According to Gillis and colleagues (2008), what maintains the behavioral polymorphism observed in a partial migrant population of dippers?

A) Migratory behavior is an evolutionary stable strategy in this population.
B) Both migrants and residents had the same fitness.
C) The behavioral polymorphism is based on a genetic polymorphism.
D) Offspring learn the migratory behavior of their parents.
E) Migratory behavior is condition-dependent.