Earth's Magnetic Poles
The variation between magnetic north (Nm) and "true" north (Ng) is around 11°. "True" North is the north of Earth's rotational axis, and the North geomagnetic pole, which is in fact the south pole of Earth's magnetic field, is located near Greenland in the northern hemisphere.
Earth's magnetic field polarity changes overtime, driven by changes in the Earth's crust - the geodynamo of iron alloys, and is recorded in certain rocks, particularly the spreading mid-ocean ridges. This switching of polarity can happen rapidly, although between periods of a few thousand years. The two magnetic poles can also wonder independently, with movements up to 40km; however these movements are generally slow, and make compass navigation viable.
Magnetoreception - Biomagetism
Magnetoreception is a sense an organism has that allows the detection of a magnetic field. This can aid in the perception of altitude, location, or direction - which is particularly useful for migration. Many forms of life have been discovered that have the ability, however the actual sensory receptor is often difficult to locate. Examples, include some invertebrates, such as nematodes and earth worms, honey bees, ants and terminates; some birds, such as homing pigeons, European robins, and chickens; some mammals, such as rodents, and potentially roe deer and even cows; and some bacteria, such as the aptly named 'magnetotactic' bacteria. Although yet to be proven, we may also have magneto-receptors in us.