World Migratory Bird Day: It is said that migratory birds fly south, but where exactly does “south” refer to?

Zhong Zhenyu

May 11 is World Migratory Bird Day. According to seasonal changes, migratory birds migrate periodically on a large scale every year, flying from the north to the far south. This migration phenomenon has aroused people's curiosity: Where is the "south" that migratory birds fly to? How do scientists study bird migration?

Migratory birds of different seasons have different destinations.

Migratory birds are birds that migrate periodically with the seasons. Every spring and autumn, they travel between their breeding grounds and wintering grounds in order to obtain enough food and avoid the harsh climate.

Take swallows as an example. When we say swallows fly south, we are not referring to the southern part of my country. Instead, they fly to the far south of the equator, to the tropical regions of the southern hemisphere. The Beijing Swift is a good example.

In Beijing's autumn and winter, the climate is cold and dry, plants stop growing and die, and insects "disappear". Without sufficient food security, the Beijing swifts have to fly to the distant "south" - southern Africa in the southern hemisphere. They pass through more than 30 countries during their migration. When they arrive at their wintering grounds, it is summer there, the plants grow lushly, the insects are active, and there is plenty of food to satisfy the survival of the Beijing swifts. They return to Beijing in the spring and summer of the following year.

In Beijing, migratory birds are divided into summer migratory birds, winter migratory birds and traveling birds. Among them, summer migratory birds fly back to Beijing from the low latitudes of the south in spring and summer every year to build nests and hatch offspring, and fly to the south for wintering in autumn and winter; winter migratory birds fly from the high latitudes of the north in autumn and winter every year to spend the winter, and fly to the north for breeding in the spring and summer of the following year; during the spring and autumn migration season, a large number of migratory birds directly fly over or briefly appear in Beijing, which are called traveling birds.

From this we can see that migratory birds do not all fly to the "south". Migratory birds of different seasons and species have different destinations.

Data from the List of Terrestrial Wildlife in Beijing (2024) show that there are 519 species of wild birds in Beijing, including more than 300 migratory birds, including 60 to 70 summer migratory birds, more than 30 winter migratory birds, and more than 200 traveling birds.

New technology reveals how migratory birds return home

Bird migration has always attracted much attention. Humans have been studying bird migration for more than 100 years. Early studies on bird migration were mainly conducted through observation and recording. In recent years, with the innovation of various technologies, advanced scientific methods such as radar, flight tracking, and remote sensing technology have been continuously used, making the results of bird migration research more accurate and intuitive, and some research results often break people's cognition.

In 2022, the results of a tracking study on the migration behavior of Beijing swifts were officially published in the international journal "Sports Ecology", accurately revealing the ecological laws of Beijing swifts' migration for the first time.

As a common swallow in Beijing, Beijing swifts like to nest in old buildings such as ancient city towers, so they are also called "building swallows". In the summer, Beijing swifts fly in groups in the air in the Summer Palace, Temple of Heaven, Beihai, Qianmen and other places, and suddenly disappear in late summer. But in the spring of the following year, they will reappear in Beijing. This happens every year.

Where did the missing Beijing Swifts go? In order to find out their migration data, on May 24, 2014, the first batch of 31 Beijing Swifts with mini photosensitive locators installed on their backs took off from the Summer Palace. In May of the following year, 13 of them flew back to the Summer Palace. Instrument storage data showed that in late July 2014, Beijing Swifts left Beijing and headed northwest into Mongolia, then turned west, crossed the Junggar Basin into Central Asia and the Arabian Peninsula, entered Africa in mid-August, and arrived in the Congo Basin in Central Africa or adjacent areas in September. After resting for 40 days, Beijing Swifts continued to move south and arrived in southwestern Africa in early November. The entire journey of about 15,000 kilometers took 111 days. Beijing Swifts spent about three months of wintering there. In mid-February of the following year, they began to return one after another and arrived in Beijing, their breeding ground, in mid-to-early April. The migration route of Beijing Swifts passes through about 37 countries and regions, with a round-trip distance of about 30,000 kilometers. This migration tracking project not only revealed the "south" where the Beijing swifts fly to, but also accurately determined the data information of the entire migration process, including where the resting places are and the daily movement distance.

With the help of scientific and technological means, scientists have conducted in-depth research on migratory birds' migration routes, times, and stopover locations, aiming to study the habitat conditions of their breeding grounds, wintering grounds, and stopover sites. This not only provides targeted data for protecting birds and their habitats, but also provides a basis for decision-making for wildlife conservation managers.

(The author is deputy director of the Beijing Biodiversity Conservation Research Center)