, Pine Siskins and several others, including Pelicans. Migrating birds need the same things all birds need, especially plenty of fresh food and clean water as they "bulk up".
For most birds, the actual flight occurs in the cool of the night, since birds navigate by the stars, so we may not see them in flight. Also at night, there are few predators and the winds are more predictable.