|Humpback whale migration routes.|
The arctic tern migrates over 44,000 miles every year, from the arctic to antarctic and back.
These are just two examples of the incredible migrations performed on earth every year. What could possibly motivate these animals to do this? Well, the exact reasons are slightly different for every migrating animal, but a unifying theme can be found in the video below. This is a video of "Net Primary Production", which is essentially a measure of the growth of plants.
The animals are chasing the plants! That's why they migrate*! So simple!!!!! And if they're not chasing the plants themselves, they're definitely chasing other plant-chasing animals.
Perhaps the most striking aspect of this video, which shows two years worth of data collected by NASA scientists, is the onset of spring/summer in the northern hemisphere where plant growth explodes in the boreal forest of northern Canada and Russia. The same effect is visible in the southern hemisphere, though it is less dramatic due to the smaller percentage of land cover. You will also notice similar effects in the oceans.
This whole pattern is of course due to the tilt of the earth's rotational axis relative to the plane in which it rotates about the sun, which causes the seasons. This epic task undertaken by awesome numbers of animals is caused by the specifics of the way our planet moves. Woah! Earth = Life = Earth
As the search for extra-solar earth-like planets continues, this sort of makes me wonder how different life would be on another planet whose axial tilt were smaller or non-existent. No migrations at all? Smaller migrations? No birds? Unicorns??
*Nature is not quite as simple as I would like, as food is not the only answer. Animals will often migrate away from food-rich areas in order to breed in places where their young will be safe from predators. In addition, scientists still haven't completely figured out the rationale behind all migratory animals' journeys. However, food and sex are the common theme that motivates most animals' activities, including migrations.