ORIGINAL ARTICLE

Jochen Dierschke Æ Franz Bairlein

Habitat selection of wintering passerines in salt marshesof the German Wadden Sea

Received: 30 April 2003 / Revised: 4 August 2003 / Accepted: 29 August 2003 / Published online: 8 January 2004� Dt. Ornithologen-Gesellschaft e.V. 2004

Abstract The salt marshes of the Wadden Sea areimportant wintering areas for some species of granivo-rous passerines, which have declined considerably sincethe 1960s. We investigated the habitat choice of allwintering passerines in eight study areas in German saltmarshes with special consideration of human impact onthese habitats. Granivorous species that almost exclu-sively winter in salt marshes, Shorelark (Eremophila al-pestris), Snow Bunting (Plectrophenax nivalis) and Twite(Carduelis flavirostris) were concentrated in the lowersalt marsh vegetation and in the driftlines, while all otherspecies preferred the high upper salt marsh communities,although Rock Pipits (Anthus petrosus littoralis) fed inmuddy areas along ditches. Shorelarks switched habitatin conditions where seeds were scarce to feed instead onarthropods in upper salt marshes. Intensively sheep-grazed upper salt marshes resemble lower salt marshes intheir vegetation and were therefore mainly visited byShorelarks, Snow Buntings and Twites. In winter, thedriftline is preferred by the two former species, while inautumn and spring more birds foraged in the salt mar-shes. Twites prefer to feed mainly on seeds of Salicornia.Areas with S. europaea are visited mainly in late autumnand early winter, while areas with S. stricta are usedthroughout the winter because of a steady supply ofseeds. Several years after embankment, polders arehardly used any more by the lower salt marsh species asthe habitat changes into freshwater marshes. Largeembankment projects since the early 1960s have in-cluded salt marshes and intertidal flats, and the resultantloss of habitat is responsible for the decline of lower saltmarsh species. For other passerine species the effects ofreclamation are unknown. The effects of intensifiedgrazing on the wintering populations of Shorelark, SnowBunting and Twite are still unresolved. Although

grazing supports lower salt marsh vegetation, the seedproduction per plant is much lower there and someimportant seed producers hardly occur. Since grazingwas reduced and embankment projects have been stop-ped, the salt marsh areas (especially lower salt marshes)have increased and so have the wintering populations ofShorelark, Snow Bunting and Twite. For the otherspecies, the consequences of habitat changes are un-known, although it is suggested that reduced grazing willsupport them. Reducing the human impact on saltmarshes will, in the long run, probably lead to a naturalsalt marsh with much variety in elevation and in itscorresponding vegetation and bird communities.Meanwhile, management by grazing might be requiredin parts of the salt marshes.

Keywords Embankment Æ Grazing Æ Salt marsh ÆWadden Sea Æ Wintering passerines

Introduction

The salt marshes of the Danish-German-Dutch WaddenSea provide habitats for huge numbers of breeding,migrating and wintering birds (e.g. Meltofte et al. 1994).However, passerine species have so far been neglectedwhen assessing the importance of the Wadden Sea. Re-cently, Dierschke (1997, 2001a) stressed the significancefor Shorelarks (Eremophila alpestris) and Twites(Carduelis flavirostris), with at least 53% and 47% of theScandinavian population wintering in this area. Addi-tionally these salt marshes contain large numbers ofwintering Rock Pipits (Anthus petrosus littoralis; e.g.Busche 1980; Bijlsma et al. 2001). Huge areas of saltmarsh have been dramatically changed since the 1960s,mainly through embankment projects in the WaddenSea and increasing numbers of cattle and sheep on theremaining salt marshes (Stock et al. 1997). The effectson plants and arthropods, as well as breeding androosting birds, have been described by several authors

J Ornithol (2004) 145: 48–58DOI 10.1007/s10336-003-0007-4

Communicated by F. Bairlein

J. Dierschke (&) Æ F. BairleinInstitut fur Vogelforschung ‘‘Vogelwarte Helgoland’’,An der Vogelwarte 21, 26386 Wilhelmshaven, Germany

(e.g. Bakker 1978, 1985; Heydemann 1983/1984; Stockand Hofeditz 2000; Thyen 2000). A dramatic decline ofShorelarks has been noted in their breeding and win-tering areas (Hilden 1987; Svensson 1990; Dierschke1997) and changes in the Wadden Sea have been con-sidered as possible causes for this decline (Dierschke1997).

The aim of this study is to identify the habitat choi-ces of wintering passerines in the salt marshes, todiscuss the effects of human interference with thesehabitats on the population of granivorous passerines,and to discuss potential management tools for the saltmarshes.

Methods

From September 1995 until April 2000, eight sites were studied inthe German part of the Wadden Sea (Fig. 1). Two differentmethods were used:

From late September to late April, all Shorelarks, SnowBuntings and Twites were counted bimonthly along driftlinestretches and their adjacent salt marshes (up to 150 m from theseawall). All stretches are described in Table 1, and see Diers-chke (2001b) for further details. As shown previously (Dierschke2001b), most birds were feeding in these areas during daylight.Birds within the driftline and those in the salt marshes weredifferentiated. However, driftlines could not always be accuratelyassessed, as they may vary considerably in height and extension.Therefore, the area between the top of the dike and the begin-ning of the salt marsh was defined as driftline, even if there wasno actual driftline visible, while birds feeding along small drift-lines within the salt marshes were counted as in the salt marshes.Thus, the width of the driftline-area was some 20 m while thatof the investigated salt marsh area was usually 150 m. The 32stretches of driftline (55.4 km in total) were grouped by a cluster

analysis with several parameters, classified from 0–3 (extent ofadjacent salt marsh, amount of upper/lower salt marsh vegeta-tion, height of the salt marsh above mean high tide line, heightof vegetation and amount of driftline), into nine groups withdifferent numbers of sites (Fig. 2). Between these groups thenumber of roosting Shorelarks, Snow Buntings and Twites werecompared in three seasons: autumn (15 October to 30 Novem-ber), winter (1 December to 15 February) and spring (16 Feb-ruary to 15 April). Data outside these seasons were only used forthe distribution of birds between salt marsh and driftline. As thebirds winter in flocks and some species are faithful to one sitethroughout the winter, the data are not independent on anindividual level. However, the aim of this study was to comparedensities between different habitats and seasons. Thus, althoughdata were pseudoreplicated, the mean densities of birds perseason and habitat were compared at different sites in the clustertree (see Fig. 2) with Bonferroni-corrected Mann-WhitneyU-tests.

At least twice per month all passerines were counted in elevensalt marsh plots (see Table 2 for characterization). Additionally, inwinter 1995-1996, five plots were surveyed in the BeltringharderKoog, a recently (1986) embanked bay, which still consists of dif-ferent stages of polder succession from salt to freshwater marshesand is therefore treated separately. The plots were grouped intothree habitats: lower salt marsh, upper saltmarsh and sanddepositing areas (the latter only for Shorelarks, Snow Buntings andTwites). Grazed salt marshes have a similar vegetation to the lowersalt marsh and were therefore placed into this group. Differences inhabitat use were tested with the Kruskal-Wallis and NemenyiTest(Shorelarks, Snow Buntings and Twites) or Mann-Whitney U-test(all other species). At Tumlauer Bucht, two of the study plotsdiffered only in their agricultural use (intensively grazed by sheepversus ungrazed since 1991). Only data with the relevant speciespresent in at least one of the plots were used. Differences in birddensities between the plots were assessed with the Wilcoxon-Wilcoxtest.

Results

Habitat choice of Shorelarks, Snow Buntings andTwites in the driftlines and adjacent salt marshes

Densities of birds (birds per km driftline) areshown for all habitats in Fig. 3, statistics are shown inTable 3.

Shorelark

Driftlines without adjacent salt marshes (groups 7–9),upper ungrazed salt marshes without driftline (group 6)and mowed salt marshes (group 2) were hardly visited atall. In upper salt marshes with large driftlines (group 3)birds were seen mainly in winter. In all seasons, the areaswith lower salt marsh vegetation and large driftline(groups 1 and 4) were commonly visited, while this holdstrue for such areas without driftline (group 5) only inautumn and spring. In areas with large driftlines (group1), Shorelarks were more abundant along the driftline inwinter, while in spring and autumn they were moreassociated with the salt marshes (Table 4). Those areaswithout driftlines therefore held Shorelarks only in au-tumn and spring (Fig. 3). However, in areas with highproportions of Salicornia stricta (group 4) more birds

Fig. 1 Location of the study sites in the German Wadden Sea (fordescriptions of the study sites see Dierschke 2001b)

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fed in the salt marsh than in the driftline. In areas withupper ungrazed salt marshes (group 3), Shorelarks werealmost exclusively feeding in driftlines, except in latespring (Table 4).

Snow Bunting

Snow Bunting was the only species which commonlyused driftlines without adjacent salt marshes (groups7–9). Areas with high densities of Salicornia stricta(groups 4 and 5) were only attractive in autumn. In allseasons Snow Buntings were more associated with thedriftlines, regardless of whether there were salt marshesor not (Table 4).

Twite

In autumn, Twites roosted mainly in areas with highdensities of Salicornia(groups 1, 4–6 and 8). Usually theywere more abundant in the salt marshes than in thedriftlines, except in the upper salt marshes (group 3).Additionally, they were more associated with the drift-line in areas with no salt marshes (group 7), but as soonas there were some Salicornia patches (group 8), thesewere preferred (Table 4).

On the island of Wangerooge, where different habi-tats are well separated, Shorelarks and Twites were mostabundant in the lower salt marsh communities, whileSnow Buntings were present nearly exclusively on thesandy beaches. The upper salt marshes were hardlyvisited at all by any of these species (Fig. 4; Kruskal-Wallis, P<0.05 in all seasons).

Habitat choice of wintering passerineswithin the salt marshes

In all seasons, Shorelarks roosted only exceptionally insalt marshes with tall vegetation. Highest densities wererecorded in salt marshes with vegetation of lower saltmarshes. Similar high densities were found on sanddepositing areas, but these were never used for roostingin winter (Fig. 5). Snow Buntings were seen only rarelyin the study plots as they were more abundant in thedriftlines (see above). They were only recorded in tallvegetation during deep snow cover and only once on thesand depositing areas, while the lower salt marsh vege-tation was visited irregularly. Twites were recordedregularly in both salt marsh types and irregularly on thesand depositing areas. In winter, they were significantlymore abundant in the lower salt marsh vegetation than

Table 1 Number of winters surveyed, length, use and vegetation types of different driftline stretches

Stretch Area Length[m]

Surveyed winters Adjacent vegetation (main species)

BK41 Beltringharder Koog 1,620 1995/1996 Mudflats, no salt marshBK43 BeltringharderKoog 825 1995/1996 Intensive sheep grazing, Salicornia and Suaeda, some areas ungrazedW6 TumlauerBucht 1,600 1995/1996–1999/2000 Mudflats, some intensively sheep grazed patches with Puccinellia

and SalicorniaW41 TumlauerBucht 2,200 1995/1996–1999/2000 Intensive sheep grazing, Salicornia and Suaeda, some areas ungrazedW42 TumlauerBucht 950 1995/1996–1999/2000 Intensive sheep grazing, Salicornia and Suaeda, some areas ungrazedW43 TumlauerBucht 1,600 1995/1996–1999/2000 Intensive sheep grazing, Salicornia and Suaeda, some areas ungrazedW44 TumlauerBucht 1,350 1995/1996–1999/2000 Intensive sheep grazing, Salicornia and Suaeda, some areas ungrazedW45 TumlauerBucht 1,700 1995/1996–1999/2000 Intensive sheep grazing, Salicornia and Suaeda, some areas ungrazedW46 TumlauerBucht 1,300 1995/1996–1999/2000 Intensive sheep grazing, Salicornia and Suaeda, some areas ungrazedJ11 Jadebusen 2,750 1998/1999–1999/2000 Mudflats, no salt marshJ12 Jadebusen 2,130 1998/1999–1999/2000 Mudflats, no salt marshJ2 Jadebusen 2,050 1998/1999–1999/2000 Ungrazed lower salt marsh, Aster, Atriplex, Halimione, Suaeda,

SalicorniaC1 Crildumersiel 1,910 1998/1999–1999/2000 Mudflats, some patches of Salicornia and SpartinaC2 Crildumersiel 900 1998/1999–1999/2000 Mudflats, some large patches of Salicornia and SpartinaC3 Crildumersiel 900 1998/1999–1999/2000 Puccinellia, intensive sheep grazing, large Salicornia fieldsE1–3 Elisabeth-Aussengroden 4,000 1998/1999–1999/2000 Upper ungrazed salt marsh, Elymus, Atriplex and AsterE4 Elisabeth-Außengroden 2,450 1998/1999–1999/2000 Upper salt marsh, mowed in late summer; TriglochinI1 Wangerooge 330 1998/1999–1999/2000 Ungrazed lower salt marsh, Salicornia, Suaeda, Halimione and LimoniumI2 Wangerooge 1,440 1998/1999–1999/2000 Ungrazed lower salt marsh, Salicornia, Suaeda, Halimione and LimoniumI3 Wangerooge 1,450 1998/1999–1999/2000 Variable ungrazed salt marsh, Salicornia, Suaeda, Halimione,

Aster, LimoniumI4 Wangerooge 1,320 1998/1999–1999/2000 Mudflats, small sandy beachI5 Wangerooge 990 1998/1999–1999/2000 Sandy beach with some gravel; eastern tip of the islandI6 Wangerooge 3,320 1998/1999–1999/2000 Large sandy beach, edged by dunesB2 Bensersiel 1,275 1999/2000 Mudflats with Salicornia, salt marsh with Puccinellia, Aster and

SalicorniaB3 Bensersiel 675 1999/2000 Mudflats, some SalicorniaB4 Bensersiel 1,375 1999/2000 Mudflats, no salt marshLN3 Leybucht 1,645 1998/1999–1999/2000 Extensive cattle grazing; Aster, Salicornia, SuaedaLN4 Leybucht 665 1998/1999–1999/2000 Sandy mudflats with Salicornia and some PuccinelliaLN5 Leybucht 1,225 1998/1999–1999/2000 Mudflats, small patches with SalicorniaLM1 Leybucht 3,500 199719/98–1999/2000 Upper ungrazed salt marsh; Elymus, Atriplex and AsterLS1 Leybucht 1,995 1998/1999–1999/2000 Upper extensively grazed salt marsh; Elymus, Atriplex and AsterLS2 Leybucht 3,395 1998/1999–1999/2000 Mudflats, small beach and small Salicornia patches

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in the other two habitats with a similar tendency inspring.

For the remaining species only the two salt marshhabitats were investigated. Rock Pipits showed higherdensities in salt marshes with tall vegetation in allseasons (Fig. 6). Densities of Skylarks were highest insalt marshes with tall vegetation, and the same heldtrue for Meadow Pipits (Anthus pratensis). Reed Bun-tings (Emberiza schoeniclus), Goldfinches (Cardueliscarduelis) and Lapland Buntings (Calcarius lapponicus)almost exclusively used salt marshes with tall vegeta-tion, although the results for the latter are not signifi-cant due to its scarcity.

Influence of grazing on wintering passerinesin salt marshes

Grazed salt marshes were only preferred by Shorelarks,while Skylarks (Alauda arvensis), Rock and MeadowPipits, Reed and Lapland Buntings as well as Gold-finches strongly preferred ungrazed salt marshes(Table 5). There was no difference in preference betweengrazed and ungrazed salt marshes for Snow Buntingsand Twites, but in the latter species a higher proportionfed in ungrazed salt marshes in autumn and spring andin grazed salt marshes in winter.

Fig. 2 Distribution of thedifferent driftline-areas in thegroups (1–9) created by acluster analysis. Location andcharacterization of the sites aredescribed in Table 1; for detailssee Dierschke (2001b). Lettersrefer to statistical tests inTable 3

Table 2 Location, size, land use, vegetation and habitat group of the study plots within the salt marshes

Plot Location Size [ha] Human use Vegetation (dominating species) Habitat-group

J3 Jadebusen 8.05 - Atriplex, Aster, Salicornia Upper salt marshW1 Tumlauer Bucht 8.97 - Halimione, Aster Upper salt marshW2 Tumlauer Bucht 11.31 Sheep grazed Salicornia, Suaeda, Festuca Lower salt marshLN1 Leybucht 14.21 Cattle grazed Aster, Plantago Lower salt marshLM3 Leybucht 8.63 - Elymus, Aster, Atriplex Upper salt marshLH1 Leybucht 12.69 Cattle grazed Fallow land vegetation Sand depositing areaE5 Elisabeth-Außengroden 3.96 - Elymus, Aster, Atriplex Upper salt marshE6 Elisabeth-Außengroden 3.80 Mowed <Xheading3>Triglochin Lower salt marshBK1 Beltringharder Koog 19.40 - Spergularia, Salicornia Embanked salt marshBK2 Beltringharder Koog 12.20 - Aster, Agropyron, Spergularia Embanked salt marshBK3 Beltringharder Koog 18.80 - Salicornia, Suaeda, Puccinellia Embanked salt marshBK5 Beltringharder Koog 4.10 Cattle grazed Fallow land vegetation Embanked salt marshBK6 Beltringharder Koog 7.90 Parking area Fallow land vegetation Embanked salt marshI7 Wangerooge 5.33 - Halimione, Salicornia, Limonium Lower salt marshC4 Crildumersiel 6.75 - Salicornia, Spartina Lower salt marshC5 Crildumersiel 14.00 - Salicornia, Suaeda, Festuca Lower salt marsh

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Fig. 3 Average densities (birds/km) and standard error of Shore-larks (Eremophila alpestris), Snow Buntings (Plectrophenax nivalis)and Twites (Carduelis flavirostris) in the different habitat groups in

autumn, winter and spring. For numbers and characterization ofthe groups see Fig. 2

Table 3 Significant differences (P) and z-values of the Bonferroni-corrected Mann-Whitney U-tests of the grouped driftline feeding byShorelarks (Eremophila alpestris), Snow Buntings (Plectrophenax nivalis) and Twites (Carduelis flavirostris). The test-number is shown inFig. 2

Test Statistic Shorelark Snow Bunting Twite

Autumn Winter Spring Autumn Winter Spring Autumn Winter Spring

A P 0.016 <0.01 <0.01 >0.05 >0.05 >0.05 <0.01 <0.01 <0.01z )8.473 )9.565 )7.617 )0.719 )0.06 )1.911 )5.255 )6.211 )4.303

B P >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05z )1.027 )1.045 )0.715 )0.092 )2.275 )0.912 )0.862 )1.406 )0.6

C P 0.032 <0.01 0.024 >0.05 >0.05 >0.05 <0.01 <0.01 >0.05z )2.913 )3.537 )2.93 )1.027 )0.098 )1.117 )3.62 )3.374 )1.287

D P >0.05 <0.01 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05z )2.356 )3.211 )2.353 )0.485 )2.013 )1.208 )0.272 )0.487 )1.415

E P >0.05 >0.05 <0.01 >0.05 <0.01 0.032 >0.05 >0.05 >0.05z )1.506 )2.416 )4.016 )0.96 )4.499 )2.863 )0.001 )0.27 )2.21

F P <0.01 >0.05 <0.01 0.024 <0.01 >0.05 <0.01 >0.05 >0.05z )5.223 )1.62 )3.219 )2.956 )3.71 )0.575 )4.217 )0.782 )0.348

G P 0.016 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05z )3.118 )2.237 )2.257 )1.476 )1.353 )0.426 )1.536 )1.099 )1.492

H P <0.01 <0.01 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05 >0.05z )3.509 )3.509 )0.595 )1.179 )0.801 )0.981 )1.055 )1.129 )1.091

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Occurrence of Shorelarks, Snow Buntingsand Twites in a recently embanked area

The study plots in the Beltringharder Koog reflectedsuccession from salt marshes to freshwater marshes inthe following sequence: lower salt marsh (BK1 andBK3), upper salt marsh (BK2) and freshwater marsh(BK5). BK6 is again a former sand depositing area,partially used as car park today. Shorelarks roostedmainly in the lower, rarely in the upper salt marsh andnever in the freshwater marsh, while the sand depositingarea was visited regularly in autumn and spring (Fig. 7).The preferences of Twites were similar, but they wereseen more regularly in the upper salt marsh and rarely

on the sand depositing areas. Snow Buntings were onlyrarely recorded in the polders.

Discussion

The investigated passerine species can be grouped intotwo categories: species preferring lower salt marsh veg-etation (Shorelark, Snow Bunting, Twite) and speciespreferring the upper salt marshes with higher vegetation(Skylark, Rock and Meadow Pipit, Lapland and ReedBunting, Twite, Goldfinch). Birds of the lower saltmarshes additionally use the driftlines to feed, while theTwite shifts its habitat from upper salt marsh in autumn

Fig. 4 Seasonal distribution ofShorelarks, Snow Buntings andTwites in different habitats onthe island of Wangerooge. Theline plot shows the averagenumber of birds present in thewhole study area

Table 4 Percentage of flocks ofShorelarks, Snow Buntings andTwites present in the driftline-area by season and clusteredhabitat group (see Fig. 2;number of flocks in paranthesis)

Cluster To 15 Oct 16 Oct to 30 Nov 1 Dec to 14 Feb 15 Feb to 15 Apr After 15 Apr

ShorelarkGroup 1 16.3 (24) 58.6 (119) 72.1 (157) 64.3 (153) 28.6 (7)Group 3 87.5 (8) 96.2 (26) 100.0 (12) 50.0 (2)Group 4 18.8 (16) 16.4 (20) 0.0 (5)Group 7+8 80.6 (15) 87.6 (11) 100.0 (7) 100.0 (1)

Snow BuntingGroup 1 65.8 (44) 68.7 (98) 77.9 (51)Group 3 100.0 (1) 100.0 (5) 100.0 (9)Group 7+8 100.0 (3) 91.7 (25) 93.0 (42) 75.0 (11)

TwiteGroup 1 36.0 (73) 48.1 (110) 52.3 (66) 100.0 (1)Group 3 100.0 (2) 54.4 (19) 81.8 (11)Group 4 0.0 (9) 15.6 (16) 33.3 (3)Group 5 12.5 (8) 16.7 (6) 0.0 (1)Group 7 66.7 (3) 75.0 (8) 100.0 (2)Group 8 11.1 (9) 20.0 (10) 66.7 (3)

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to lower salt marsh in winter and back to the upper saltmarsh in spring.

The results indicate that the lower salt marsh vege-tation provides habitat particularly for species that areconcentrated on salt marshes in winter, while the uppersalt marsh vegetation is visited by species using a varietyof habitats. The exception to this is the Rock Pipit, but itmainly uses the ditches in the upper salt marsh, where itprobably feeds on small molluscs (Dierschke 2002a).Salt marshes in eastern Britain harbor several speciesthat are hardly ever found wintering in the Wadden Sea(Wren Troglodytes troglodytes, Dunnock Prunella mod-ularis, Stonechat Saxicola torquata, Blackbird Turdusmerula, Linnet Carduelis cannabina, Greenfinch C. chl-oris) while others (Skylark, Reed Bunting, Goldfinch)occur in considerably higher densities (Brown andAtkinson 1996). These species winter mainly in shrubs ofthe Suaeda-vera-community, which does not occur in theWadden Sea. However, species preferring lower saltmarsh communities appear for some reason to be muchmore common in the Wadden Sea than in British saltmarshes: the Shorelark is a scarce wintering bird inBritain with only 200–400 individuals annually (Fraseret al. 1999), while 6,500–9,000 birds winter in theWadden Sea (Dierschke 2001a). Also, ScandinavianTwites are scarce in Britain (Davies 1987), but 22,000–35,000 winter in the Wadden Sea. The wintering popu-lations of Snow Buntings in Britain and in the WaddenSea are of similar size (Dierschke 2001a).

Shorelarks, Snow Buntings and Twites all feedmainly on seeds of lower salt marsh plants in winter,when seeds are concentrated in high densities in thedriftlines. Driftlines exposed to the main wind directionare larger and usually contain higher densities of seedsthan others (Dierschke 2001b). Granivorous birds areknown to concentrate in areas with higher seed abun-dance (Moorcroft et al. 2002), explaining the higherdensities of birds in these driftlines. Twites often take theseeds directly from the plants while sitting on them,whereas Shorelarks and Snow Buntings mainly consumeseeds from the surface. Salicornia plants growing onmudflats with a silty surface are therefore mainly visitedby Twites, while Shorelarks and Snow Buntings onlyvisit these plants when they are growing on intertidalflats with a sandy surface (Dierschke 2001b). However,in autumn and spring, profitable seeds are scarce, be-cause they are either still on the plants and often not yetripe (autumn) or they are already germinating (spring).During this time Shorelarks and Snow Buntings alsoconsume arthropods (Dierschke 2002b) and a habitatshift from the driftlines to the salt marshes and sanddepositing areas may be recorded, which is most pro-nounced in Shorelarks. An analysis of droppings fromShorelarks showed that they feed on seeds and smallbeetles in the salt marshes, but on the sand depositingareas, where halophyte seeds are scarce, the foodconsists nearly exclusively of arthropods (Dierschke2002b).

Fig. 5 Average densities and standard error of Shorelarks,Snow Buntings and Twites in autumn, winter and spring inplots with lower (light grey) and upper saltmarsh vegetation (darkgrey) and sandy depositing fields (black). *P<0.05, **P<0.01,***P<0.001

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The species preferring the lower salt marsh commu-nities have all been recorded as declining since the 1960s.Svensson (1990) and Hilden (1987) described a dramaticdecline in the Finnish and Swedish breeding numbers ofShorelarks. Dierschke (1997) additionally found similardeclines for Twites and probably also for Snow Bun-tings. The reasons for the decline are so far unknown,but being specialized on similar habitats and similarfood in the wintering grounds (Dierschke 2002b),changes there are very likely to be responsible for the

decline, as granivorous passerines are at least partiallyregulated by winter food availability (Smith et al. 1980;Dunning and Brown 1982; Schluter and Repasky 1991;Peach et al. 1999).

After extensive floods in the Netherlands in 1953 andin Germany in 1962, coastal protection was intensifiedand large embankment projects were carried out. Al-though considerable areas had been embanked before,the priorities were changed. The aim of embankmentprojects before the 1960s was to reclaim land for agri-cultural use, and therefore mainly upper salt marsheswere reclaimed. Afterwards, since the aim was coastalprotection, complete bays were embanked, includingupper and lower salt marshes as well as mudflats, tocreate a straight coastline (Dieckmann 1988; Kramer1991). The result was a straight coastline with less salt

Table 5 Distribution of some passerine species in grazed and ungrazed salt marshes at Tumlauer Bucht (statistics by Wilcoxon-Wilcoxtest)

Species n (cases/ind.) Grazed (%) Ungrazed (%) P z

Shorelark Eremophila alpestris 19/590 94.7 5.3 0.001 )3.419Skylark Alauda arvensis 61/861 14.0 86.0 <0.001 )6.013Rock Pipit Anthus petrosus 78/845 8.1 91.9 <0.001 )7.584Meadow Pipit Anthus pratensis 34/285 13.9 86.1 <0.001 )4.883Twite Carduelis flavirostris 35/3,287 69.8 30.2 0.714 )0.367Goldfinch Carduelis carduelis 6/76 0.0 100.0 0.027 )2.207Snow Bunting Plectrophenax nivalis 7/219 75.0 25.0 0.38 )0.845Lapland Bunting Calcarius lapponicus 13/38 7.7 92.3 0.002 )3.139Reed Bunting Emberiza schoeniclus 13/24 0.0 100.0 0.001 )3.307

Fig. 6 Average densities and standard error of Skylarks (Alaudaarvensis), Rock (Anthus petrosus littoralis) and Meadow Pipits(Anthus pratensis), Goldfinches (Carduelis carduelis), Snow andReed Buntings (Emberiza schoeniclus) in autumn, winter and springin plots with lower (light grey) and upper saltmarsh vegetation(dark grey). *P<0.05, **P<0.01, ***P<0.001

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marshes and mudflats and hardly any bays left wheresalt marshes might develop again. The loss of habitatmay have resulted in lower numbers of birds winteringthere. The polders still held salt marsh vegetation forsome years, especially Salicornia which was prominenton the former mudflats, while the former salt marshessoon turned to freshwater marshes (Brehm 1979; VanNoordwijk-Puijk et al. 1979; Wolfram et al. 1998). TheseSalicornia fields provide excellent habitat for Twites, butless so for Shorelarks and Snow Buntings, as there areusually no prominent driftlines in the polders. Thismight explain the earlier start of the decline in thesespecies (Dierschke 1997).

The effects of intensified grazing are much more dif-ficult to assess, as there is controversy about the conse-quences on the vegetation. Generally, grazing leads toa retrogressive succession of salt marsh vegetation,

entailing growth of lower salt marsh communities onelevations where they would normally be displaced byupper salt marsh plants (Bakker and Ruyter 1981;Bakker and de Vries 1992; Bertness 1991). Intensivegrazing often causes patches without vegetation cover,where seeds are more visible and which are thereforeoften preferred by granivorous passerines (Moorcroftet al. 2002). Intensive grazing also usually leads to thedominance of a few plant species (e.g. Brongers et al.1990), including some important seed producers forShorelarks, Snow Buntings and Twites (e.g. Salicorniaeuropaea, Suaeda maritima). However, the seed pro-duction per plant is much lower in intensively grazedareas and certain plants that produce energy rich seeds,such as Atriplex sp., hardly occur (Brongers et al. 1990;Brown and Crockford 1997; Dierschke 2001b). Addi-tionally, Salicornia europaea, which dominates in grazed

Fig. 7 Average densities andstandard error of Shorelarks(upper graph) and Twites (lowergraph) in different habitats ofthe Beltringharder Koog.Brackets indicate significance atP<0.05 (Kruskal-Wallis /Nemenyi test)

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salt marshes, sheds its seeds during winter, whileS. stricta, a typical species of the pioneer zone, keeps ahigh proportion of its seeds until spring (Konig 1960;Dierschke 2001b). Therefore, Twites often visit grazedsalt marshes only in autumn, while the pioneer zoneprovides food all winter (Dierschke 2001b).

Man-made sedimentation areas and grazing haveresulted in salt marshes with a low variability in eleva-tion and a high density of straight-lined ditches. Aban-doning grazing, therefore, often leads to a monotonousvegetation typical for the elevation of the salt marsh.Although the growth of Elymus sp. is dependent onmany factors, such as soil composition, wetness, geno-type of the Elymus-plants, competition with Halimioneportulacoides and presence of herbivorous birds andmammals (Dierßen et al. 1991; Bakker 1993; Heinzeet al. 1999; Bockelmann and Neuhaus 1999; Kiehl et al.2003), large parts of salt marshes where grazing is dis-continued are often soon covered by Elymus plants. Thisvegetation is hardly visited at all by Shorelarks, SnowBuntings and Twites, and Elymus seeds are rarely con-sumed by them (Dierschke 2002b). However, reducedgrazing in the German Wadden Sea has led to a higherproduction of profitable seeds in the lower salt marshes(Dierschke 2001b) and also influenced the vegetation ofadjacent grazed areas (Gettner 2003), but the effects onthe wintering population of granivorous passerines haveyet to be studied in detail.

In the 1980s, the German part of the Wadden Seawas classified as a national park, and protection hassince been improved. Grazing has been discontinued orreduced in large areas and no further embankmentprojects have been started. There has since been anincrease in lower salt marsh areas (Stock et al. 2001),and in the wintering populations of Shorelark, SnowBunting and Twite (Dierschke 2001b). For other win-tering passerines, there are no data available on thepopulation trends in the Wadden Sea, but it seemsprobable that the wintering population of species thatprefer the upper ungrazed salt marshes have increasedas well. In the long run it is to be expected that thereduction of the human impact on the salt marshes willlead to more natural salt marshes with a high vari-ability in elevation and with corresponding vegetation,even on a local scale, providing habitat for largenumbers of different bird species. However, as long asthese processes continue, management of some uppersalt marshes through grazing might be required in themeantime, at least on a local scale, to retain a highvariability of habitats.

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