Sedge Warbler 38. schoenobaenus

Until 1961, the year in which Moreau published his ground-breaking review of the problems surrounding Mediterra- nean-Saharan migration, very little evidence had been available for the presence of large numbers of Palearctic migrants in West . Sedge Warblers were known to winter east of Chad, and only because they were seasonally well distributed in the western Mediterranean was it considered ‘pretty certain’ that they wintered across the whole of Africa south of the . Large-scale ringing on the breeding grounds, especially after the introduction of mist nets in the 1970s, began to change this monumental gap in our knowledge, albeit fi tfully, because even ringing more than a million Sedge Warblers, over 600 000 of which had been ringed in Britain alone (Wernham et al. 2002), produced few recoveries from West Africa (Zink 1973, Glutz von Blotzheim & Bauer 1991). In the 1960s, the advent of ringing expeditions to wintering grounds, so that species could be studied there, improved the chances of recovery substantially. Furthermore, they provided a wealth of data on habitat use, stopover strategies, timing of fattening and arrival and departure dates. These expeditions continue to this day. The recent introduction of stable isotope ratio techniques to establish the proportions of trace elements in feathers, thus providing clues to individual ’ breeding grounds, staging posts and wintering grounds, is an invaluable step in defi ning population distributional patterns and changes (Hobson et al. 2004). Such techniques have the potential to revolutionise our understanding of the peregrinations of migrant birds, especially those with small chances of recovery.

4436 Livvinng onn thee edge Breeding range netsov & Titov 2001). This should translate into low stopover site fi- delity, as suggested by Bibby (1978), but exceptions do occur: in a The breeding range of Sedge Warblers extends from Great Britain to reedbed near Madrid in central Spain, individual birds were recap- the Yenisey River in West Siberia, between 45° and 65°N for the tured frequently during autumn (outward) ànd spring (return) mi- most part. Breeding limits lie between the July isotherms of 12° and gration, indicating site fidelity (Bermejo & de la Puente 2002). The 30°C in the boreal, temperate, Mediterranean and steppe climate higher speed of migration compared with Reed Warblers (55 km zones. In the early 1990s, the European population was estimated at versus 39 km per day) and the lower fat loads of Sedge Warblers at 4.4-7.4 million pairs, biased towards northern and eastern coun- departure (southern , Bensch & Nielsen 1999) indicate that tries, including Russia (BirdLife International 2004a). The entire the latter must accumulate energy reserves on their way south, as is population winters in Africa south of the Sahara. known in western (Bibby & Green 1981, Bargain et al. 2002) and is thought to occur in the Pannonian Lowlands in Central Eu- rope (Gyurácz & Csörgö 1994, Procházka & Reif 2002). Fat reserves Migration accumulated by juveniles during migration in southern Hungary theoretically sufficed to cover some 1400 km, enabling nonstop The essentially single-brooded Sedge Warblers are, from the mo- flight across the Mediterranean (Gyurácz & Bank 1996). However, ment the young have reached independence, in a hurry. They start Scandinavian birds captured in northern carried only moder- leaving the breeding sites from late July and early August onwards ate or low amounts of fat (Basciutti et al. 1997). Minor daily increas- (Finnish and Swedish breeding sites; Koskimies & Saurola 1985, es in body mass were recorded for this region, as were very short Hall 1996), or slightly later (mid-August, Central ; Procházka stopovers, and so there can be no doubt that these birds must use & Reif 2002). On average, adult peak migration is between 4-7 days fattening areas further south in Italy or in northern Africa. This (in some years up to 22) ahead of that of juveniles, a feature typical throughout Europe (Insley & Boswell 1978, Bibby & Green 1981, Røs- tad 1986, Litérak et al. 1994, Gyurácz & Csörgö 1994, Basciutti et al. 1997, Bermejo & de la Puente 2002, Zakala et al. 2004). Juveniles may at first disperse in any direction, including north, before heading towards the wintering grounds (Gyurácz & Csörgö 1994, Basciutti et al. 1997, Procházka & Reif 2002). The adults, whose body masses re- main consistently higher than those of juveniles, head directly for their wintering quarters and are more adept in finding food bonan- zas en route. One of the main prey, aphids, show steep seasonal de- clines, to the point that aphid abundance in southern Europe has already dropped below levels that are worth exploiting by the time Sedge Warblers begin to migrate (Bibby & Green 1981). Further- more, aphid abundance differs substantially between areas and years, from almost non-existent to superabundant (Bibby & Green 1981, Bargain et al. 2002). Adult experience in locating and exploit- ing patchy food supplies may enhance migration speed and effi- ciency of fat loading. However, it is an oversimplification to consider aphids as the sole diet of migrating Sedge Warblers. At Bouche d’Ognon near Lac de Grand Lieu in France, Bibby & Green (1983) considered Sedge Warblers fattening on mayflies as an interesting exception, but the extensive research of Chernetsov & Manukyan (2000) on the Baltic’s Courish Spit showed unambiguously that this species actually forages on a wide range of small invertebrates as- sociated with aquatic or moist habitats and reeds, i.e. on Chirono- midae, beetles, spiders, aphids and aphid consumers (mostly small Fig. 249 European origins of 372 Sedge Warblers recovered or captured parasites and predators). Depending on seasonal variations in between 4° and 37°N. To avoid cluttering the figure, lines for birds abundance, the species easily switched from one group of abundant ringed or captured in the Delta (red square) are omitted; in- invertebrates to another. Sedge Warblers are clearly efficient in find- stead, red dots indicate ringing sites of 33 birds recaptured in Senegal ing unpredictable patches possessing a high variety of arthropods, or 107 birds ringed in Senegal and recaptured subsequently in Europe. and in tracking the phenology of these ecological complexes (Cher- From: euring.

SSeddge WWarrbler Acrocceephaluss scchoennobaenus 437