Migratory behaviour (animal)



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, including birds, mammals, fish, reptiles, amphibians, insects, and crustaceans. The trigger for the migration may be local climate, local availability of food, the season of the year or for mating reasons. 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 as birds migrating south for the winter, 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.

Definition
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 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) 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 Types
Migration can be either obligate, meaning individuals must always 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).

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. Similarly, some jellyfish make daily horizontal migrations, traveling a few hundred metres across a lake.

Multiple generation migration
In some insect species, such as the monarch butterfly, the whole migration is not carried out by one individual. Instead the butterflies mate and reproduce on the journey, and successive generations travel the next stage of the migration.

Human cultural responses to animal migration
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. 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. Another example is the swallow, which at various times was suggested to hibernate either underwater, buried in muddy riverbanks, or in hollow trees.

Migration, Generally

 * Aidley, D.J. (1981) Animal migration. Cambridge University Press.
 * Dingle, H. (1996) Migration: The Biology of Life on the Move. Oxford University Press.
 * Baker, R. R. (1978) The Evolutionary Ecology of Animal Migration. Holmes & Meier Publishers.
 * Gauthreaux, S. A. (1980) Animal Migration, Orientation, and Navigation. Academic Press.
 * Rankin, M. (1985) Migration: Mechanisms and Adaptive Significance: Contributions in Marine Science. Marine Science Institute.
 * Milner-Gulland, E.J., J.M.Fryxell, and A.R E. Sinclair (2011) Animal Migration: A Synthesis. Oxford University Press.

Taxon-specific Migration

 * Alerstam, T. (1990) Bird migration. Cambridge University Press.
 * Berthold, P. (2003) Avian migration. Springer Publishing.
 * Elphick, J. (1995) The atlas of bird migration: tracing the great journeys of the world's birds. Random House.
 * Greenberg, R. and Marra, P. P. (2005) Birds of Two Worlds: The Ecology and Evolution of Migration. Johns Hopkins University Press.
 * Drake, V. A. and Gatehouse, A. G. (1995) Insect migration: tracking resources through space and time. Cambridge University Press.
 * Harden Jones, F. R. (1968) Fish migration. St. Martin’s Press.
 * Lucas, M. C. and Baras, E. (2001) Migration of freshwater fishes. Blackwell Science.
 * McKeown, B. A. (1984) Fish migration. Timber Press.