1) TOPOGRAPHY--mainly daytime migrants. Hawk Mountain is a famous example. Raptors follow the south-west to Northeast mountain ranges. Because they really want to be going more or less south, they pour through passes rather than going over the mountains. Thousands of hawks can be seen at Hawk Mountain during a single day. Certainly the final stage of migration must be dependent on topography. Although homing pigeons in Boston seemed to be following interstates, they do not need sight to find their way home: researchers put frosted contact lenses over their eyes and they still found their way home!
2) POSITION OF SUN--birds might have an internal clock (we have one too, that's why we get messed up traveling). At a given time of day, the bird might know where the sun should be. The bird then travels in that direction is the sun isn't where it ought to be.
3) STARS--Researchers in planetarium studies can shift the night sky, and orientation shifts. Birds mostly cue on NORTH STAR and those rotating nearby. Note that 1000-2000 years ago, star positions were not where they are now, so birds must be able to adapt to celestrial changes. Young raised in isolation oriented correctly BUT MUST BE RAISED OUTSIDE. Interestingly, 99% of lost birds are juveniles. In wood warblers: the farther from N-S route is the normal migration route, the more likely they are to show up in California. Most of these go southeast in the fall, so perhaps landing in California means they have a mirror image of proper migration route.
4) MAGNETIC FIELD--circling the earth, it can be used by several species. In disturbed fields, birds don't orient properly. How do they perceive the field? Some fish have electro-receptors, but birds? In any case, birds still orient in a cueless cage but, put a steel shell around cage, and you get randomness. If you artificially alter magnet field in sky-opened cage, you CAN cause alteration of orientation. Recently magnitite has been discovered in birds' inner ears, and this may explain their sensitivity to the magnetic field.
5) POLARIZED LIGHT: could be used if birds perceive it.
6) OLFACTION: birds might not be all that bad at smelling--some new data suggest pigeons may home by olfaction!
Try Googling any of these subjects for more information.