History

One of the first scientific tools to track migratory animals involved a metal band fitted around the leg of a bird (Nebel 2010). This method has become known as banding (or ringing) and was first used in 1890 by the Danish biologist Hans Christian C. Mortensen. He applied zinc bands to two European starlings (Sturnus vulgaris), but then realized that their weight altered the starlings' behavior, and switched to aluminum bands instead. Over the next decade and a half, he continued to band starlings, and by 1906, Mortensen had banded a total of 1550 individuals. By 1930, bird banding had become prevalent throughout Europe, North America, India, Australia, and New Zealand (Preuss 1989).

The recovery of banded birds has been primarily used to identify migration routes (Bairlein 2001). The first comprehensive review on recoveries of banded birds was written by von Lucanus (1919). He compiled about 3000 recoveries of 127 bird species, and used these data to identify several flyways across Europe. Over the next decade, hundreds of recoveries were collected and entered in the first 'Atlas' (Schütz & Weigold 1931). More than 6,800 recoveries of about 230 species were presented in the atlas.

Bird banding was revolutionized in the early 1950s, when mist-nets became widely available, and cannon-netting started to be used for capturing shorebirds and waterfowl. As a consequence, the number of recoveries of banded birds increased drastically, particularly in passerines. In the mid-1960s, the European Union for Bird Ringing (EURING) was founded, which is now the coordinating organization for European bird-banding schemes (Bairlein 2001). Other national organizations exist around the world that coordinate and regulate the catching and banding of birds, such as the Australian Bird and Bat Banding Scheme (http://www.environment.gov.au/biodiversity/science/abbbs/), or the North American Bird Banding Program (http://www.pwrc.usgs.gov/bbl/).

Bird Capture

The method used to catch birds primarily depends on the size of the bird and its habitat. The most widely used capture method for small birds is mist-netting (Figure 1), which can be used in nearly all terrestrial habitats. Mist-nets are typically made of terylene netting and are suspended in the air strung between vertical poles. Under the right conditions, the fine meshing is virtually impossible to see, and flying birds or bats will get caught in the net. Note that mist-nets are of little or no use for larger species such as pigeons, wildfowl, gulls, or raptors (Gosler 2004).

Larger birds that tend to roost close to the edge of the water, such as gulls, waterfowl and shorebirds, are typically trapped with cannon-nets, which are powered by gunpowder (Figure 2). Projectiles are attached to the edge of the net with ropes and are fired simultaneously from small cannons. The net is then projected across the birds that are roosting or foraging on the ground, and which are then trapped underneath it. Cannon-netting has the potential to catch a large number of birds at once. Note that depending on the country where the catching takes place, there might be a legal issue with using gunpowder (Gosler 2004).

Other capture methods are cage traps, where a bird enters the trap, usually in search for food, and trips a door-release mechanism; noose-carpet traps, where a bird can be trapped with a carpet of monofilament nooses; or nest box traps for catching parenting passerines. In some cases, adults can also be caught at night by hand, when the birds are roosting. This can sometimes be done with small passerines in nest boxes, or with seabirds on land by using a long-handled hand net held above the head (Gosler 2004).

Capturing birds is often time consuming and requires substantial training, as the safety and welfare of the birds are of uttermost importance. In many countries, a license needs to be obtained in order to catch and band birds (Gregory et al. 2004). It needs to be remembered that — whatever method is used — trapped birds tend to be stressed, and are often exposed to heat, wind, or rain, and may suffer from hyper- or hypo-thermia. Rapid extraction from the net or trap is therefore essential (Gosler 2004).

Types of Bands

After capture, a band — nowadays made of aluminum or stainless steel — is usually placed on the bird's leg (Figure 3). Bands are available in a wide range of sizes, made to fit anything from a hummingbird to a swan. For large birds such as waterfowl, bands can also be worn as a neck collar. The metal band is engraved with a unique code that identifies the individual and also usually — depending on its size — with a return address. If the bird is either recaptured or found dead, and date and location of the find are reported, it is possible to infer routes and timing of migration, and even obtain estimates of longevity (Gosler 2004). Recovery rates are, however, usually very low: in many small species, less than 0.1% of banded individuals tend to be recovered, while in larger species, recovery rates can each up to 20% (Newton 2008).

In addition to the metal band, coloured plastic bands are also often used (Figures 4 & 5). The benefit of using combinations of coloured bands is that, unlike the number engraved on the metal band, they can be read with telescopes, or even binoculars, without having to catch the bird. While they cannot be used to identify individuals (unless the study population is very small), they can provide information on — for example — stop-over duration, or movements of subsets of a population, such as age cohorts. Care needs to be taken when reading colour combinations, as some plastic bands fade or even change colour over time, especially under tropical conditions (Gosler 2004).

Data Generated by Bird Banding

Most of what we know today about migratory routes, stop-over sites, and wintering areas can be attributed to the continuing work of regional and national bird banding programs (Åkesson & Hedenström 2004). Often, the results are presented as migration atlases, such as the ‘Swedish Bird Ringing Atlas' (Fransson & Pettersson 2001) or the ‘Migration Atlas: Movements of the Birds of Britain and Ireland' (Wernham et al. 2002). Banding schemes also often publish annual reports with the most recent findings, for example Ringing & Migration is the journal of the British Trust for Ornithology Ringing Scheme (http://www.bto.org/ringing/). It has been publishing findings on banding and migration studies since 1975, and focuses on birds occurring in the Western Palearctic (Europe, North Africa, and West Asia).

An early banding study by Perdeck (1958) provided insights into the way migrating birds navigate. European starlings and chaffinches (Fringilla coelebs), migrating south in the autumn from the Baltic to their winter quarters in northern France and southern England, were caught on route in the Netherlands and released to the south-east in Switzerland. By observing where the banded birds were resighted, Perdeck discovered an important difference between adult and juvenile migrants: adults were able to compensate for the displacement, and most were reaching the intended winter quarters by changing directions, while juveniles continued using the same migratory direction and thus ended up in a new wintering area. In spring, however, the displaced young starlings managed to return to their traditional breeding area (Perdeck 1985, 1983). These experiments provided evidence of fundamentally different navigational strategies in first-time and experienced migrants (Wiltschko & Wiltschko 2003).

Another example of the insights that can be gained by the recovery and resighting of bands is provided by Wilson et al. (2007). In Australia, two populations of the Bar-tailed Godwit (Limosa lapponica), a long-distance migratory shorebird, occur. During the non-breeding season, about a third of the global population migrates to Australia. Starting in the early 1990s, the Australasian Wader Studies Group (http://www.awsg.org.au/) and the Victorian Wader Study Group (http://home.vicnet.net.au/~vwsg/) have been placing a single coloured leg-flag on Bar-tailed Godwits caught in Australia-orange in Victoria, green in Queensland, and yellow in north-west Australia. In the 15 years after the banding activities began, 17,170 individual Bar-tailed Godwits were captured and banded, and 65% of these received a leg-flag. Fewer than 2,000 individuals with a leg-flag were resighted or recaptured. Nevertheless, it was possible to draw some interesting conclusions. Based on the resightings, Wilson and colleagues (2007) were able to show there was very little exchange between the two populations, and it also became clear that timing and routes of migration were quite different. In combination with the differences in morphology that Wilson et al. demonstrated, their study provided strong evidence that Bar-tailed Godwits migrating to north-western Australia belong to the subspecies L. l. menzbieri, which breeds in north-eastern Russia, while those that migrate to south-eastern Australia belong to the subspecies L. l. baueri, which breeds in northern and western Alaska. Detailed knowledge of migratory pathways is an important aspect of ensuring that a species is protected throughout its migratory range.

Estimating Population Size and Survival Rate

Survival rate and population size are parameters that play an important role in designing conservation programs for threatened wildlife populations (Beissinger & McCullough 2002). They can be estimated by capturing and marking individuals so that they can be identified at subsequent encounters — the ‘Capture-Mark-Recapture Method' (Nichols et al. 2004). To obtain an estimate of the population size, a group of individuals from a specific location is captured, marked (e.g., with a band) and released into the wild. After a certain period of time, a second group of individuals from the same location is recaptured, and the total population size can then be estimated based on the number of captured and the number of marked individuals (White & Burnham 1999). Collecting mark-recapture data over longer periods of time allows estimating survival rates of a species or a population (e.g., Sandercock 2003, Johnson et al. 2010).

An example of a banding program that has been established to generate data on avian survivorship is MAPS: Monitoring Avian Productivity and Survivorship Program (www.birdpop.org/maps.htm), which is run by the Institute for Bird Populations. MAPS is a North America-wide network of hundreds of mist-netting stations that are constantly maintained. The data obtained by banding on such a large scale using standardized methods are used to contribute to our understanding of the ecology of North American land bird populations, and of the factors leading to changes in their populations, and they help guide conservation efforts.

Outlook

The introduction of bird banding has unveiled many of the former mysteries of the movements of migratory birds (Bairlein 2001). Despite the recent advances in methodologies such as radar (Mein & Nebel 2012), stable isotopes (Zimmo et al. 2012), biotelemetry (Hay & Nebel 2012), and satellite telemetry (Perras & Nebel 2012) that can be used to track moving animals, Bairlein (2003) argues that these methods will not replace bird banding to study migration, but that banding will continue to play a pivotal role in achieving conservation projects (Bairlein & Schaub 2009). One of the great advantages of bird banding is that — compared to many of the more recent technologies — it is cheap, easy to do, and it attracts a very large number of skilled volunteers, who both benefit from, and contribute to, the study and conservation of migratory birds. The potential benefits of involving ‘ordinary' citizens in science and conservation projects has by now been recognized widely, and bird-banding projects are ideal avenues for the involvement of ‘citizen scientists'.