: Migration

Migration generally is the movement of anything from one place to another, separated by long distances. "Anything" can be replaced with animals, data, chemical molecules, and even humans. The migration of organisms may be along longitudes(north⇔south direction), or along altitudes(up⇔down direction). Migrants migrate to find better homes, food, or breeding grounds.

Why and How do organisms migrate? 

One of the best examples of 'migration' is the migration of birds. They migrate over long distances. Birds migrate due to weather changes, or to find a better home. They cover thousands of miles in a year, in search of a new home as weather changes. Weather is the most influential factor in bird migration. They also migrate in search of better breeding grounds(better places to protect the viability of the eggs).

The sooty shearwater travels about 40,000 miles a year, between northern and southern hemisphere.

Wikipedia: sooty shearwater

 

Organisms can detect even the slightest changes in the environment. Changes in daybreak are one of the best indications to track the changing weather. Changing weather is one of the indicators for organisms to start their migration. As leaves fall in autumn, cold sways its arms over the terrain. Failing to survive in such harsh conditions, some organisms hibernate and others migrate. Some organisms aestivate or migrate in hot weather conditions(like visiting hill stations during summer).

Why do Organisms migrate along Longitudes(North-south direction)?

The answer is simple. As you might know, the earth is divided into six climatic zones(tropical, temperate, arctic, and antarctic). As weather conditions change with changing latitudes in the north-south direction, most of the organisms migrate along the north-south direction.

Why do Organisms migrate along altitudes(up-down direction)?

As we mention altitude, we speak of organisms that climb or descend mountains or hills. As winter falls, so does the temperature at the peaks. So organisms descend to the foot of the hill to escape the extremely cold weather conditions. As spring falls and temperatures become normal, organisms climb back onto the top. Why don't they settle at the bottom forever? This can be explained by the brilliant diversity of plant species one gets to see as one ascends to the top of the mountain. As Plant species vary with altitude, so do the eating habits of organisms living around the place. One can argue that the land at the bottom of the hill is insufficient for all the various organisms to thrive in. Therefore, some organisms are obliged to return back to their homes.

An older technique developed by George Lowery and others to quantify migration involves observing the face of the full moon with a telescope and counting the silhouettes of flocks of birds as they fly at night.

How risky is it to migrate?

Migration is risky. It has its cons and what is the backup plan? In some species, not all members of the flock migrate. Some members stay back(also most of the time, climatic conditions are not too harsh) so that their species don't go extinct in case the migrating flock doesn't find a better home. Even if migrating birds find a better home, not all migrating members make it up to the destination. Migrating organisms die on their way(probably because they fail to find food while migrating, the long distances they travel, and changing weather from place to place are major factors).

Lamprey migrate from their coastal feeding grounds into freshwater, to get ready to spawn, during the autumn and spring. Autumn migrants are sexually undeveloped, while spring migrants enter from the sea in spawning condition.

Lamprey is blocked by garbage at the river mouth.

How do Organisms know when to migrate?

The timing, in most cases, is decided by the length of the day. As seasons change, so does the length of the day(not at poles). Another factor that influences the timing is the accumulation of fat in their bodies. As birds are stationed at a place for some time, the fat percentage in their bodies increases. This gives an indication that they are ready to migrate over long distances, as fat serves the purpose of food reserve. In some birds, the timing is decided by genetics.

How do organisms know where to migrate?

Many birds have been shown to use sun compasses. Navigation is based on a combination of abilities such as the ability to detect magnetic fields (magnetoreception) and use visual landmarks as well as olfactory cues. The ability to successfully perform long-distance migrations can probably only be fully explained by accounting for the cognitive ability of the birds to recognize habitats and form mental maps.

The award for fastest bird goes to the great snipe: It flies around 4,200 miles at speeds of up to 60mph!

 

Migration and Genetics:

Avian migration is a complex life history event. It requires integrating a wide variety of neurological, physiological, and behavioural traits so that they can move between habitats separated by long distances.  Although scientists have long suspected that many short-lived migratory birds rely upon their genes to direct their seasonal migrations – and despite several recent studies showing that large portions of the songbird genome are associated with migration in some way – identifying specific genes out of the many hundreds of possibilities that may be involved in migration is complicated. 

A gene associated with a neurodegenerative disorder in humans has been found to play a role in songbird migration. The gene had been previously identified as VPS13A. This is very speculative at this point, but it could also be involved in clearing reactive oxygen species, which are free radicals that can build up during a prolonged migration and damage cells.

References:

1. https://en.wikipedia.org/wiki/Bird_migration
2. https://simple.wikipedia.org/wiki/Migration
3. https://www.nature.scot/plants-animals-and-fungi/fish/freshwater-fish/lamprey
4. https://www.audubon.org/news/9-awesome-facts-about-bird-migration
5. https://www.forbes.com/sites/grrlscientist/2019/10/25/bird-migration-is-written-in-their-genes/
6. David P. L. Toews, Scott A. Taylor, Henry M. Streby, Gunnar R. Kramer, and Irby J. Lovette (2019). Selection on VPS13A linked to migration in a songbird, Proceedings of the National Academy of Sciences, 116(37):18272-18274 | doi:10.1073/pnas.1909186116