Migration is a term that is most commonly used to describe how large groups of animals move from one place to another. Migrations are associated with a change in living conditions that affect the individual stages of animal development.
Migrations have pronounced adaptive nature; This function has emerged during evolution and it can be observed in many different species in the animal kingdom.
The migration of most animals is associated with seasonal changes in nature ( droughts, hot and cold temperatures ), which deprive the animals of food.
Some species make one journey in their lifetime. Typical example would be the European river eel. The adult eels live in fresh water, when they reach the time to mate they take on a journey to the Sargasso sea, which in straight line is around 8000 km. ( Almost 5000 miles ) There, on depth of 1000 meters and with specific water temperature, they release their caviar and die. The larvae, carried by the Gulf Stream, reach Europe and the Mediterranean Sea, from which they enter into fresh water.
There are two types of migration – active and passive.
Examples of seasonal movements are best seen in birds and fish. Nowadays, there are about 8,500 species of birds that are known to science, most of whom are sedentary. Nevertheless, they are prone to migration, when the time comes for mating. Seasonal migration happens with the approach of winter, when birds fly to a softer, warmer climate to mate and in search of food.
Diurnal (diel) vertical migration is a way of movement that some water organisms use. This migration occurs when species move up from the mesopelagic zone ( or hypolimnion zone for lakes ) to the epipelagic zone and return when the night comes. The name originates from the Latin “dies” day, which points out that it happens in a 24-hour period. This is known to be the biggest migration in terms of biomass in the world.
But only two fish change their familiar surroundings – a salmon and European eel.
Here is something interesting:
At the same time when salmon starts to spawn, brown bears migrate. Leaving the forest, settling into rivers teeming with salmon. Thus, it turns out that they follow their food supply.
Periodic animal migration can be divided into two subtypes: horizontal and vertical.
The horizontal migration of animals is associated with the movement in search of food. For example, the gray whale moves from the North Atlantic Ocean to the subtropical, tropical part of it, which at this time is full of plankton – the main food source of the whale.
Vertical migration is also present in alpine animals that come down in the winter in the forest, and in the summer, as the snow melts, climb back up the mountain.
There is such a thing as age migration of animals. Such movements are revealed best by the example of large carnivores.
Let’s take the Tigers as an example. A solitary animal with a large territory, which they leave at a certain age. When cubs are born they live with the female until the age of sexual maturity ( typically around 3-4 years after they are born ), after that the males are separated and leave the family in search of their own territory.
Examples of insect migration
Insects also undergo such journeys. The most popular examples of such travelers are locusts, butterflies, dragonflies and beetles. Even though, a clear definition of an insect migration is particularly difficult task, because all insects move to some extent.
Generally, migration is marked by well specified destination that needs orientation and navigation of some sort. Insects that fly need to make constant corrections while doing so. It has been shown that many migrating insects can sense and direction and thus make those corrections in midflight.
Many of the migratory species have some polymorphic forms. The migration period is marked by their well-developed wings. Polymorphism like that is present in grasshoppers and aphids. Scientists suggest that such adaptations would be more prominent in insects that live in environments where the seasonal changes affect the quantity of resources.
Here are some interesting facts:
The common V formation that birds use when flying over great distances reduces drag and helps them to stay afloat more easily. Each bird reaps the benefits that come along with this formation except the first. After some time the formation changes so that the first bird is replaced by one of the other members. Similar effects come into play when we use a glider. The tip of the apparatus is experiencing the normal forces of gravity and air resistance, but after that the air pushes the back of the glider upward, thus, maintaining its height indefinitely if we have enough rising air.
In terms of biology any group of fish gathered together for social reasons are refered as shoaling and if their momevents are coordinated in the same direction is considered schooling. For instance close to one quarter of the fish shoal for their entire life and about a half of them shoal for just a period of time. Shoaling behaviour leads to various benefits such as defence against predators ( increasing the chances of detection and lowering the posibility of individual capture ), enhanced foraging and improving the success rate of finding a mating partner. Its is also known that fish form shoal membership to increase the efficiency of their hydrodynamics.