Bird ringing (UK) or bird banding (US) is the attachment of a small, individually numbered metal or plastic tag to the leg or wing of a wild bird to enable individual identification. This helps in keeping track of the movements of the bird and its life history. It is common to take measurements and examine the conditions of feather moult, subcutaneous fat, age indications and sex during capture for ringing. The subsequent recapture, recovery, or observation of the bird can provide information on migration, longevity, mortality, population, territoriality, feeding behaviour, and other aspects that are studied by ornithologists. Other methods of marking birds may also be used to allow for field based identification that does not require capture.^[1]

The earliest recorded attempts to mark birds were made by Roman soldiers. For instance during the Punic Wars in 218 BC a crow was released by a besieged garrison, which suggests that this was an established practice. Quintus Fabius Pictor used a thread on the bird's leg to send a message back. In another case in history, a knight interested in chariot races during the time of Pliny (AD 1) took crows to Volterra, 135 miles (217 km) away and released the crows with information on the race winners.^[2]

Falconers in the Middle Ages fitted tags on their falcons with seals of their owners. In England from around 1560 or so, swans were marked with a swan mark, a nick on the bill.^[3][4]

Storks injured by hunting arrows (termed as pfeilstorch in German) traceable to African tribes were found in Germany and elsewhere as early as 1822, and constituted some of the earliest definitive evidence of long-distance migration in European birds.^[5]

In North America John James Audubon and Ernest Thompson Seton were pioneers although their method of marking birds was different from modern ringing. To determine if the same birds returned to his farm, Audubon tied silver threads onto the legs of young eastern phoebes in 1805, although the veracity of Audubon's dates and methods has been questioned,^[6] while Seton marked snow buntings in Manitoba with ink in 1882.^[7] Ringing of birds for more extensive scientific purposes was started in 1899 by Hans Christian Cornelius Mortensen, a Danish schoolteacher, using aluminium rings on European starlings. Mortensen had tried using zinc rings as early as 1890 but found these were too heavy.^[8] The first ringing scheme was established in Germany by Johannes Thienemann in 1903 at the Rossitten Bird Observatory on the Baltic Coast of East Prussia. This was followed by Hungary in 1908, Great Britain in 1909 (by Arthur Landsborough Thomson in Aberdeen and Harry Witherby in England), Yugoslavia^{[clarification needed]} in 1910 and the Scandinavian countries between 1911 and 1914.^[9] Paul Bartsch of the Smithsonian Institution is credited with the first modern banding in the U.S.: he banded 23 black-crowned night herons in 1902.^[10][11][12] Leon J. Cole of the University of Wisconsin founded the American Bird Banding Association in 1909; this organisation oversaw banding until the establishment of federal programs in the U.S. (1920) and Canada (1923) pursuant to the Migratory Bird Treaty of 1918.^[12]

Bird ringing is the term used in the UK and in some other parts of Europe and the world. Bird banding is the term used in the United States. Organised ringing efforts are called ringing or banding schemes, and the organisations that run them are ringing or banding authorities. Birds are ringed rather than rung. Those who ring or band birds are known as ringers or banders, and they are typically active at ringing or banding stations.

Birds may be captured by being taken as young birds at the nest, or as adults, captured in fine mist nets, baited traps, Heligoland traps, drag nets, cannon nets, or by other methods. Raptors may be caught by many methods, including bal-chatri traps.

When a bird is caught, a ring of suitable size (usually made of aluminium or other lightweight material) is attached to the bird's leg, has a unique number, and a contact address. The bird is often weighed and measured, examined for data relevant to the ringer's project, and then released. The rings are very light, and are designed to have no adverse effect on the birds – indeed, the whole basis of using ringing to gain data about the birds is that ringed birds should behave in all respects in the same way as the unringed population. The birds so tagged can then be identified when they are re-trapped, or found dead, later.

When a ringed bird is found, and the ring number read and reported back to the ringer or ringing authority, this is termed a ringing recovery or a control.^[13] The finder can contact the address on the ring, give the unique number, and be told the known history of the bird's movements. Many national ringing/banding authorities now also accept reports by phone or on official web sites.

The organising body, by collating many such reports, can then determine patterns of bird movements for large populations. Non-ringing/banding scientists can also obtain data for use in bird-related research.

At times in North America, the bands have just a unique number (without an address) that is recorded along with other identifying information on the bird. If the bird is recaptured the number on the band is recorded (along with other identifying characteristics) as a retrap. All band numbers and information on the individual birds are then entered into a database and the information often shared throughout North American banding operations. This way information on retrapped birds is more readily available and easy to access.

• 
  Mist nets for bird ringing, Malta.
• 
  Male common redstart with a ring, Malta.
• 
  Weighing ringed bird, Malta.
• 
  Bird ringing is not without hazards to the ringer! Here, a red-backed shrike is attacking the person ringing it.

Mist nets

Mist nets are fine mesh nets with shelves that create pockets to temporarily restrain birds. Mist nets come in a variety of mesh sizes, heights, lengths, weights, materials, and colours. The mesh size of the net is calculated differently in different countries; in the US and Canada the given mesh size is equivalent to two sides of a mesh square, while in the UK it is equivalent to one side. Ringers must choose a mesh size that appropriately targets the desired species, with smaller birds requiring a smaller mesh size. Nets can range from 1.5 to 18 metres in length, and from 1 to 3 metres in height depending on the number of panels. The weight of the thread and the ply (number of strands) can also vary. Common construction materials include nylon, polyester, and monofilament. Mist nets are typically black, but may also be shades of green or brown. While mist nets are capable of capturing a variety of species, they require supervised training in order to use properly and when they are deployed must be checked frequently.^[14][15]

Leg gauge

The ring size is determined by using a leg gauge. This is placed around the bird's leg, which determines the diameter of the leg. After identifying the size of ring needed, it is then placed on around the leg with the help from the ringing pliers.^[17] In Australia, ring sizes range from 1 to 15, plus special sizes for birds whose leg shapes require special rings, such as parrots and pelicans.^[18]

Certain bird species are for various reasons unsuitable for ringing. In some countries, such as Australia, there exist laws prohibiting ringing of such species.^[18]

Some very large birds, such as ratites, are difficult to ring because the cost of making a ring which is capable of securely fitting their strong, heavy legs is prohibitive. At the other extreme, the smaller species of river and tree kingfishers, todies and certain lories, have such narrow tarsi that a ring placed around the bird's foot may impose danger to blood circulation. For some gamebirds, such as the Indian peafowl, spurs on the legs interfere with the rings, which thus can cause injury to the birds.

Special rings are needed for long-lived seabirds, such as Manx shearwaters, which can live for over 50 years. The corrosive effects of sea water, combined with wear, result in traditional aluminium rings only lasting around 4 years; for these birds, much tougher and more corrosion-resistant incoloy, monel, or stainless steel alloy rings are now used.^[19]

Many species of cockatoo, which even if able to be ringed, require special rings to fit the unique shape of their legs. With softer metals like aluminium, they can bite off the rings with their powerful bills; tougher alloys also need to be used for these rings.^[20] The ability to overcome this problem varies between species, and with some such as the Gang-gang cockatoo, it is known to be too dangerous to attempt banding. New World vultures also cannot be banded on their legs because they urinate onto their legs, causing corrosion of the bands into a powdery oxide that sticks to the vulture's leg and injures the bird. Dippers are also dangerously handicapped by ringing because the rings induce drag that makes it extremely difficult for them to catch prey in fast-flowing water.

Among species which can be safely ringed, there are major limitations among nomadic species of the deserts of the Eastern Hemisphere and cardueline finches of the taiga. The highly unpredictable movements inherent in these species' lifestyles means that recovery rates are extremely low,^[21][22] especially given generally low population densities within their habitats.

Many institutions that ring birds offer demonstrations for the public, where experts ring live birds while highlighting the steps of the process and answering questions from the public.^[33] Educating visitors about the technique helps to spread accurate information about it to the public.^[34] While live bird ringing is not the only method of educating the public on bird conservation, it can be an especially engaging and unique method for visitors.^[35]

Ringing activities are often regulated by national agencies but because ringed birds may be found across countries, there are consortiums that ensure that recoveries and reports are collated. In the UK, bird ringing is organised by the British Trust for Ornithology. In North America the U.S. Bird Banding Laboratory collaborates with Canadian programs and since 1996, partners with the North American Banding Council (NABC).^[36] Waterfowl hunters may report the ring number of the bird they killed or observed, and find out the details of that specific bird such as breed, age, and ringing location. Bird rings are often seen as a prize because they are still relatively rare.^{[citation needed]} The European Union for Bird Ringing (EURING) consolidates ringing data from the various national programs in Europe.^[37] In Australia, the Australian Bird and Bat Banding Scheme manages all bird and bat ringing information.^[38] while SAFRING manages bird ringing activities in South Africa.^[39] Bird ringing in India is managed by the Bombay Natural History Society. BirdRing rings in the Neotropics and Africa. The National Center for Bird Conservation (CEMAVE) coordinates a national scheme for bird ringing in Brazil.^[40][41][42]

• Tracking animal migration