The mysteries of bird migration – still much to be learnt Franz Bairlein ABSTRACT Bird ringing has unveiled many mysteries of avian migrations, notably routes and destinations. However, there is still much to be explored by the use of ringing and other marking techniques. Satellite tracking, geolocation and global positioning systems are new tools, as well as particular chemical and molecular markers which appear to be very useful in the study of bird migration by delineating origin of birds and connectivity between breeding and non-breeding grounds. Understanding of bird migrations also gained much from captive studies about the internal mechanisms in the control of bird migration, but we still lack knowledge about external factors, such as food availability, weather, competitors, parasites or diseases.This paper summarises ongoing studies on Northern Wheatears Oenanthe oenanthe to illustrate the benefit of such an integrated approach. Future migration research must aim much more at comparative research and a more integrated approach at various spatial and temporal scales, and linking various sub-disciplines. It is also important to realise that migration is only one part of the life-cycle of a migrating species.Thus, linking migration and breeding is another future challenge, for both basic science and conservation of migratory birds. or centuries, the seasonal arrival and The study of bird migration by ringing departure of bird species to and from Our knowledge of bird migration improved Ftheir breeding grounds remained a dramatically with the development of bird mystery. Although these events were described ringing, first practised by the Danish school by early observers such as the wall and floor teacher Hans Christian Cornelius Mortensen in painters of ancient Egypt, Aristotle and the 1899 (Jespersen & Tåning 1950; Bairlein 2001). Emperor Friedrich von Hohenstaufen (who For the first time, birds were ringed in a con- recognised that, in the northern hemisphere, certed effort to unravel the mystery of their birds moved south in autumn and north in movements, although bird ringing had occa- spring), very little information was available sionally been used for that purpose in earlier about particular origins or destinations, times (Bairlein 1999). After Mortensen, system- although there were some intriguing clues. One atic ringing for the study of bird migration was example concerns a female White Stork Ciconia first introduced by Johannes Thienemann in ciconia that was observed and later shot at its 1903, at the newly founded (in 1901) ‘Vogel- nest in Mecklenburg, northeast Germany, on warte Rossitten’ on the Courish Spit, on the east 21st May 1822; embedded in the bird was an shore of the Baltic (Stresemann 1951). 80-cm long spear of the type used by tribes in Although ringing was much criticised by animal central Africa (plate 48; Kinzelbach 2005). This welfare protestors at that time, its development was the first evidence that migrating White for the study of migration continued, and with Storks winter in tropical Africa. great success. Also in 1903, bird ringing began 68 © British Birds 101 • February 2008 • 68–81 The mysteries of bird migration at the Hungarian Centre for Ornithology. In showing some 9,200 recoveries of 151 species. 1909, Hugo Weigold started ringing birds on After about 100 years of bird ringing, various the island of Helgoland, Germany, while H. F. migration atlases have now been published, Witherby and A. Landsborough Thomson first either nationally (Yamashina Institute for introduced ringing in Great Britain that same Ornithology 1996; Fransson & Pettersson 2001; year. It was also in 1909 that the first birds were Wernham et al. 2002; Bakken et al. 2003 & ringed in the USA, but systematic ringing in 2006; Bønløkke et al. 2006) or internationally North America started only in 1920, with the (Zink 1973–1985; McClure 1974; Zink & Bair- collaboration of the US Fish and Wildlife lein 1995); these and the many papers which Service and the Dominion Wildlife Service of have analysed recoveries of single species or Canada. Organised ringing began in Switzer- species groups have unveiled many of the land in 1911, in Sweden in 1912, in The Nether- former mysteries of bird movements (Bairlein lands and France in 1914, and in Finland in 2001). 1916. Subsequently, bird ringing gradually The hundreds of thousands of recoveries of developed as a routine technique used by avian ringed birds are supplemented by abundant scientists worldwide and many countries observational data on the occurrence and founded ‘Ringing Centres’. spatial and temporal distribution patterns of Since migrating birds ignore political migratory species (e.g. Glutz von Blotzheim & boundaries, international collaboration in the Bauer 1966–1998, Moreau 1972, BWP, Keast & study of bird migration was essential. Conse- Morton 1980, Curry-Lindahl 1981, Brown et al. quently, in 1963, the national Ringing Centres 1982–2000, Hagan & Johnston 1992, Poole et al. in Europe founded the ‘European Union for 1992–2002, Rappole et al. 1995, Greenberg & Bird Ringing’ (EURING); they agreed on a Marra 2005, Wisz et al. 2007). Consequently, the common code to computerise ringing and annual movements of many bird species, recovery data, and to gather recovery data in a together with their non-breeding distribution, centralised database. The EURING database at least for birds breeding in the northern hemi- was established and maintained at the Dutch sphere, are fairly well known. Ringing Centre in Heteren until 2005, when it was moved to the BTO in Thetford, Norfolk. Currently, the EURING database contains details of some 2.3 million recoveries (Chris du Feu pers. comm.), an extraordinary resource for the analysis of bird movements (for details see www.euring.org). Such an effort would not have been possible without the many enthusiastic volunteer ringers whose spare-time activities are so important for avian science. These volunteers are trained to extremely high levels, the training being co-ordinated by national ringing centres, and participate in targeted scientific projects. This degree of collaboration between profes- sional and amateur ornithologists is unique among biological sciences worldwide. For many decades, the major interest in and objective of bird ringing was to understand the migration routes and non-breeding distribu- tion of birds. As early as 1910, Thienemann published the first 35 recoveries of ringed White Storks, while, in 1929, von Lucanus compiled several hundred recoveries of 127 species and identified the major migration Courtesy of Prof. Dr R. Kinzelbach, Rostock, Germany routes of European birds. The first ‘Migration 48. White Stork Ciconia ciconia carrying central- Atlas’ was published by Ernst Schüz and Hugo African spear.This individual was shot at its nest in Weigold in 1931, and contained 262 maps Mecklenburg, northeast Germany, in May 1822. British Birds 101 • February 2008 • 68–81 69 The mysteries of bird migration New techniques In addition to electronic wizardry, recently In recent years, new techniques have supple- established chemical and molecular markers mented bird ringing and even widened its scope may be used to establish the origin of migrants for establishing migratory routes (Bairlein 2003). and to delineate bird migration routes (Webster One of the most widely used of the new method- et al. 2002). The earth’s surface varies in its ologies for tracking the routes of individual chemical composition. Through diet, birds migrants is that of satellite telemetry, and carry a signature of that chemical composition numerous studies have been conducted. This in their tissues. Stable isotopes are found to enables a much more detailed spatial and tem- function as natural markers and provide new poral resolution of avian migrations and helps to insight into the location histories of highly identify migratory routes, stopover sites and win- mobile animals by delineating the origin of tering grounds, especially of birds for which com- birds feeding in areas where diets differ in paratively few recoveries are available or could be isotope composition (e.g. Hobson & Wassenaar obtained, such as larger or rare species. The tech- 1997, Alisauskas et al. 1998, Bensch et al. 1999, nique has so far been applied only to compara- Hobson 1999, Chamberlain et al. 2000, Ruben- tively large species (e.g. storks (Ciconiidae), stein et al. 2002, Hobson 2003, Lott et al. 2003, cranes (Gruidae), geese (Anatidae), raptors), Bearhop et al. 2005, Yohannes et al. 2007). Simi- owing to the weight of the transmitters, but larly, trace-element composition of plumage miniaturisation of transmitters and improved can be used to identify the origins of migrating receiver sensitivity is likely to enable application birds (e.g. Parrish et al. 1983, Szép et al. 2003). to smaller species. Geolocation (GLS) and Global Positioning System (GPS) are two other new tools Innate migratory behaviour to track migrating birds on a worldwide scale In recent decades much has also been revealed (von Hünerbein et al. 2000; Weimerskirch & about the endogenous control of avian migra- Wilson 2000; Gauthier-Clerc & Le Maho 2001; tions (for reviews see Alerstam 1990, Gwinner Wilson 2001). Geolocation is based on real-time 1990, Berthold 1996, 2001, Bairlein 2002, Bair- measurement of ambient light intensity to deter- lein et al. 2002, Berthold et al. 2003). For their mine geographic co-ordinates, while GPS receives first outbound migration, young migrants data from satellites for calculating a bird’s posi- appear to be equipped with an innate knowl- tion. Initially, these techniques required an edge about timing, distance, direction and ener- archival tag on the bird to collect the data and getic demands. They are capable of finding their subsequent recapture of the bird and recovery of way by using external means of orientation – the logger. However, recent developments to link the sun, the stars, or the earth’s magnetic field GLS and GPS to satellite transmitters allow the (e.g. Wiltschko & Wiltschko 2002, 2003). stored data to be downloaded without recapture.