Gradual how? If there's no overall magnetic moment then surely navigation on magnetic field sensing organs will simply fail, if it switches/moves - even gradually - and stays the same strength then the animal likely ends up in entirely the wrong place with the wrong foodstuffs and climate. Can't see that either of those "gradual processes" is going to work well.

Certainly the magnetic field has flipped before but do we know that animals that use magnetism for migratory guidance were able to "adapt quickly". Can you walk us through the evidence?

You're assuming that this is the only method of navigation they have, instead of it being one tool within their navigation tool belt. Also, I'm sure magnetic sensory organs have some natural error to them that they are constantly re-calibrating for. If the reversal happens within the bounds of natural error and recalibration, then I suspect they would have little trouble adapting.

Correct, animals such as homing pigeons primarily rely on sight and landmarks. There are no known animals that exclusively use magnetic fields to navigate.

The Monarch Butterfly uses it when the sun isn't available: http://en.wikipedia.org/wiki/Monarch_butterfly#Migratory_The...

Apparently it "sees" the magnetic fields, but it also relies on the sun. I don't think it's been studied if it can reorient itself at birth if the poles are switched or if it's "hardcoded".

There are some bacteria

https://en.wikipedia.org/wiki/Magnetotactic_bacteria

This is a good point, although I submit that the magnetic direction is irrelevant for their purpose.

I wonder how they manage when flying at night. Do those birds that are more capable night-flyers have superior magnetic senses?