Around the end of the nineteenth century, research on bird migration received a major boost with the start of scientific bird ringing, which is still the mainstay of migration studies around the world. This activity began with the efforts of a school master, Hans Christian C. Mortensen, in Denmark in 1899, but it quickly spread to other places in Europe, North America and elsewhere. A ring (or band) is a light but tough metal band which can be placed loosely around the leg of a nestling or adult bird, with different sizes for different species. The British scheme currently uses rings of 20 sizes, with internal diameters of 2-26 mm. Each ring carries a unique engraved number, identifying the individual bird, and an address to which a recovery can be reported. The bird can be identified unequivocally, and its whereabouts are thus known at least twice in its life - at ringing and recovery. In general, birds ringed as nestlings are of most value because their precise natal locality is known, whereas birds ringed as adults may be of less certain provenance; depending on when and where they were caught, they may have been local breeders, winter visitors or passage migrants. Some recoveries of ringed birds are provided by other ringers who trap the birds alive and release them again, while other recoveries are provided by hunters or by other members of the public who may report the birds dead or injured.

The recovery rates of ringed birds are generally low: in many small species less than 0.1% of ringed individuals are ever reported again, but in larger species, especially those that are hunted, the proportion can rise above 20%. Of course, for ringers operating repeatedly in the same place, local recapture rates can be very much higher, rising to nearly 100% in some species, but such local records reveal little about bird movements. In general, therefore, getting useful information about migration in this way depends on ringing very large numbers of individual birds, from which varying proportions may be subsequently reported from elsewhere. Moreover, because nestlings suffer higher mortality rates than older birds, many more nestlings than adults must be ringed to provide a given number of recoveries.

Another problem is that recovery rates can vary enormously along migration routes, according largely to the density and literacy of the local human population. For example, of nearly 300 000 House Martins Delichon urbica ringed in Britain, just over 1000 (0.4%) have been recovered. More than 90% of these reports were from within Britain and Ireland, and so were of little help in indicating migration routes, while only one came from Nigeria, within the presumed wintering range (I. A. Hill, in Wernham et al. 2002).

In 1903, following the pioneering work of Heinricke Gatke on Heligoland Island in the southern North Sea (Box 2.2), a modern-style bird observatory and ringing station was established at Rossitten (now Rybachi) on the Courland Spit in the southern Baltic, a site where migrant birds are concentrated. Subsequently, many other bird observatories were established at other sites in Europe and North America and most are still in operation. Together, they provide a network of well-placed sites, where migrants can be observed and, more importantly, trapped and ringed in large numbers. During the early twentieth century, many countries came to operate their own institutionalised ringing schemes, in most of which ringing was carried out largely by amateurs operating in their home areas, but also making ringing expeditions to more remote areas. Nowadays in Europe, the various national ringing schemes are linked by EURING, which coordinates techniques and the electronic handling of data, unifies standards and formats, and stimulates projects and analyses on a pan-European basis. All ringers are trained, tested and licensed before they can operate alone.

Many of the techniques used to trap birds are developments of ancient methods used to catch birds for food. One important development was a giant funnel trap, big enough to enclose bushes, known as the Heligoland trap, because it was first constructed on Heligoland Island. At the end of the funnel is a glass-fronted catching box into which birds are driven (Figure 2.2). However, the numbers ringed increased greatly in the 1950s with the development of more efficient trapping methods, including mist nets and cannon nets, which increased the range of species that could be caught in large numbers. Mist nets are essentially walls of fine, almost invisible netting, each up to 20 m long and up to 2 m high. Each net is erected on

Box 2.2 Heligoland Bird Observatory

The first bird observatory, of very different style from those of today, was established on the island of Heligoland (German Helgoland) in the southeastern North Sea, about 60 km west of Denmark and about 80 km north of the German town of Wilhelmshaven. The observatory became famous mainly through the work of one man, Heinrich Gatke, who spent more than 50 years on the island, observing and shooting birds. The skins were sold to museums and private collectors, providing a useful supplement to the income of Gatke and his local collaborators. In the process, Gatke amassed a great deal of information on the timing and volume of bird migration, and on the occurrence of vagrants on the island. The business of skin collecting meant that particular emphasis was paid to rarities, as in much of modern bird-watching. His famous book, Heligoland as a Bird Observatory, was translated into English and published in 1895. Until the spring of that year, he had recorded 398 different bird species on the island. The book is full of fascinating information, and most of his ideas and interpretations have stood the test of time, although in the absence of proper measuring devices, he greatly overestimated the speed and altitude of bird migration.

The bird observatory still survives on Heligoland, but like other modern observatories, it has become a centre for ringing and scientific study. It is the original home of the so-called Heligoland trap, a large horizontally placed wire-netting funnel, big enough to enclose many bushes, and through which birds can be driven and caught in a glass-fronted box at the end.

Moving Images Birds Caught Traps
Figure 2.2 Drawing of a Heligoland bird trap, a large funnel through which birds can be driven and caught in a glass-fronted box at the end.

poles, and set against a background of trees and shrubs to ensure that the net does not show against the sky. Any small bird that hits the net slides into a pocket of net formed by one of three or four shelf strings, which are threaded horizontally at different levels through the length of the net.

Figure 2.3 Cannon-netting of Oyster-catchers Haematopus ostralegus.

A different method was developed for catching waders, waterfowl or others that gather in large concentrations on the ground. A cannon- or rocket-propelled net is placed furled on the ground near where birds assemble (a roost or baited feeding area). The several rockets, or projectiles from cannons, are then fired simultaneously, pulling the large net rapidly over the unsuspecting birds.

By the end of the twentieth century, using a variety of trapping methods, more than 200 million birds had been individually ringed worldwide, giving hundreds of thousands of recoveries, revealing the movement patterns of different populations. Over the years, several 'atlases' of bird movements, based on ringing data, have been published (e.g. Schuz & Weigold 1931, Zink 1973-85, Wernham et al. 2002, Bakken et al. 2003).

Ringing activities tend to be concentrated in particular regions, where opportunities and interest levels are high. Although many of the ringed birds then move on, the subsequent recoveries are probably biased, as mentioned above, towards areas with high-density, literate human populations. Even in so well studied a region as western Europe, spatial variation in the reporting of ring recoveries could give an unrepresentative idea of migration patterns. Over this whole area, much migration occurs on a northeast-southwest axis, as amply confirmed by ring recoveries; but almost certainly ring recoveries greatly underestimate the amount of migration that occurs on a northwest-southeast axis. This is because the chances of getting ring recoveries from southeast Europe are much lower than from elsewhere on the continent. Yet other information from migrants caught in central and eastern Europe indicates that many species show predominantly southeast directional preferences in autumn, their movements not being picked up to any significant degree by subsequent ring recoveries (Busse 2000, 2001).

Care is therefore needed in the interpretation of ring recoveries, although they can still be useful in defining the flyways and wintering areas of particular breeding populations, the annual and seasonal timing of movements, and any sex and age differences in movements that might occur within species (Chapter 15). Some of the most geographically complete information on migration relates to North American waterfowl. It results from a planned, geographically dispersed ringing effort over many years, and subsequent recoveries provided from all parts of the continent by millions of hunters.

Overall, ring recoveries comprise our main source of information on bird movements. Taken together, they have revealed a network of bird migration routes that encompass all habitable parts of the globe, and that are travelled annually by millions of migrating birds. It has sometimes been possible to set up coordinated collaborative projects in a wide range of localities along a migration flyway, in which many observers collect data on the same species in a standardised way. The EURING projects on Barn Swallow Hirundo rustica and other European-African songbird migrants provide examples.

One drawback of ringing is that the ring can only be re-read if the bird is in the hand, alive or dead. Not surprisingly, therefore, researchers have been keen to develop methods that enable the re-sighting of marked birds without the need to trap them. Marking has been achieved in many different ways depending partly on the species, such as colour rings on the legs, large rings bearing numbers or letters that can be read through a telescope, coloured or numbered neck collars or wing tags. Such colour-marking schemes have greatly increased the rate of information gain for some species, especially waterfowl and waders, and have often yielded multiple records of the same individuals at different places. They have given more accurate information than ring recoveries on the speeds of migration and the duration of stopovers. The information yield from such schemes is, of course, greatly increased if observers along the potential migration route are alerted to look out for tagged birds. For example, in the Black-tailed Godwit Limosa limosa in Britain only 2.5% of ringed birds were ever recovered, but following the introduction of a colour-marking programme and additional observer input, more than 80% of marked birds were subsequently reported, many at several different places on the migration route. In the same way, the use of colour leg-tagging has greatly increased our knowledge of shorebird migration in Eastern Asia-Australasia and in North-South America, providing information on the timing and speed of migration, and of the locations of important stopover sites (Minton 2003).

Birds trapped for ringing can be sexed and aged, enabling differences in timing and other aspects of migration between sex and age groups to be identified. Individuals in the hand can also be measured and weighed, providing information on weight gain and fat deposition in different species, which can be related to the types of journeys undertaken. Laboratory analyses of carcasses have revealed that substantial changes in body composition accompany migration, not simply the gain and loss of fat (Chapter 5). In some species caught on migration, measurements (mostly wing and bill) can give some idea of provenance, enabling the passage periods of different populations to be assessed. Nowadays this type of information can often be augmented from ring recoveries or studies on DNA or isotope markers (see later). Blood samples can provide information on the levels of specific hormones, metabolites or red blood cells, all of which can help in understanding migration. Details of plumage development also reveal how moult is fitted into the annual cycle of different species, along with breeding and migration (Chapter 11).

From early in the twentieth century, bird ringing led to the experimental manipulation of birds in order to learn more about their navigation abilities. Large-scale displacement experiments, in which birds were caught in one locality and released in another far away, were done to see whether birds could re-find their home areas, or how translocation affected their migrations, using subsequent ring recoveries to provide the necessary information. No less than 24 such large-scale experiments, involving a wide range of species from Barn Swallows Hirundo rustica to White Storks Ciconia ciconia, were done in the first half of the twentieth century at Vogelwarte Rossitten on the Courland Spit (Schuz et al. 1971), followed by others in other parts of Europe and in North America (Chapter 9). Recoveries of birds trapped, ringed and released without displacement provided the control comparisons. Such experiments revealed much about the orientation and navigational abilities of birds, and about differences in behaviour between young and older individuals. In more recent years, much work on orientation and navigation has involved homing pigeons, which are easy to keep and handle. Or it has involved wild birds which were trapped at migration times and their directional preferences assessed in 'orientation cages', after which they were released to continue their journeys (see later). Such captive birds were sometimes subjected to simulated displacements by adjusting the celestial or magnetic cues to which they were exposed, and their directional preferences then re-assessed.

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