Migration encompasses four related concepts: persistent straight movement; relocation of an individual on a greater scale (both spatially and temporally) than its normal daily activities; seasonal to-and-fro movement of a population between two areas; and movement leading to the redistribution of individuals within a population. Migration can be either obligate, meaning individuals must migrate, or facultative, meaning individuals can "choose" to migrate or not. Within a migratory species or even within a single population, often not all individuals migrate. "Complete migration" is when all individuals migrate, "partial migration" is when some individuals migrate while others do not, and "differential migration" is when the difference between migratory and non-migratory individuals is based on age or sex (for example).