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Movements of European stonechats Saxicola torquata according to ringing recoveries Helm, Barbara; Fiedler, Wolfgang; Callion, John

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Download date: 01-10-2021 Movements of European Stonechats Saxicola torquata according to ringing recoveries

Barbara Helm1, Wolfgang Fiedler2 & John Callion3

Helm B., Fiedler W. & Callion J. 2006. Movements of European Stone- chats Saxicola torquata according to ringing recoveries. Ardea 94(1): 33–44.

Ringing recoveries of European Stonechats increased more than tenfold to 1162 since the last in-depth analysis in 1965. This paper presents a new overview over all recoveries and detailed analyses of Stonechats ringed during the breeding season in Britain, Benelux countries, and Germany. Stonechats showed stronger migratory tendencies than previ- ously thought. of known origin from all parts of Europe were recorded in north African winter quarters. Local overwintering was only observed in Britain, France, Spain, and Italy. Benelux Stonechats were fully migratory, just as their German conspecifics. British Stonechats were partially migrant; according to conservative estimates almost half (42.1%) of the British Stonechats migrated. Instead of segregating into residents and migrants they travelled over a continuous range of dis- tances. Some reached north African winter ranges but median routes were shorter than those of Benelux and German Stonechats. Surprising- ly, natal dispersal and post-fledging movements were longer in British partial migrants than in continental obligate migrants. These move- ments tended to be biased in migration directions. Sexes and age classes showed identical migratory behaviour, an unusual observation among individually migrating birds. Furthermore, migration of Stonechats did not change consistently over the last decades, counter to predictions. In conclusion, European Stonechats are predominantly migratory but par- tial migration and summer movements seem to be surprisingly plastic.

Key words: Stonechat, migration, differential migration, post-breeding movements, ringing

1Max Planck Institute for Ornithology, D-82346 Andechs, Germany ([email protected]); 2Max Planck Institute for Ornithology, Vogelwarte Radolfzell, D-78315 Radolfzell, Germany ([email protected]); 3Bank Cottage, Allonby, Maryport, Cumbria CA 15 6QH, UK ([email protected])

INTRODUCTION (Pulido et al. 1996, Brown & Brown 2000, Bearhop et al. 2005) while being based on plastic- In our changing world rapid modifications of bird ity in others (Przybylo et al. 2000). To study the migration have been reported (Burton 1995, particulars of migratory species, ringing programs Sutherland 1998, Berthold 2001, Fiedler 2003). are valuable tools as they provide information on Avian movements are diverse. The observed the birds’ origin and life-histories. The Stonechat changes may thus involve genetic change in some Saxicola torquata has been ringed intensively for 34 ARDEA 94(1), 2006

decades and is therefore a good species to investi- classes, typically sex and age (Lack 1943/44, gate migration behaviour (Dhondt 1983, Callion Cristol et al. 1999). Differential migration has been 2002, Urquhart 2002). For partially migratory suggested to be the rule among individually Stonechats increased overwintering and shorter migrating birds, but the pairwise wintering of migration distance have been proposed as a conse- Stonechats makes them a possible exception quence of climate change. Assuming that migra- (Kalela 1954, Cristol et al. 1999). So far, migration tion is genetically determined, selection should differences have been tentatively suggested for have favoured decreasing migratory behaviour Stonechat age groups but not sexes (Johnson over the mild winters of the last decades (Dhondt 1971, Riddiford 1981, Bueno 1991). 1983, Callion 2002, Berthold 2001, Urquhart Our current knowledge about Stonechat migra- 2002). tion is based on a ringing analysis by Van Hecke The Stonechat is a species that displays varia- (1965b) of a small database of 93 recoveries and tion in migration behaviour among geographic on few other, mostly localized studies (Thomson populations. Stonechats belonging to the sub- 1956, Van Hecke 1965a, Zink 1973, Dhondt 1983, species S. t. rubicola are considered resident in Bueno 1991, Callion 2002). Since Van Hecke’s south Europe and obligate short-distance migrant analysis the total number of recoveries has steeply in central and east Europe (Zink 1973; but see risen to 1162. Using this hugely extended data set Wink et al. 2002 for revised ). In con- we re-analysed Stonechat ringing recoveries to trast, west European Stonechats have been address several topics. Firstly, we reassess overall regarded as a separate subspecies S. t. hibernans migration patterns. Next to the general overview and are thought to be partial migrants (i.e. part of we present an in-depth study of Stonechats from the population stays on the breeding grounds Britain, Benelux countries, and Germany. These whereas other individuals migrate; Berthold three focal populations are represented by suffi- 2001). They were classified as ‘basically sedentary’ cient recoveries for detailed analysis. Secondly, in Britain (Thomson 1956, Van Hecke 1965b, within these focal populations we review the year- Johnson 1971, Lack 1986), although regular move- round spatial patterns, including post-breeding ments to north Africa were observed (Zink 1973). movements and dispersal, and thirdly, test for the Recent reviews still regard the majority of British predicted decreasing migration distance and ten- Stonechats as residents, but report frequent sea- dency. Fourthly, we compare migration patterns sonal movements on local scales (Cramp et al. between the sexes and age groups within the focal 1988, Callion 2002, Urquhart 2002). Northern groups. birds often move to mild, predominantly coastal regions, resulting in graded winter densities (Van Hecke 1965a,b, Lack 1986, Cummins 2001). Migra- METHODS tion of Benelux Stonechats and their relationship to S. t. hibernans is under debate (Cramp et al. The schemes of EURING (European Union for Bird 1988, Walker 2001, Wink et al. 2002). Overwinte- Ringing; www.euring.org) contributed 1162 ring- ring birds were suspected to be British migrants ing records until 2003. We excluded local recover- (Van Hecke 1965a) or, alternatively, Benelux resi- ies of one Asian and three African Stonechats dents (Dhondt 1983). (Algeria, Morocco, Tunisia), and combined all A life-history peculiarity of Stonechats is the other records in Fig. 1. The quality of records dif- formation of heterosexual pairs upon arrival on fered widely. However, additional information pro- the winter grounds, followed by vigorous territor- vided in the ringing code helped us select records ial defence (Roedl 1994, 1999). This habit makes which were sufficiently accurate for detailed them an interesting species to examine differential analysis. We excluded birds if finding condition migration, i.e. segregated movements according to was ‘not freshly dead’ or involved captivity, and Helm et al.: STONECHAT MIGRATION 35

Figure 1. Ringing and recovery sites of all European Stonechats. The schemes of EURING contributed 1162 ringing records until 2003. ringing recovery only retained data from birds for which ringing could have either remained on the breeding and finding time were accurate to within one grounds or have already moved on. week. To assure spatial accuracy we excluded birds The more detailed analyses covered only focal that were moved or whose recovery site was regions for which sufficient recoveries of birds with known less accurately than 10' (geographical coor- common background were available. Focal popula- dinates). Data for 1073 recoveries were analysed tions included Stonechats ringed on the British in detail. Sex of birds was specified only if unam- Isles, in Benelux countries, and in Germany during biguously reported by ringer and/or finder. For the breeding season, and two British spring recov- ringed nestlings age and place of origin were eries of birds ringed in autumn in Spain, presum- known exactly. Hatching date was set 10 days in ably on migration (totals in Britain 220, Benelux advance as nestlings are usually ringed in the sec- 316, Germany 141). Classifying birds as migrants ond week of life. Breeding, migration, and winter- or residents is crucial to assess partial migration. ing seasons were defined according to Cramp et al. Lack (1943/44) regarded birds as ‘resident’ if pre- (1988), Glutz & Bauer (1988), and Urquhart sent on the breeding grounds while conspecific (2002). The breeding season was set to last from migrants were recovered at winter sites. Migratory April through August, and wintering from Novem- status thus depends on defining breeding and win- ber through January. Overlap between breeding ter grounds. A frequently used criterion to define and migration left some ambivalence. For instance, migrants in Britain is their departure from the Isles birds ringed at the end of the breeding season in winter (Urquhart pers. comm.). However, this 36 ARDEA 94(1), 2006

definition cannot be transferred to other popula- The longest reported distance was 2513 km tions, e.g. Benelux Stonechats. It also excludes flown by a Belgian nestling recovered in Morocco movements within Britain, although distances in its first winter (Fig. 2A). An immature ringed in within the Isles can exceed those to the continent. The Netherlands on 27 August 1964 was found We therefore define migration based on flown dis- freshly dead only twelve days later in Morocco, tance by empirically deriving a divide in travelled after a journey of 2450 km. North African winter- distance between residents and migrants. Birds ing has been recorded for Stonechats from all main recovered in winter closer to home than the divide European regions, including Spain, France, the are regarded as residents, and those that flew fur- focal countries, and Austria, Czech Republic, and ther as migrants (see Results). Slovakia (Fig. 1). Central and east European Changes in migratory behaviour over time were Stonechats were fully migratory and moved in examined by dividing the data into two intervals. south-westerly directions, except for a Hungarian The main climb of global temperatures started nestling captured in Greece in November. Sample around 1980 (Houghton et al. 2001). Accordingly, sizes for a closer analysis of north African winter we assigned recoveries until 1979 to one group quarters were low but suggest more easterly win- and more recent recoveries to a second. This cut-off tering of east European Stonechats (mean coordi- point yielded roughly equal group sizes. We nates east Europe 36.4°N, 5.3°E; n = 4; Britain analysed changes in the proportion of migrants and 35.6°N, 1.3°W; n = 8; Benelux 34.5°N, 3.W°E; n = in loxodromic (straight lines on geographical mer- 34; Germany 35.7°N, 0.4°E; n = 47). African win- cator-representation) distance flown. Flown dis- ter ranges differed significantly in longitude and tances differed in variance among populations and latitude (log-linear model to account for presence were therefore analysed over ranks (Conover & of birds, longitude: deviance ratio 40.3; df = 20; Iman 1981) by restricted maximum likelihood P < 0.005; latitude: deviance ratio 40.7; df = 20; methods (REML). We present non-parametric sta- P < 0.004). tistics but for comparative purposes also mean ± European winter ranges comprised Spain, SD, although the latter measures are problemati- Portugal, France, Belgium, Britain, Italy, and cally sensitive to outliers (Paradis et al. 1998). Greece. True residency, i.e. local overwintering, Maps were produced using GIS Arc View 3.0. was recorded only in Spain, France, Britain, and Italy. Whereas Italian Stonechats were only locally recovered, the other populations were partial migrants. Of 12 French winter recoveries, three RESULTS were found in France, six in Spain and three in north Africa (median distance 958 km; range Migration of European Stonechats 0–1806 km). Four Spanish-bred Stonechats showed Stonechat recoveries started in 1923 and were considerable movement within Spain and to reported at low rates typical of birds. Morocco (median distance 639 km; range 0–660 Based on schemes which provided ringing and km). Crossings of the Mediterranean are common. recovery numbers, an overall proportion of 0.56% Migrants recovered on islands originated from birds was recovered (746 of 135,599). Recovery France, the focal countries, and Hungary. On was lowest in Germany (0.42%) and highest in Malta migrants arrived in October and November The Netherlands (1.30%). Maximal ages are with about a quarter staying for the winter (J. around 10 years, based on two ringed adults Sultana pers. comm.). 1.95% of c.1850 Stonechats recovered with high accuracy eight and nine years ringed on Malta until 1993 returned for at least after banding and on a Spanish immature recov- one (up to four) additional migration periods. ered 12 years and 8 months after ringing without Return dates of these birds were relatively precise: finding details. 20 of 24 autumn arrivals occurred within two Helm et al.: STONECHAT MIGRATION 37

weeks of their prior capture date (median devia- than British migrants, and their winter areas were tion 7 days). Adults dominated among returning less spread out (U = 185.0; P = 0.001; n = 69; χ 2 migrants (adult males 30%; adult females 27%; Bartlett’s test for homogeneity of variances: 2 = young males 6%; young females 14%; n = 138). 10.7; P = 0.005; n = 70). Mean vectors of the focal groups were clearly directed (Fig. 3). The Focal populations: British, Benelux, and vector of British migrants was outside confidence German Stonechats limits of Benelux and German populations, which MOVEMENTS THROUGHOUT THE YEAR in turn oriented in almost identical directions Individual Stonechats differed greatly in the timing (British 158.6°; n = 41; Benelux 204.6°; n = 75; and extent of their movements (Fig. 2). Winter German 203.6°; n = 59; Raleigh test P < 0.005 in recoveries ranged from the British Isles to North all cases). Africa, without records from Benelux countries In spring (Fig. 2 B) some birds approached the (Fig 2A). Among British Stonechats, the flown dis- breeding grounds as early as February while others tance distribution showed a clear peak at short were still in North Africa. By March all British distances and a long tail over a surprisingly contin- Stonechats but only some of the Benelux and uous range. Of 38 winter recoveries, 22 birds had German Stonechats were caught on the breeding stayed within 107 km, while the others flew grounds. In April no migrants could be identified. between 392 km and 1925 km. The gap between During the main breeding season (Fig. 2 C) recov- 107 km and 392 km was used to distinguish ery distances probably indicate dispersal but could migrants from residents and yielded a migrant pro- also still relate to migration. An adult found in portion of 42.1%. If defined as leaving Britain, Belgium in June had been ringed in Britain in migrants amounted to 39.5%. Five additional birds April (cf. Van Hecke 1965b). A British Stonechat were recovered in Spain, Algeria, and Morocco was found breeding in The Netherlands in its sec- (distance 2012 km), but recapture times were not ond year of life (716 km from natal ringing site), precisely known. British migrants took more and another was freshly killed in Spain in May south-easterly routes than continental birds. (1426 km from natal ringing site). A German first- In contrast, all Benelux and German Stone- year bird was caught in early May in Algeria. Two chats wintered far from the breeding grounds, no Stonechats had dispersed over c. 200 km within closer than the south French coast. Overall, the Germany by their second year, and three had continental populations migrated over similar dis- moved short distances into Benelux countries. tances (Table 1; U = 260.0; P = 0.318; n = 54). Benelux birds were not found at considerable dis- They wintered further from the breeding grounds tances from ringing sites in summer.

Table 1. Movements of Stonechats from three focal populations in which sufficient recovery data allowed detailed analysis.

Population Benelux British migrants German natal dispersal (km) median (range) 2.2 (0-106) 15.2 (0-1426) 10.0 (0-1574) mean ± SD (n) 11.7 ± 22.1 (29) 76.8 ± 276.1 (26) 77.2 ± 278.2 (30) post-fledging (km) median (range) 0.0 (0-36) 2.1 (0-53) 4.6 (0-10) mean ± SD (n) 2.4 ± 6.2 (39) 11 ± 15.7 (31) 2.8 ± 3.9 (7) migration (km) median (range) 1774 (865-2513) 1139 (392-1925) 1702 (1073-1897) mean ± SD (n) 1726 ± 313 (37) 1212 ± 493 (16) 1661 ± 222 (17) 38 ARDEA 94(1), 2006

BENELUX BRITISH ISLES GERMANY

November A December January

February B March April

Figure 2. Recovery locations of Stonechats from focal populations around the year (Benelux countries, British Isles, and Germany). Dots present recovery locations, lines refer back to the ringing site (solid line: recovered during the same migration period; dotted line: recovered during subsequent migration period). Months are indicated by different sym- bols. Periods distinguished are winter (A), spring (B), summer (C), and autumn (D). C and D on next page.

χ 2 Natal dispersal, i.e. recovery distance of pulli Germany 75.0%; Benelux 51.6%; 2 = 9.38; P = in later breeding seasons, differed between popu- 0.009; n = 92). Sex had no significant effect on lations (Table 1; Kruskal-Wallis analysis H = 9.6; dispersal (median distance, male 8.9 km; female df = 2; P = 0.008; n = 87). British Stonechats 16.1 km; U = 335.5; P = 0.624; dispersal percent- χ 2 travelled longer distances than German and Bene- age, male 82.1%; female 73.1%; 1 = 0.64; P = lux conspecifics (U = 537.5; P = 0.017), which in 0.423; n = 54). Mean directions tended to be turn did not differ from each other. The proportion biased along migration axes. For all young, the of pulli recovered more than 1 km from the ringing mean direction was 227°. Directions of Benelux site (‘dispersers’) was also highest in Britain, inter- and German Stonechats were mostly oriented mediate in Germany, and lowest in Benelux coun- towards south-west and north-east, those of British tries (dispersal percentage in Britain 88.5%; Stonechats towards south-east and north-west. Helm et al.: STONECHAT MIGRATION 39

BENELUX BRITISH ISLES GERMANY

May C June July

August D September October

Figure 2. Continued

χ 2 Post-fledging movements, movements of pulli young 45.9%; adult 29.4%; 1 = 1.6; P = 0.211; during their first summer, showed similar tenden- n = 102). Between breeding seasons, however, cies: British immatures flew longer distances than adults were less likely to disperse and flew shorter continental Stonechats (Kruskal-Wallis analysis distances than young (median distance first-year H=5.5; df = 1; P = 0.019; n = 77) and dispersed 8.9 km; adult 0 km; U = 846.5; P < 0.001; disper- χ 2 at a slightly but non-significantly higher proportion sal proportion young 67.4%; adult 29.0%; 1 = (Britain 51.6%; Germany 42.9%; Benelux 38.5%; 22.6; P < 0.001; n = 132). χ 2 1 = 1.17; P = 0.279; n = 77). Summer move- After local movements in summer, the first bird ments of adults were represented by few recover- on migration was found in September as far south ies. We therefore pooled the records from all focal as southern France (Fig. 2D). In October Stone- populations for comparison with natal dispersal. chats of all focal populations had reached winter Within a breeding season adult movements did not quarters while others were still at breeding sites, differ from those of young (median distance both 0 revealing pronounced individual differences in the km; U = 629.5; P = 0.407; dispersal proportion timing of migration. 40 ARDEA 94(1), 2006

BENELUX BRITISH ISLES GERMANY 0° 0° 0° 40 8 20

20 4 10

270° 90° 270° 90° 270° 90°

180° 180° 180°

Figure 3. Orientation of migratory flights of the focal populations in the Benelux countries, on the British Isles and in Germany. Vector length gives the number of records for a given direction. Thin line marks are the mean vector ± 95% confidence interval.

DIFFERENTIAL MIGRATION distances increased significantly in the continental Winter distributions of sex and age classes indicate populations after 1979 (Benelux U = 53; P = no differential migration (Fig. 4). The sexes resem- 0.019; n = 37; German U = 10.0; P = 0.037; n = bled each other in migration distance (median 17) but decreased non-significantly in British males 1467 km; mean ± SD 1542 ± 21 km; fe- Stonechats (U = 21.0; P = 0.299; n = 16). The males 1458 km; mean ± SD 1286 ± 26 km; U = proportion of British migrants declined non-signifi- 28.0; P = 0.559; n = 19). Similarly, flown dis- cantly from 56.3% to 31.8% after 1979 (likelihood tance did not differ between age groups (median ratio = 2.28; df = 1; P = 0.131; n = 38). For first year 1712 km; mean ± SD 1600 ± 21 km; pooled residents and migrants distances decreased adult 1614 km; mean ± SD 1591 ± 19 km; U = after 1979 (U = 103.5; P = 0.031). 887.0; P = 0.734; n = 90). In partially migratory British Stonechats there was also no tendency for age-related migration differences (median distance DISCUSSION young 1190 km; mean ± SD 1225 ± 23 km; adult 1266 km; mean ± SD 1207 ± 23 km; U = 21.0; European Stonechats are to a greater extent migra- P = 0.949; n = 15; migrant percentage young tory than hitherto assumed (Van Hecke 1965b, Zink χ 2 44.0%; adult 33.3%; 1 = 0.38; P = 0.536; n = 1973). Local differentiation of their migratory 37). We found no relationship between hatching behaviour was partly confirmed: we observed par- date and flown distance in first-year migrants (lin- tial migration in Spain, France, and Britain, full ear regression; all F57,1 = 1.44; P = 0.236; British residency in Italy, and exclusive migration in all F24,1 = 0.20; P = 0.658). other populations. Among partial migrants, i.e. populations consisting of both migrants and resi- TIME TRENDS dents (Berthold 2001), flown distances were sur- Overall wintering distance did not differ between prisingly large and migrants were approximately as birds recovered after 1979 and those discovered frequent as residents. Unlike other partial migrants, earlier (REML analysis: Wald statistic = 1.39; df = British Stonechats did not travel shorter distances 1; P = 0.239). The populations differed clearly in than migratory populations but flew all the way to flown distance (Wald statistic = 19.05; df =2; North African winter quarters (e.g. Lack 1943/44, P < 0.001) and tended to behave differently over Berthold 2001). Furthermore, British Stonechats time (interaction time and population: Wald statis- wintered along a continuous range of distances tic = 5.4; df = 2; P = 0.067). In greater detail, instead of segregating into residents and migrants. Helm et al.: STONECHAT MIGRATION 41

male female adult first-year A British Islands B British Islands Benelux Benelux Germany Germany

Figure 4. Winter location of focal populations by sex (A) and age (B).

This result conflicts with the idea of a clear-cut a rigorous definition of winter (November to dimorphism in migratory behaviour. Our account January; Fig. 2A) no local birds were recovered in may underestimate migrant behaviour because we Benelux countries. Furthermore, the correlations excluded not fully accurate winter records, because between numbers of wintering Stonechats and we narrowly defined ‘wintering’ (November to preceding winter temperatures per se (Dhondt January), and because reporting rates may be 1983) offer weak support for local overwintering. higher for British residents than Mediterranean Winter frequencies could covary with numerous migrants. In autumn, Spanish and French Stone- factors, for instance during breeding or migration chats emigrated from regions used as winter quar- at different locations. Our data tentatively support ters by British and central European conspecifics, a link to British Stonechats (Van Hecke 1965a). although local pairs are expected to have prior- Although no British-ringed birds were recovered in occupancy advantages over their territories. Field winter, we found anecdotal evidence of natal dis- studies could clarify whether their emigration is persal from Britain into the Benelux breeding pop- related to the influx of northern migrants. ulation. Such birds – and possibly their offspring – Benelux Stonechats were clearly migratory, could explain occasional overwintering as well as matching German migrants in distance and timing the close genetic affinity between S. t. rubicola and of movements (Fig. 2, Table 1). The origin of win- S. t. hibernans (Urquhart 2002, Wink et al. 2002). tering birds in Benelux countries thus remains Analyses of time trends in migration yielded obscure. Our data do not support Dhondt’s (1983) contradictory results. Overall migration distances hypothesis of local overwintering which was based remained constant, while in greater detail journeys on recoveries in October when migrants are often had lengthened for continental migrants but short- still near breeding sites (Fig. 2D; Cramp et al. ened for British partial migrants. If corroborated 1988, Glutz & Bauer 1988, Urquhart 2002). Under by more data these developments could reflect 42 ARDEA 94(1), 2006

locally different responses to environmental chal- studies from winter quarters: reductions of Stone- lenges. Possible changes in migration were also numbers affected young disproportionately. suggested for Eastern Stonechats (S. t. maura Roedl (1994, 1999) estimated that under excep- group; Wink et al. 2002), based on increased sight- tionally harsh circumstances 30% of Stonechats, ings in central Europe predominantly in autumn mostly young males, did not find partners and (e.g. Robertson 1977, Glutz & Bauer 1988, Urqu- vanished from Israelian winter grounds. Provided hart 2002, E. Urquhart pers. comm.). These sight- that these birds survive to move on, differential ings are currently hard to interpret because diag- migration may occur as a density-dependent nostic criteria of Stonechat taxa are under debate behaviour under harsh conditions (Kalela 1954). (P. Barthel pers. comm.). A surprising deviation from ‘typical’ patterns European migrants wintered around the Medi- was found for dispersal. Whereas shorter dispersal terranean Sea. South-easterly and south-westerly has been associated with shorter migration route directions, respectively, took British and continen- (Baker 1993, Paradis et al. 1998), British partial tal Stonechats to similar winter quarters (Glutz & migrants dispersed further than continental mi- Bauer 1988). East European Stonechats reached grants. British birds could differ from continental North Africa at least in part by crossing the Medi- relatives because the Isles provide an unusual terranean Sea, as indicated by passage captures locality. Immature British Stonechats aggregate to (Van Hecke 1965b, Glutz & Bauer 1988, Bueno small moulting flocks and preferably move to the 1991) and island records. The ‘small island’ project coast (Urquhart 2002). Movements of observed (Massi et al. 1995, Messineo et al. 2001) reports flocks ranged between 2 and 35 km from natal passing Stonechats on most islands, sometimes in sites (J. Callion, unpubl. data). This behaviour considerable numbers during peak passage. Site could thus enhance dispersal and simultaneously tenacity has been reported from breeding and win- genetic exchange, as none of the observed flocks ter sites (recapture proportions 35 to 40%; comprised any siblings (Callion 2002). Further- Frankevoort & Hubatsch 1966, Roedl 1994, 1999, more, dispersal could be promoted by flexible Sultana & Gauci 1982). Data from Malta indicate movements, considered typical of partial migrants that at least some Stonechats, similar to other (Lack 1943/44), which may be associated with passerine species, also return to convenient migra- migration. Dispersal tended to be oriented along tion stopovers (Cantos & Telleria 1994, Catry et al. migratory axes. Anecdotal evidence, for example 2004). Accurate return dates over up to four years of British Stonechats recovered in the summer in furthermore suggest conservative route choice. The Netherlands and Spain, suggest that birds Stonechats deviated from ‘typical’ migrants by could terminate migration en route under con- an absence of differential migration between the ducive conditions. More generally, studies of dis- sexes, possibly related to wintering in heterosexual persal may tend to underestimate opportunistic pairs (Johnson 1971, Roedl 1994, 1999, Cristol et termination of migration as a side-effect of remov- al. l999, Urquhart 2002). We also found no evi- ing ‘migration bias’. For instance, by excluding dence for a longer migration of young Stonechats long dispersal distances in migratory directions, suggested by autumn sightings from coastal loca- Paradis et al (1998) may have in turn biased their tions (Van Hecke 1965b, Johnson 1971, Riddiford findings against detecting links between migration 1981, Bueno 1991). A prevalence of juveniles and dispersal. before crossing open water, termed ‘coastal effect’, Taken together, European Stonechats appear to has been described in other species and was behave more migratory than previously assumed. related to lack of experience, rather than true However, several traits, including variable migra- migration differences (Ralph 1971, 1978). How- tion distance and dispersal patterns, suggest plas- ever, a tendency of young Stonechats to migrate ticity of their migratory behaviour to yet unknown further than adults finds some support in field environmental factors. Helm et al.: STONECHAT MIGRATION 43

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