NO we don't know exactly when it will flip again.

The last major flip was hundreds of thousands of years ago. When the field does flip it takes about 1000 years to do so... so any animals will have plenty of time to adjust.

If we look back at the last 100 millions years or so, we see that there have been periods of time measured in a few millions years in which the polarity has been the same... then there are periods as brief as a few 100,000 years where it flipped between states. Basically the cause is related to the turbulent flow in the liquid molten metallic core of the Earth. The field of study is called MAGNETOHYDRODYNAMICS and its very complicated and is studied by using a supercomputer to simulate turbulent flow in the rotating very hot molten core of the EARTH. Study lots of physics to work this problem out!!!

I am a biologist, not a geologist, so I don't know a lot about magnetic field reversals. It's my understanding that the last reversal was 100,000 to 200,000 years ago or so, and that reversals probably take place slowly, over many years, not overnight.

I do know a few things about animal navigation, however. You're right that many animals seem to be able to use magnetic fields to navigate long distances, including birds and some sea turtles. The way this magnetic sense seems to work is that the animal makes a mental map. For example, a sea turtle knows what the magnetic field strength is like a the island where it was born, so it can go swimming off through the ocean and when it's ready to head back to that island again, it just follows the magnetic field to the spot where the field strength is right.

It's a little hard to imagine how this works since we don't have anything like a magnetic field sense of our own, but you can imagine it's like living in a big city with numbered and lettered streets. If you know you live on 50th St. and D Ave, I could drop you anywhere in the city, like at 15th & Q, and you could follow the letters and numbers back to 50th &D. Since we don't know exactly how field reversals work, it's hard to predict what would happen to these unsuspecting animals.

I think your suggestion is right -- if the reversal happened overnight, a lot of birds would wake up and fly in the wrong direction. Researchers have done experiments where they manipulate the magnetic field inside a birdcage, and this is exactly what happens. However, it seems that real magnetic field switches happen over many years. So there is a period of weaker and weaker fields, then a period of virtually no magnetic field, and then a strengthening to the new polarity. This reversal should happen over the course of many generations of animals. So animals will gradually find that their maps are getting less and less accurate, but it won't be a very drastic change. Plus, each new generation of animals has to start making its own map from scratch anyway, so it won't be a big deal if the field is different from what it was when their parents made their own map. When the polarity switch is finished, the current generation of animals will have magnetic maps that work just as well as those in the generation before the switch.

Now of course there will be a point in the middle of the switch at which there is no magnetic field, and it will be impossible for animals to navigate using that sense. I'm not sure what will happen then; chances are there will be a lot of animals that will die because they can't navigate properly. But the capacity for magnetic navigation is transferred genetically from generation to generation, so when the magnetic field starts coming back, the animals that are alive then will be able to use it. One thing I can tell you is that some of the species alive today that use magnetic sense (like sea turtles) were around 200,000 years ago when the last switch happened, and they didn't go extinct then. So whatever happens when the field switches, it seems that species are able to weather the switch.

If you'd like to learn more about magnetic fields, here's a good website:
magnetic fields

Well, in a strict sense, we don't know it will happen again, but the evidence is very strong that it will do so at least once in the next million years. As it happens, iron oxides are relatively common materials on the earth, and can be deposited by sedimentation from water or air. As they are already magnetic, they tend to orient to the prevailing Magnetic field during sedimentation. In some places, sedimentation has occurred for a very long time -- I recall that there is a small mountain range in Africa in which the local magnetic field (from the local rock) changes back and forth dozens of times as you walk across the layers. Another source of evidence is in the ocean floor building across the mid-Atlantic ridge. As you know, the Atlantic Ocean is spreading slowly as the plates grow from the ridge which runs roughly north/south. As the floor forms, it is initially molten -- and so tends to align with the prevailing magnetic field direction. There have been several studies of the magnetization of the ocean floor -- which can be accurately dated and has reversed many times as you move away from the ridge.

Most larger animals use the magnetic field in concert with other senses -- and the field is likely to be very weak for some time during a shift. (It is a planetary sized field after all!). There are extinctions and new species events which some people believe correlate to periods of shifting (which also allow solar wind to propagate into the tropics instead of into the poles). So there is higher likely-hood of radiation induced mutation at such times.

Please check the USGS and Scientific American sites for more information on continental drift and the magnetic field. (Good questions!)

The magnetic field of the earth gets stronger and weaker with time (nobody knows why). When the field gets very weak, a more complex, weaker field, takes over, and when the strong two-pole field appears again, sometimes it is oriented in the opposite direction.

Normally, the earth's magnetic field cycles back and forth irregularly on a timescale of many tens of thousands to a million years. However, the field right now seems to be decaying, and is doing so at such a rate that it may disappear or even reverse within our lifetimes.

Since the magnetic field changes occasionally, I can only expect that they are prepared for this inevitability (if they weren't they would have gone extinct the last time the magnetic field changed). Nonetheless, it might cause a considerable degree of confusion. That said, the Sun always rises in the east and sets in the west, and the angle of the Sun with the horizon and the time of year tells you how far north or south you are, so I expect that many animals could use that as a directional guide.