http://www.universetoday.com/18977/2012-no-geomagnetic-reversal/
http://science.nasa.gov/science-news/science-at-nasa/2003/29dec_magneticfield/

In short:
Although there appears to be a current downward trend in magnetic field strength, the current magnetic field is still considered to be “above average” when compared with the variations measured in recent history. According to researchers at Scripps Institution of Oceanography, San Diego, if the magnetic field continued to decrease at the current trend, the dipolar field would effectively be zero in 500 years time. However, it is more likely that the field strength will simply rebound and increase in strength as it has done over the last several thousand years, continuing with its natural fluctuations.

The pole kept going during the 20th century, north at an average speed of 10 km per year, lately accelerating "to 40 km per year," says Newitt. At this rate it will exit North America and reach Siberia in a few decades.

They've also learned what happens during a magnetic flip. Reversals take a few thousand years to complete, and during that time-contrary to popular belief-the magnetic field does not vanish. "It just gets more complicated," says Glatzmaier. Magnetic lines of force near Earth's surface become twisted and tangled, and magnetic poles pop up in unaccustomed places. A south magnetic pole might emerge over Africa, for instance, or a north pole over Tahiti. Weird. But it's still a planetary magnetic field, and it still protects us from space radiation and solar storms.

First of all it is constantly changing and birds still exists and move to the right location, also if you look at the coloured map at this link of how the magnetic field varies, then you clearly see that birds do have to navigate through changing fields all the times. Birds do follow visual clues too.

http://en.wikipedia.org/wiki/Earth%27s_magnetic_field

The only way to significantly change the axis of rotation in a short period of time would be through an impact.

I agree, only a real collision could do that.

There is no other force that is strong enough to do that to the earth. Not gravity from a passing planet, and not magnetism from the sun, or from a passing planet.

Even if there was a big enough force. Earth is a sphere the forces would not have a hold on something except for a bit of gravity assist.
If earth would be very irregular shaped and the other massive mass moves slowly relative to earth so the forces can get a hold on the irregular shape then something could happen.

My question is, if there were such a spontaneous pole shift overnight, what would be the results?

That would actually a good thing protecting us from the Sun because the weaker moment is very short.

But for the rest only your compass pointing in the wrong direction and maybe birds migrating in the wrong direction. That is it.
Your GPS will probably point to the right direction since it does not use magnetism to determine your position.

The thing is that the magnetic field is very very very weak. The magnets sticking on the refrigerator is far more powerful, and no tidal waves occur when you take the magnet from your refrigerator.

But it can't physically change in a day, or a year or hundred years.

I'm going to answer your question as best I can.

The consequences of a magnetic pole shift are fairly well known, since we have a geologic record indicating that this has happened many times in geologic history. These changes in the magnetic field of the earth are recorded in the orientation of iron crystals in igneous rocks on the sea floor. The phenomenon is known as 'magnetic striping'. These events do not appear to be tied to any extinction events. In other words, unless you have a compass, you probably wouldn't even notice that it was happening. It is thought that some animals may sense and use magnetic fields for orienting during migrations, but I don't know that this has ever been tested. In a magnetic pole reversal, it takes a long time for the field to move (hundreds to thousands of years) so it may be that any animals who use magnetic fields in this way just adjust. If you are asking about the consequences of an rapid field reversal, I would speculate that some animals may become disoriented, but beyond that, I don't think that we would see much, if any, consequences on earth.

The consequences of a rotational pole shift, on the other hand, would be significant… if it were possible, and its not. At http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970401c.html the speed of the earth's rotation at the equator is calculated at about 1,000 miles per hour. So let's imagine what would happen if we changed the direction of the earth's rotation: Everything on the earth is also travelling with it, so a sudden change in direction would cause everything on the earth (air, water, rocks, houses, etc) to fly horizontally in the 'old' direction at 1,000 miles per hour. Various authors have proposed various scenarios that they say would cause this shift, including magnetic fields from the sun, or from passing rogue giant planets, or the gravity of a passing planet, and things of that nature. However, none of these ideas have ever been presented with the math. The fact is that the angular momentum of a rotating sphere can be calculated based on its mass and its speed. The angular momentum of the earth is immense, far greater (millions of times greater) than every nuclear weapon ever built put together. The only way to significantly change the axis of rotation in a short period of time would be through an impact. There is no other force that is strong enough to do that to the earth. Not gravity from a passing planet, and not magnetism from the sun, or from a passing planet. The consequences would be disastrous if it were possible, but the consequences of the sun going supernova would also be disastrous. It's fortunate that neither of these are possible.

A geomagnetic pole shift is a reversal in polarity of the earth's magnetic field. These events do happen roughly every 1,000,000 years. From the fossil record, we can tell that such shifts have had no perceptible impact on our planet's evolutionary biology. If such a shift occurred "overnight," as you suggest, I suspect it would be a little more noticeable. Any animals that navigate by way of the magnetic field might become confused. Compasses would point south. Navigation equipment would have to be recalibrated. The overall magnetic field strength might drop during the flip, which might cause a very brief increase in radiation that reaches Earth's surface. I can't see any circumstances under which this would be a cataclysm or have any long-standing effect on the planet's climate.

Note that even the small minority of scientists who proposed the "rapid pole shift" hypothesis only claim that the reversal occurred over a few years, not in one spontaneous overnight event.

The "other" pole shift you see on 2012 woo sites is a rotational pole shift, wherein our planet or its core flips upside down and starts to spin in the opposite direction. This simply isn't possible without an enormous transfer of energy, for which no doomsayer has been able to provide a plausible mechanism. If such a thing were to happen, the required energy would result in a heat release that would boil away the oceans, if not outright melt the planet. Unless scientists have been completely blind to the contents of our Solar System over the last few centuries, such an event simply is not possible. Even if it were, the 2012 survivalists (e.g. Patrick Geryl) would still be hoaxers, because there would be no survival under those circumstances.

As to the possibility of a axial tilt? That's just a no. Magnetic shifts DO happen… roughly every 700,000 years or so, but axial tilts never do. The saying goes: "Weebles wobble but they don't fall down!" Neither does the Earth, and its moment of angular momentum is a bit higher than a "weeble"… whatever they are.

Don't worry about where you posted. If Bill thinks you should be elsewhere, he will just move your post. No fuss at all.

I apologize again if my post in inappropriate for this forum. I'm not attempting to cause any trouble, this just seems like a reserved and intelligent community who would answer my questions based on scientific theory rather than a science fiction novel. Thanks for any information you can provide me!