Not only from north to south, but also from east to west, in a circle... "Where do birds go?"

As the northern hemisphere enters winter, a spectacular migration is taking place in the sky.

Red-crowned cranes in Yancheng National Nature Reserve, Jiangsu Province. Source: Xinhua News Agency

Of course, similar large-scale migration movements will also take place not only in the sky, but also in the ocean and on land, such as the "migration" of fish and the upcoming "Spring Festival travel rush" of the Chinese people.

Migrating sockeye salmon (Photo credit: Sekarb/Dreamstime.com)

Generally speaking, birds that migrate are called migratory birds, while those that stay and breed in the same place all year round are called resident birds.

But the definition between the two is not very strict, because if the habitat of migratory birds is destroyed or food is reduced, they will also consider migrating to other places. However, this has a bit of the flavor of "leaving their hometown" and is forced.

Robins in the UK, both resident and migratory birds. Image source: Erni/Shutterstock.com

Migratory birds are very "realistic". They also need to worry about food and nesting places, but in most cases, the change of seasons will prompt them to actively move to warmer climates and find places that can meet these two "necessities".

Migrating shorebirds - Red-necked Sandpiper Photo credit: Martin Stokes

Therefore, bird migration is actually the long-distance migration activity of birds that is regular, has a definite purpose and direction as the seasons change.

Your ancestors are guiding you

There is no clear conclusion on the origin of bird migration behavior, but many scholars believe that the alternating ice ages on the earth caused birds that originally lived in higher latitudes to migrate south. After the glaciers retreated northward, the birds instinctively migrated back to their breeding grounds at high latitudes, thus forming migratory behavior.

Another view is that bird migration is due to the pressure of natural selection. Since the migration journey is often long and dangerous, migratory birds need to fly thousands or even tens of thousands of kilometers. Therefore, only birds that survive the migration can occupy an advantageous position in the competition for survival. It is this pressure that creates bird migration behavior.

The migratory bird said, I am "lying down", you can do whatever you want. Photo credit: Marisela De Santa Anna

But no matter which one it is, ancient birds have left a valuable treasure for modern birds, that is, the routes they flew.

Scientists call this ancient route imprinted in the sky the "migratory bird flyway", also known as the "flyway".

Map of the northward migration route of the Bar-tailed Godwit from Wikipedia

Modern birds can follow the wings of their ancestors, flying from the starting point to the end point, and this route is the entire distance of their migration, including breeding and non-breeding grounds, as well as resting and feeding points along the way.

There are currently 10 major "migratory bird routes" in the world , six of which cross the Eurasian continent, Africa and Australia: the Atlantic Flyway, the Black Sea-Mediterranean Flyway, the Asia-East Africa Flyway, the Central Asia-India Flyway, the East Asia-Australasia Flyway and the Western Pacific Flyway.

There are four in the American continent, including the Atlantic Flyway, the Mississippi Flyway, the Central Flyway and the Eastern Pacific Flyway.

From left to right: Pacific, Central, Mississippi and Atlantic Flyways Source: Public Domain

Seeing this, curious friends may wonder, with so many migratory birds in the world, are only 10 routes enough?

First of all, the coverage area of ​​each migratory route is actually very wide. For example, the East Asian-Australasian Flyway covers an area from the Zhejiang coast to the Sichuan Basin in China.

And birds of different species may follow similar routes; birds from one area may merge with other flocks before flying off to different destinations.

In addition, not every bird will fly along the migration route from the beginning to the end. Some birds migrate a shorter distance and may stop to spend the winter from Beijing to Hunan, such as mallards.

Mallard duck in flight. Photo credit: Tom Koerner/US Fish & Wildlife Service

And birds like the Arctic tern migrate from the Arctic Circle to the Antarctic Circle, flying from the "end of the world" to the "cape of the sea", across almost the entire earth.

Arctic tern: Yes, I am the "King of Rolls"! Photo credit: Bob Feldman Myshot

Birds flying in opposite directions

Generally speaking, many birds migrate twice a year. When spring comes, they fly north, breed in temperate or frigid zones in the summer, and return to warmer southern areas to overwinter in autumn and winter. Of course, in the southern hemisphere, the direction is opposite.

This is the most common migration pattern of birds: migrating according to "latitude" , or migrating in the north-south direction, such as the geese flying south that many people are familiar with.

Image source: Thermos

In addition, there are some unusual migration patterns.

In Eurasia, some passerine birds no longer migrate south to their wintering grounds in Africa, but instead fly west to southern France to spend the winter. This east-to-west migration pattern does not belong to the traditional north-south "bird migration route", but rather migrates along "longitude" .

Some researchers have found that the reason for the emergence of this new route may be the impact of climate change. At the end of the 20th century, the temperature in Western and Southern Europe rose, making these areas more suitable for birds to spend the winter than before. They can settle down without flying farther. This is a less risky way for birds to migrate .

A Richard's Pipit (Anthus richardi) stands on a meadow in France for the winter. Photo courtesy GAETAN JOUVENEZ

The two migration patterns mentioned above both involve flying in a straight line, but some birds migrate along one route when heading north and then "circle" back along another route when heading south, forming a circle, so it is also called "circular migration". Many seabirds and shorebirds follow this migration pattern .

A hummingbird migrating in a circular pattern. Image source: chirpforbirds

There are also special cases that occur in individual birds, namely reverse migration , that is, migrating in the opposite direction of the normal migration pattern. The reason for this may be that the young birds did not keep up with the main group during their first migration and lost their way, or encountered storms during the migration and deviated from the original migration route.

A flock of birds encountering a storm. Image source: chirpforbirds

Birds that deviate from normal migration patterns usually do not survive because they are in an environment that is not suitable for wintering. Only a small number of birds can survive in non-wintering sites and then fly back to their breeding grounds the following year. In the next winter, they will relocate and return to their original wintering grounds.

Fly together or separately?

There is also an interesting phenomenon during the migration of migratory birds.

Flocks of birds like to "deploy their troops in formation". For example, migrating geese sometimes form a "person" shape and sometimes form a "one" shape.

The benefit of doing this is that it can reduce the energy cost of flock flight. Research has shown that wild geese can save 12%-20% of energy when flying in a V-shaped formation compared to when they fly alone, which helps the flock to sustain longer flight time.

The position of the wings of these greylag geese shows that they rely on each other's strength to fly. Image source: Anagramm

In addition, maintaining a formation also allows each bird to see the position of its companions and avoid collisions between birds. After all, "safety accidents" can easily occur when there are too many individuals.

However, this problem does not exist for some large broad-winged birds of prey, as most of them travel alone and complete their migrations alone. They are usually active during the day because the thermals created by the sun heating the air can support them to fly long distances.

Vulture flying in the sky. Source: Wikipedia

However, it is difficult for these birds to cross large bodies of water because thermal air only forms over land and the birds themselves cannot maintain active flight over long distances, so the straits of the sea have become a must-pass route for many migratory birds.

Due to this characteristic, many poachers will wait here and kill migratory birds in large numbers.

In fact, bird migration is never smooth.

Especially as humans become more and more involved in nature, birds are more likely to collide with man-made objects during migration, such as glass windows and railings of buildings, power lines and signal towers, collisions with vehicles, and the continuous loss of birds' resting places.

Bird marks hitting the window Image source: Wiki Commons

It is no exaggeration to say that every migration of migratory birds is a hymn of courage. For this reason, we hope that in the cold winter, humans can pay more attention to nature and provide migratory birds with a better migration environment.

References:

1. Newton, I. (2003). Geographical Patterns in Bird Migration. In: Berthold, P., Gwinner, E., Sonnenschein, E. (eds) Avian Migration. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-05957-9_14

2.A new westward migration route in an Asian passerine bird

https://doi.org/10.1016/j.cub.2021.09.086

3. CJ Cutts, JR Speakman; Energy savings in formation flight of pink-footed geese. J Exp Biol 1 April 1994; 189 (1): 251–261. doi: https://doi.org/10.1242/jeb.189.1.251

4.Kasper Thorup (2004) Reverse migration as a cause of vagrancy, Bird Study, 51:3, 228-238, DOI: 10.1080/00063650409461358

5.The Flyway Concept: What it Is and What it Isn't

https://www.researchgate.net/publication/268036693_The_Flyway_Concept_What_it_Is_and_What_it_Isn't

6. China’s migratory bird flyway protection network established

http://www.news.cn/mrdx/2022-11/11/c_1310675694.htm

7.https://www.britannica.com/science/migration-animal/Birds

Author: Fish

Winner of the Silver Award for Outstanding Science Popularization Works of China Science Writers Association

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