What is that thing tied to the bird's leg?

Bird migration has always been an important topic in ornithological research. Of the more than 10,000 bird species in the world, at least 20% are migratory birds. Migratory birds travel back and forth between wintering grounds and breeding grounds at fixed times of the year and along roughly fixed routes.

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Studying bird migration not only allows us to understand the physiology, population size, behavioral rhythm, migration routes, habitat status and changing trends of birds, so as to formulate better research and protection strategies, but also provides important references in epidemic prevention and control, airport site selection and route setting. In addition, studying the mechanism of bird migration, such as positioning methods, metabolic changes, etc., is also an important topic in bionics.

Studying bird migration has a long history

In fact, more than two thousand years ago, there were records in "Lüshi Chunqiu" about "geese flying north in the first month of spring" and "geese coming back in the first month of autumn". That is to say, at least in the pre-Qin period, our ancestors knew that some birds were migratory.

In the story of Su Wu herding sheep in the Han Dynasty, there is an allusion to "swan geese delivering letters". Although swan geese swim for a long time, letters tied to their feet without waterproofing will be soaked and rotten; and swan geese migrate between wintering grounds and breeding grounds about once every six months. As a species at the bottom of the food chain, there is a risk of being preyed on, so it can be said that there is no advantage in delivering letters, but this did not prevent the ancients from making romantic associations based on observations of nature.

Hongyan丨Photo source: contracted photographer/IC photo

In Europe, at least before the 18th century, many people believed that birds hibernate in winter, including Aristotle and even Linnaeus, the founder of the binomial nomenclature of plants and animals. It was not until around 1820 that someone discovered a wounded white stork with an African native spear in Germany that they realized that birds would migrate long distances.

The birth and use of banding

Initially, people often conducted fixed-point observations at several locations where migratory birds often passed or stopped. Using optical equipment alone would cause the least interference to wild birds, but it was impossible to accurately grasp the movements of populations and individuals.

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In 1899, Danish ornithologist Mortensen put aluminum rings with systematic numbers on the legs of some birds. This was the beginning of human application of scientific ringing.

Nowadays, metal rings are still the most widely used bird rings in the world. Many countries have their own bird ringing centers. The code on each ring is unique, which is equivalent to a human ID card. There are many different types of rings, the most common ones are leg rings, neck rings, wing tags, etc.

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Foot ring

Most leg rings are made of aluminum, which has the advantages of being easy to shape, wear-resistant, corrosion-resistant, and light. But it has a weakness that it is too small and will rotate. If we want to read the number on the ring completely from a long distance with a telescope, it is almost impossible to complete the task. If you want to confirm the ring number, you must capture the animal again.

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For some wading birds with very long legs or tarsus (the part between the legs and toes of birds, usually without feathers, and with scaly keratin on the epidermis), flags can also be used, which are a mark that is easier to observe than ordinary metal rings.

Neck Ring

Many waterfowl, such as swans and geese, keep their feet below the water surface when swimming, and their legs and tarsus are very short. Even when they are on land, they often move around in the grass. If you put a leg ring on such birds, the ring will be blocked most of the time and cannot be observed. Fortunately, waterfowl such as swans and geese usually eat mainly plant-based food, and their esophagus will not expand significantly. We can choose a corrosion-resistant, brightly colored, soft and light neck ring.

Image source: Paixin Creative

In fact, although the neck of a bird looks thick, most of its volume is occupied by feathers. The actual muscle and bone parts are very thin, only about half or 2/3 of the visual effect. Of course, the neck ring should not be too tight. After wearing it, it should be able to rotate freely; but it should not be too loose. A too loose neck ring will repeatedly hit and rub the neck during the movement of the bird, which is easy to cause abrasions to the bird.

Wing emblem

If the research object is a bird like a pelican, cormorant, heron, or stork that swallows large fish and has a greatly expanded esophagus, we often use wing tags. Wing tags are made of brightly colored plastic sheets with conspicuous numbers and letters sprayed on them. The wing tag needs to be stuck on the bird's wing membrane. The advantage is that the ring number on it is very clear and easy to read. Even if the research object is far away from you, you can still see it intuitively with a telescope, and we often don't need to capture the animal again. The disadvantage is that it will bring a certain risk of infection to the bird because it needs to penetrate the wing membrane, and when it is worn on the wings, it may create a certain resistance to the bird's flight.

Image source: Paixin Creative

The emergence of the banding method has provided us with good data for roughly studying the number of bird populations, community composition, migration patterns and even lifespan, but it still has many shortcomings. For example, it may be necessary to repeatedly capture wild birds, the recapture rate of the same bird is low, and it is still impossible to effectively monitor birds whose activity range exceeds the known observation range.

Technological development makes tracking easier

As technology advances, we are beginning to use other tracking methods when studying bird migration.

Radio telemetry

In the early years, we used radio telemetry, which involves attaching radio transmitters to the tail feathers of birds and then having researchers track them from behind with radio receivers.

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Advantages: In breeding or wintering grounds, as long as the birds do not move at high speed and over a large area, the movements of the research subjects can be accurately monitored. Moreover, as the birds molt in the current season or the next season, the signal devices will fall off and can be easily recovered.

Disadvantages: It is impossible to track birds for a long time, and if the birds need to migrate long distances, it is difficult for us to catch up with their speed using existing land transportation, so it is difficult to maintain an effective receiving distance and the signal will be lost quickly.

Chip or GPS positioning method

GPS positioning can easily track long-distance migratory birds, especially those that need to migrate across countries or even continents.

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Advantages: The signal is relatively stable, and even if the birds being studied are far away from the researchers, they can be tracked continuously, which is of great significance for studying the migration patterns and life history of a certain type of bird, and there is no need to capture them.

Disadvantages: Since the signal transmitter relies on solar panels for power, it is generally heavy and can only be used to monitor medium to large birds. In addition, solar panels have high requirements for the duration of direct sunlight. Once birds hide in dense forests for a long time, they may lose signals due to insufficient power supply.

Subcutaneous chip method

This method is not commonly used, but it can also temporarily record all the bird's movements in the chip.

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Advantages: Very light.

Disadvantages: Birds need to be captured, and whether the chip is placed or removed, the birds need to undergo surgical operations, and there is also a certain risk of infection.

In ornithological research today, several tracking methods are often used simultaneously, and the wearing of bands and trackers must be done by professionals.

Image source: contracted photographer/IC photo

We study birds not only to benefit humans, but also to better protect birds. In the process of research, we must not only pay attention to the scientific nature of the methods, but also pay attention to protecting the welfare of wild animals.

About the Author

The cat demon in the bird's nest, a senior wildlife conservationist.

Editor: Ren Yalin

The article is owned by Chongqing Science and Technology Museum. Please indicate the source when reprinting it.