World Migratory Bird Day丨This migration journey that concerns survival is more than just magnificent!

When the leaves fall and the flowers turn yellow, the autumn wind blows, and the migratory birds are about to start their journey. Some of them eat frantically to store enough fat, while others begin to shed their breeding feathers and replace them with smoother feathers. In short, the migratory birds are ready for this journey.

At this time, we also usher in World Migratory Bird Day. Migration is the mission of migratory birds, a commitment to protect their environment throughout their lives, even if it may cost them their lives.

1. Is the migratory habit of migratory birds an innate instinct?

Migratory birds are birds that migrate regularly between their breeding areas and wintering areas, accounting for about one-third of all bird species. From the cold Arctic to the hot and desolate desert, from the vast plains to the vast ocean, migratory birds can be seen flying high in the sky.

The migration of migratory birds is a positive adaptation instinct to environmental changes. This migration is characterized by regularity, directionality, and large groups. The migration time of various migratory birds rarely changes each year, and the migration route is also fixed all year round, and often flies along a certain terrain, such as rivers, coastlines or mountains. Many migratory birds migrate south and north through the same route, but each migratory bird has a different migration route.

Copyright image, no permission to reprint

Why do migratory birds make these seasonal migrations? How did migration evolve? This has long been a topic of debate among scientists. However, current research results cannot determine when the ancestors of birds began to migrate, and it is not realistic to track the migration history of ancient birds.

The reasons for bird migration are also very complex. This instinct of migratory birds is not only due to genetic and physiological factors, but also the result of long-term adaptation to external living conditions, and is closely related to changes in living conditions such as climate and food. Migration can avoid the severe cold in the north in winter and the scorching heat in the south in summer, and can obtain sufficient food such as insects, rodents, fruits and seeds of plants at the right time and place, especially to meet the best conditions for them to build nests, lay eggs and breed offspring.

However, the migration distances of migratory birds are mostly different. For example, some species can almost cross the world, while other species only migrate on a hillside, going back and forth between higher altitudes and valleys. Moreover, not all migratory bird routes are north-south, some are east-west, and some have more tortuous routes.

Therefore, there is still no definite answer to the reason why migratory birds migrate. It is generally believed that migratory birds migrate to adapt to the climate and cope with food shortages. It may also be due to changes in external conditions such as changes in light that cause changes in the hormone regulation of birds and affect their migratory behavior. For example, the rise in spring temperature in the south will promote the development and maturation of bird gonads, thereby prompting them to migrate to cooler areas to reproduce.

2. Built-in "navigation" system, you can find your way even if you travel thousands of miles

Arriving at the destination is not as simple as flying in a straight line. Migratory birds need to face the impact of wind, rain, lightning and other natural factors on their flight routes, and sometimes they have to make certain changes in order to eat or avoid unsuitable environments. Do migratory birds have already drawn navigation maps in their minds during the migration process?

Scientists began to conduct relevant research mainly through a series of bold experiments in the middle of the last century. After long-term research, scientists found that the perception system of migratory birds mainly comes from the following aspects: First, vision. Migratory birds determine the position and direction of long-distance flights by observing iconic landmarks such as mountains and waters, and using celestial bodies (the position of the sun, moon and stars in the sky).

Copyright image, no permission to reprint

Scientists have also found that some migratory birds use the "sun compass" which is more reliable than landmarks to orient themselves during migration, and use "clock shift" to prove that they have an inner sense of time. Even when the sky is covered by clouds, they can adjust their course accordingly by observing the changes in polarized light. When flying at night, they use the stars to determine their direction. According to legend, in Finnish mythology, the Milky Way is called the "bird's path" because they noticed that migratory birds rely on the Milky Way to guide them during migration, and the Milky Way is also considered to be the real home of birds.

The second is the sense of smell. When approaching a target and needing more accurate positioning, smell, infrasound and sound clues become more important. Birds are very sensitive to the smell of places they have been to before, and their memory is very strong, so they can recognize the unique smell of the migration destination.

In addition, magnetic fields, polarized light, ultraviolet rays, etc. are also important bases for helping migratory birds fly thousands of miles without getting lost.

In the late 1970s, scientists discovered that birds’ brains contain small amounts of lodestone, a substance similar to magnetite, which forms a “compass” in their brains. Further, scientists discovered that there are specific chemicals in birds’ eyes that can sense and even see the Earth’s magnetic field, and these chemicals are connected to areas of the bird’s brain that process visual information. In a bird’s field of vision, the magnetic field or magnetic direction may form a bright or dark spot, and when the bird turns its head, its field of vision will move accordingly. In a recent study, scientists tested the magnetic sensitivity of Cryptochrome4 (Cry4) in the eyes of a migratory bird and found that this protein is the “living compass” that allows migratory birds to use the Earth’s magnetic field for navigation. The Cry4 protein of migratory birds is more sensitive to magnetic fields than the Cry4 protein of non-migratory birds. This study further revealed that magnetically sensitive molecules do exist in migratory birds.

Copyright image, no permission to reprint

In recent years, in addition to traditional field observations and banding, scientists have also begun to widely use various advanced technologies such as radar, global satellite positioning systems, photosensitive geolocators, remote sensing technology, molecular biology experiments, etc. to track and study the flights of migratory birds. They have basically figured out the main migration routes of birds around the world and have a better understanding of the navigation mechanisms of migratory birds.

3. The survival crisis faced by migratory birds is far greater than you think

Every year, hundreds of millions of migratory birds travel back and forth between different places on Earth, which is one of the most stunning and fascinating natural phenomena on Earth. But for migratory birds, this journey is a life-and-death struggle for survival.

Migratory birds have to overcome the hardship of long-distance flights, the severe challenges of nature, and the threats from natural enemies and human activities. Therefore, migration is not as interesting as people imagine for migratory birds, but a game for survival.

Copyright image, no permission to reprint

During migration, missing the best time or place to forage, food shortages, strong winds and rainstorms, and other factors may devour these creatures. The early or late migration time also hides dangers: setting off too early means that the living environment in the north is still covered with ice and snow, and setting off too late means there is a risk of encountering storms. In today's global warming, the frequency of extreme climate events has increased, and the uncertainty of habitats and food resources caused by climate change will lead to the disorder of the life history of migratory birds, and then disrupt the coupling relationship of phenology or migration patterns, posing a fatal threat to the survival of migratory birds, which may have exceeded their ability to bear.

The dangers of the world do not end there. In addition to indiscriminate hunting, human activities such as urbanization, lake reclamation, human destruction, and pollution of habitats are a silent "hunting of migratory birds." In recent decades, many migratory birds have died due to accidentally colliding with lighthouses, chimneys, bridges, power lines, or man-made buildings, especially the shocking "bird strike" incidents - in fact, bird strikes have even been considered the second largest cause of bird deaths caused by humans.

The main culprits for the "mass collision" of birds are the reflection and light transmission of building glass during the day and the lights at night. In a metropolis with tall buildings, there are glass curtain walls everywhere. Birds may not be aware of the existence of glass during the day due to the reflection and light transmission of glass, or because the glass of the building reflects the sky and green land, birds may misunderstand when flying; at night, they are more likely to be confused by artificial lights, lose their sense of direction and crash into tall buildings, which is especially dangerous when it is foggy.

The best way to prevent migratory birds from flying into buildings is to reduce light pollution in cities as much as possible and avoid using large areas of glass as exterior walls. Based on research and practical experience, scientists suggest building new glass curtain walls that can project patterns or arrange patterns visible to birds on the outside of the glass to serve as a warning to birds. Flat patterns must look like three-dimensional objects with a sense of space, so that they will avoid flying through the middle because the "space" between adjacent objects is too small.

Winter turns to spring, and life never ends. In the process of human economic development, how to protect migratory birds from harm and let them share the blue sky with humans is still an important issue that needs to be taken seriously.

author:

Li Xiangtao, Researcher at the National Museum of Natural History, Honorary Executive Director of the Chinese Ornithological Society

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.