“For San Blas the stork you will see and if you do not see it, snowy year” or “For San Juan the stork begins to fly” are some examples of typical sayings that refer to the migration of, in this case, storks, such as indicative of a specific time of year. Like all sayings that have their origin in popular knowledge, it is a very interesting idea, since these migrations of animals are very common and there are many who carry them out, but do you know why they do it exactly?
What is animal migration
First of all, what is migration? What does it consist of? Migration is neither more nor less than the displacement of a population periodically from its original habitat to another where they settle for a certain period of time. These migrations are phenomena that occur in nature to maintain the ecological balance or balance of all the ecosystems that participate in this event, both the departure and arrival ecosystems as well as those intermediate ones through which these migrant individuals pass.
These animal migrations are studied in various fields, such as ecology, biogeography or zoology.
Migrations usually last several days depending on the distance they have to travel and are different between the different species that carry them out. For example, birds often use the trade winds and other air currents, while other smaller animals such as amphibians or invertebrates also take advantage of water currents.
On the other hand, migrations are not usually individual events, but animals tend to migrate in groups . However, what is really unknown even today is how these species that travel great distances are able to orient themselves until they reach their destination , there are various theories such as, in the case of birds, the orientation of the Sun and the stars. Theories that take the Earth’s magnetic field as a reference are also considered.
Once we know what migrations are, the big question comes. Why do animals migrate? It seems that this behavior is something instinctive in the species that perform it.
Migrations are linked to certain events to fulfill different purposes, but as a common denominator to all of them we find the main objective of all forms of life: survival .
The 3 fundamental objectives of migration are the following:
- The differences in temperature between the different seasons of the year. This is perhaps the most well-known reason of all, and it is that species migrate due to these changes in temperature, which become very low during winter and, on the contrary, very high during summer, so that species they move away and ensure their ability to survive in other areas during these extreme periods.
- Reproduction and foraging . It may not be so obvious, but many animals are forced to move away from their usual habitats to mate safely or to look for food in other areas where competition is not as high.
- Run away from your predators . Again it is not one of the first reasons that comes to mind, but it is logical. The animals move away from those areas where they feel threatened by other species and look for new shelters where they can protect themselves.
There are many species of animals that migrate , from birds to reptiles and fish, among others. Next, we are going to delve a little deeper into each of the groups of animals that migrate and discuss some examples. Attentive!
Birds are the ones that usually travel the longest distances during their migratory journey, such as shearwaters (seabirds that can fly up to 910 km a day), swallows or the small golden plover .
In addition, birds can be classified according to when they migrate, that is, what time of year they decide to move away in a specific place. Taking the Iberian Peninsula as a reference, we can distinguish between wintering birds that flee from the cold north and spend the winter here on the peninsula, such as the common crane ( Grus grus ); summer birds that, on the contrary, flee from the heat of Africa like the common swallow ( Hirundo rustica ) and spend the summer here; or birds of passage.
Other examples of migratory birds are:
- The white stork , which migrates to Africa in winter avoiding crossing the Mediterranean Sea, which is the most striking part of its migration period. The reason? It seems that the winds from which they are helped that we discussed above only occur over land areas.
- The Canada goose , one of the typical ones that form V groups in the skies during migration. This species prefers and seeks warm climates, as well as better availability of food sources.
Some species of fish also migrate, the most famous being the Salmon migration . Many species reproduce in fresh water, where the young of the fish develop that once they reach adulthood migrate to the sea where they grow, feed and mature sexually. Finally they return to the rivers, which they cross against the current to reproduce in them.
Something similar, although, on the contrary, occurs with European and North American eels . In their adult state they live in rivers, but they go to the Sargasso Sea during the reproductive season.
Other examples of species that migrate annually are herring, cod or flounder .
Amphibians and reptiles
Some species of sea turtles , such as green turtles, carry out migrations. They live on the coasts near Brazil, but they travel great distances to spawn on the beaches of Ascension Island (in the South Atlantic Ocean). These hatchlings quickly go to the sea and, over time, appear on these Brazilian coasts where the adult specimens live and feed. Some examples are the loggerhead turtle, the olive ridley turtle, the olive ridley turtle, the leatherback turtle (the longest, more than 4,831 km from their nesting beaches).
On the other hand, newts, frogs, toads and salamanders also carry out annual migrations of a few kilometers. In these cases, migration occurs from those places where they hibernate during the winter to rivers or ponds where they reproduce later.
Mammals that migrate
Some well-known examples of migrating mammals are wildebeest, zebra, caribou, antelope, or elephants . These animals migrate to find food sources and wells of water to survive.
Another example is that of seals, the California sea lion or whales , such as the humpback whale, the southern right whale or the gray whale. These whales can travel up to 20,000 kilometers during their migrations between their feeding and breeding grounds.
Although we usually forget about them, invertebrate animals also carry out migrations related to the search for food and the reproductive season.
A particular case is that of plankton , which performs “vertical” daily migrations. During the night the plankton is found in the most superficial layers of the water of the seas and oceans where it feeds and with the arrival of the day it descends up to 1,200 meters to flee from its predators and reduce its metabolism due to the lower temperature of the waters. deeper, saving energy.
Some examples of the more well-known migrating invertebrates are:
- The crabs , which move to salty waters to lay their eggs, covering distances of up to 240 kilometers.
- Lobsters , known for the great damage they cause to many crops in their path.
- Butterflies such as the monarch butterfly ( Danaus plexippus ), which you can see in the image below, whose migration lasts longer than the life of any other species of butterfly (it can live up to 9 months) and that takes them to travel more than 5,000 kilometers from the northern United States and Canada to Mexico and California. Migration begins between August and October and they remain at their destination until March, at which time they return north. They are thus one of the few insects that can make transatlantic voyages.
- Dragonflies belonging to the genera Libellula and Sympetrum , which migrate in massive groups of individuals. Pantala flavescens stands out with the longest transatlantic migratory trips of all insects (more than the monarch butterfly). Travel between India and North Africa, roundtrip, traveling up to 15,000 kilometers.