When a bird flies south for the winter is it learned or inherited?

Yes, in most if not all birds flying south is an instinctual, inherited behavior.

In fact someone recently did a study in Europe which found that this fact is contributing to a possible speciation event, some birds, which have found a food source in Britain in the winter, are arriving in mating grounds sooner and are passing the trait of going on to Britain along with other traits, to their young. This slowly causes a divergence of populations which may eventually become separate species.

http://scienceblogs.com/notrocketscience/2009/12/british_birdfeeders_split_blackcaps_into_two_genetically_dis.php