About Animal migration ~ Unexlained natural phenomenon?

 Animal migration

Many animals migrate thousands of miles of land and sea, all without the use of a GPS device. How do animals take these amazing journeys without getting lost? No one really knows, though there are many theories. According to an article in The Independent that focused on pigeon migration, some believe the birds navigate the Earth using visual landmarks or their sense of smell to determine their location. More bizarre-sounding theories include the concept that pigeons use magnetism to determine if they’re north or south of home; another is that the pigeons use morphic resonance, a theory by Rupert Sheldrake, to refer to what he calls the "the basis of memory in nature ... the idea of mysterious telepathy-type interconnections between organisms and of collective memories within species." 

Animal migration is the relatively long-distance movement of individuals, usually on a seasonal basis. It is a ubiquitous phenomenon, found in all major animal groups, includingbirds, mammals, fish, reptiles, amphibians, insects, and crustaceans.^[1] The trigger for the migration may be local climate, local availability of food, the season of the year or for mating reasons.^[2] To be counted as a true migration, and not just a local dispersal or irruption, the movement of the animals should be an annual or seasonal occurrence, such asbirds migrating south for the winter; wildebeest migrating annually for seasonal grazing; or a major habitat change as part of their life, such as young Atlantic salmon leaving the river of their birth when they have reached a few inches in size.

Migration can take very different forms in different species and as such, there is no simple accepted definition of migration. One of the most commonly used definitions, proposed by Kennedy^[4] is

“Migratory behavior is persistent and straightened out movement effected by the animal’s own locomotory exertions or by its active embarkation upon a vehicle. It depends on some temporary inhibition of station keeping responses but promotes their eventual disinhibition and recurrence.”

Migration has also been described as a term that describes the four related concepts:^[1]

1. persistent, straight, movement behavior
2. relocation of an individual on a greater scale (both spatially and temporally) than its normal daily activities
3. seasonal ‘to-and-fro’ movement of a population between two areas
4. movement leading to the redistribution of individuals within a population.

 Migration can be either obligate, meaning individuals must migrate, or facultative, meaning individuals can choose to migrate or not.

Within a migratory species or even within a single population, often not all individuals migrate. Complete migration is when all individuals migrate, partial migration is when some individuals migrate while others do not, and differential migration is when the difference between migratory and non-migratory individuals is based on age or sex (for example).^[1]

While most migratory movements occur on an annual cycle, some daily movements are also referred to as migration. For example, many aquatic animals make a vertical migration (Diel vertical migration), travelling a few hundred metres up and down the water column.^[5] Similarly, some jellyfish make daily horizontal migrations, traveling a few hundred metres across a lake.

Before the phenomenon of animal migration was understood, various folklore and erroneous explanations sprang up to account for the disappearance or sudden arrival of birds in an area. In Ancient Greece, Aristotle proposed that robins turned into redstarts when summer arrived.^[9] The barnacle goose was explained in European Medieval bestiaries and manuscripts as either growing like fruit on trees, or developing from goose barnacles on pieces of driftwood.^[10] Another example is the swallow, which at various times was suggested to hibernate either underwater, buried in muddy riverbanks, or in hollow trees.