It seems as if birds use magnetic north as part of their navigation system, with geographical and other cues (such as the smell of various landmarks and maybe even celestial navigation and more) to supplement the magnetic information. I would guess that succeeding generations of birds would fine tune their various means of navigation to adjust for the magnetic pole motion, as migration is a mix of learned and instinctive behaviour.

Not all long distance migrators use magnetic navigation - geese for example appartently do it by other means and little magnetic caps don't throw them off they way pigeons get turned around by the caps.

An interesting article here.