The Earth's magnetic field is generated by electric currents in the core-mantle boundary, so if it changes position or magnitude there isn't a damn thing we can do about it. As for migratory birds, all we can do is try not to further wreck their chances of survival through our own activities. On the other hand, this could be a factor driving evolution, particularly if seasonal survival is based on a birds' ability to follow magnetic field lines to a new food source, and that food source is only marginally acceptable to some fraction of the bird population. One year they land where food is plentiful. The next year they land further west, where 20% of the population dies from indigestion. The are replaced by the offspring of birds that *do* survive, and so forth.