i haven't had a chnace to read to read Dr Bill's link.. but i recently read Prey--Michael Critions book about nano machines that escaped to the wild, and how they were programed.
the small amount of magnetic material might be the clue.
at some point (temperature, but more likely shorter day light hours) monarchs start to migrate to longer days. (ie south. )
as they do, they pass through some (many?) magnetic anomalies.. and each one activated the magnetic material in their brains. eventually (some what by luck, some what by geograpy (monarchs don't like flying over large bodies of water) they find them selfs in texas (moving south and west (opposite their north east spring migration (again, why west? programed to fly to longer days, and setting sun? (and in spring longer days and rising sun?)
flying further south from texas puts them in the gulf.. and they don't like flying over water.. so they continue west, (and south.. and suddenly-- they are sensing a specific magnetic anomoly--one their brains calls 'home')
since all you have to do is have programs that say:
Fly north and east so long as there are X or more hours of daylight (and of course the further north they get, the longer the day)
When there are fewer than X hours of daylight fly south and west. (east and west being driven by the sun rise/set) Avoid large bodies of water.
If you encounter this specific magnetic field, home towards it.
pretty simple, the program can be instinctual, and pass from generation to gereration with out the need for a big brain, or for maps. and it would work.
(not anyless beautiful for being understandable. )