Bird assemblages of an Iberian cereal steppe

Agriculture, Ecosystems and Environment 78 (2000) 65–76

Bird assemblages of an Iberian cereal steppe

A. Delgadoa,1, F. Moreirab,∗a Liga para a Protecção da Natureza, Estrada do Calhariz de Benfica, 187, P-1500 Lisbon, Portugal

b Centro de Ecologia Aplicada ‘Prof. Baeta Neves’, Instituto Superior de Agronomia, Tapada da Ajuda, P-1399 Lisbon Codex, Portugal

Received 22 January 1999; received in revised form 29 June 1999; accepted 12 August 1999

Abstract

The cereal steppes of Castro Verde (Portugal) support many bird species of high conservation value on which agriculturalpractices can have significant impacts. The aim of this study was to describe the seasonal variations in bird assemblagesusing the agricultural landscape mosaic and to explore the potential ecological implications of changes in land use on birdpopulations. Bird densities and habitat variables were measured in four habitat types (cereal fields, fallow land, stubbles, andploughed land) during 1997/1998, and detrended correspondence analysis used to summarise information on the structure ofbird assemblages across habitats in the winter, breeding and post-breeding periods.Pluvialis apricariaL., Vanellus vanellusL., Alauda arvensisL. andAnthus pratensisL. were abundant during winter. During the breeding season, bird assemblageswere quite distinct across land use types, with species typical of cereal fields (e.g.Miliaria calandra L.), fallow land (e.g.Melanocorypha calandraL.) and ploughed land (e.g.Anthus campestrisL.). After the breeding season, the overlap in birdassemblages increased again, withM. calandra, M. calandraL. andCiconia ciconiaL. being common. Species used differenthabitats along the year, showing the importance of maintaining the traditional agricultural landscape for preserving the diversityof bird species. The potential impacts of several types of agricultural changes (e.g. agricultural intensification, agriculturalabandonment and increase in stocking levels) on the bird populations were discussed on the basis of the results obtained. Anychange in the current farming system will probably lead to a decrease in steppe bird diversity. ©2000 Elsevier Science B.V.All rights reserved.

Keywords:Landscape; Cereal cultivation; Bird assemblage structure; Impacts of agricultural changes

1. Introduction

Low-intensity farming systems of Europe havea high bird diversity (Tucker, 1997) and, amongthese, the pseudosteppes of the Iberian Peninsula

∗ Corresponding author. Tel.:+351-1-3616080;fax:+351-1-3623493.E-mail address:[email protected] (F. Moreira).

1 Present address: Centro de Ecologia Aplicada ‘Prof. BaetaNeves’, Instituto Superior de Agronomia, Tapada da Ajuda, P-1399Lisbon Codex, Portugal.

hold a large proportion of species with unfavourableconservation status (Suárez et al., 1997; Tucker,1997). Iberian pseudosteppes include semideserts,‘páramos’ and cereal steppes (Martınez and Purroy,1993; Tellerıa et al., 1988a, b). The most commonpseudosteppe in western Spain and Portugal is thecereal steppe, created in flat, open landscapes by theextensive cultivation of cereals on a rotational ba-sis, resulting in a mosaic of cereal fields, stubbles,ploughed and fallow land (usually used as sheep pas-tures) (Suárez et al., 1997; Moreira, 1999). Cerealsteppes constitute a stronghold for several threatened

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66 A. Delgado, F. Moreira / Agriculture, Ecosystems and Environment 78 (2000) 65–76

bird species, such as the great bustard,Otis tardaL.,the little bustard,Tetrax tetraxL., the sandgrouse,Pteroclesspp. and the lesser kestrel,Falco naumanniFleischer (Tucker and Heath, 1994; Suárez et al.,1997).

Cereal steppes represent an economically marginalfarming system, with yields of less than half the aver-age European Union yields (Suárez et al., 1997). Con-sequently, land use of cereal steppes has been chang-ing in the last decades because of agricultural intensi-fication (especially irrigation), land abandonment andafforestation (Tucker and Heath, 1994; Suárez et al.,1997). Thus, this landscape is threatened by habitatdegradation and loss, which can have significant im-pacts on the bird populations (e.g. Baldock, 1991; Diazet al., 1993; Santos, 1996). The present Common Agri-cultural Policy (CAP) of the European Union promotesthe maintenance of such farmed landscapes throughagri-environmental regulation based on the compen-sation of farmers for maintaining agricultural prac-tices allowing the conservation of threatened species(Robson, 1997).

Several authors (e.g. Baldock et al., 1993; Beaufoyet al., 1994; Telleria et al., 1994; Suárez et al., 1997)have drawn attention to the fact that the scientificbasis for establishing practical management actionsaimed at maintaining or increasing the bird popula-tions are extremely scarce. Many studies of birds ofpseudosteppes have focused on particular species (e.g.Martınez, 1994), season (e.g. Martınez and Purroy,1993; Martınez and De Juana, 1996) or type of land-scape (e.g. Tellerıa et al., 1988a; Hódar, 1996a, b), butfew (e.g. Tellerıa et al., 1988b in central Spain) haveattempted to characterise the seasonal use by bird as-semblages of the various components of the mosaiccover created by the extensive cultivation of cereals.This type of data is, however, needed for adequatelyevaluating the consequences of changes in agriculturalpractices on bird populations.

The objectives of this paper were:1. to describe the seasonal variation in land use of

an agricultural mosaic of the largest cereal steppein Portugal;

2. to describe the bird assemblages using eachland-use category in this mosaic; and

3. to explore the implications of changes in agricul-tural practices on both the agricultural landscapemosaic and the bird populations.

2. Methods

2.1. Study area

The study site was an area of 5240 ha locatedapproximately at the centre of the cereal steppe ofCastro Verde (ca. 37◦43′N, 7◦57′W), the most impor-tant area for steppe birds in Portugal (Grimmet andJones, 1989). The traditional agricultural system usedin this region is as follows: each farm is divided intoparcels, each of which lies under cereal cultivationfor two years, after which the land is left fallow for2–3 years. The parcel is then ploughed to re-initiatethe rotation cycle. Fallow land is generally used aspasture for sheep. The altitude ranges from 150 to240 m, the annual average temperature from 15.5 to16.0◦C, and the annual rainfall from 500 to 600 mm.The site was located in the Meso-Mediterraneanbioclimatic stage (Rivas-Martınez, 1981), tree coverbeing restricted to some plantation of olive trees(Olea europaeaL.). Areas of scrub (Cistus ladaniferL.) occurred in rocky outcrops with no agriculturaluse.

2.2. Land use

The availability of the different land-use classeswas assessed by mapping land use in the whole studyarea (scale 1 : 25 000) in November 1996, using aerialphotography and field checks of all farms. From thenuntil August 1997, a monthly survey was made toregister land-use changes. The area with each landuse type was measured with a planimeter. In eachseason, i.e. winter (November to February), spring(breeding season, i.e. March to May), and summer(post-breeding, i.e. June to August) the percentage ofthe area with each land use was calculated by averag-ing the monthly estimates. Nine land uses were con-sidered i.e. three types of cereals (wheatTriticumspp.,oatsAvenasp. and barleyHordeumsp.), two types offallows (recent, i.e. fallows from cereal in the previ-ous spring, and old), ploughed land and three typesof stubbles (one per each cereal). A small percentageof dry leguminous crops (ca. 2%) was not includedin the calculations. For the purpose of this paper, datawere pooled into cereals, fallows, ploughed fields andstubbles.

A. Delgado, F. Moreira / Agriculture, Ecosystems and Environment 78 (2000) 65–76 67

2.3. Bird counts

Bird counts were carried out within patches of agiven land use type, using the Jarvinen and Vaisa-nen (1975) line transect sampling. Observations wererecorded within, and beyond, a main belt of 25 m onboth sides of the observer. Only birds within the sam-pled patch of habitat were registered.

Transects were defined in the land cover map in or-der to account for two basic conditions: (1) a mini-mum length of 500 m, and (2) assuring a sample sizeof at least 10 transects in each land-use type.

Counts were made during the first 3 h after sun-rise. Mean travel speed (±SD) was 1.94± 0.39 km/h(n= 358). Transect length ranged from 500 to 1125 m(mean= 782± 165 m).

Cereal field transects were surveyed monthly andthese data were grouped in the following time peri-ods: winter (December to February), breeding (Marchto May) and post-breeding (June to August). Fallowfield transects were counted in winter (in Novem-ber and again in January), breeding (April) andpost-breeding (July). Ploughed land and stubble tran-sects were counted twice in their season of occurrence(respectively breeding and post-breeding).

2.4. Habitat variables

Data on vegetation height and percentage coverwere collected in fallows and cereal fields in Novem-ber (December for cereal fields), January, April andJuly. Along each transect used for bird counts, theheight of the vegetation was measured over the first100 m of the transect at 10 sites (every 10 m) witha ruler to the nearest cm. The percentage cover infallows was measured at 5 sites (20 m distance) alongthe transect. At each site, a 10-m tape was stretchedin a random direction and the length of the tape withvegetation cover noted to the nearest 5 cm. For cerealfields, due to difficulties (potential crop damage) instretching the tape, a quadrat of 50× 50 cm2 was usedfor estimating visually the cover of vegetation (Hayset al., 1981).

2.5. Data analyses

In half of the birds species considered for dataanalyses (11 out of 22), no individuals were regis-

tered within the main belt in at least one season.Thus, data for each transect were pooled (sum of de-tections within, and beyond, the belt) and expressedas (birds km−1). Species occurring in<5% of all thecounts were not analysed. Counts of both species ofthe genusGaleridaoccurring in the area (thekla lark,G. theklaeBrehm, and crested lark,G. cristata L.)were pooled, due to difficulty in identifying someindividuals.

Data for each transect were analysed grouped intoseason. Winter and post-breeding counts were logtransformed to minimise the influence of large flocksin multivariate analysis (see below). When more thanone count was made in each transect, data were aver-aged prior to transformation. For the breeding seasonthe maximum count was used for species using thehabitat for nesting and the average for species usingthe habitat for feeding (e.g. white stork,Ciconia cico-nia L., and cattle egrett,Bubulcus ibisL.), as it was as-sumed that the maximum count was the best estimateof breeding densities whereas the average would bethe best indicator of intensity of use as feeding habitat.

Detrended correspondence analysis (DCA) (Gauch,1982), was used to summarise the information onbird assemblages across the several components ofthe landscape. This ordination technique involves thederivation of new axes that maximally account for thestructure of the points in a multi-dimensional space,making possible the reduction of dimensionality withminimal loss of information. These new axes max-imise the correspondence between habitats and birdsspecies, so that transects with similar bird assemblagecomposition are placed together, as are species withsimilar patterns of abundance across transects. DCAwas carried out using the programCANOCO(TerBraak, 1987), with the option for detrending bysecond-order polynomials.

For habitat variables, each land-use type was char-acterised by the average vegetation height and cover.

Non-parametric tests (Siegel and Castellan, 1988)were used to compare:1. transect coordinates of each habitat type in the

axes of DCA;2. bird densities across habitats; and3. habitat variables in cereal fields and fallows.

On account of the high number of tests when compar-ing bird densities across habitats, the sequential Bon-ferroni technique (Rice, 1989) was used to control the


probability of incorrectly rejecting one or more truenull hypothesis. Analyses were performed using theSPSS software (Norusis, 1992).

3. Results

3.1. Land use and habitat variables

Seasonal variations in the agricultural landscapemosaic are shown in Fig. 1. During winter, the agricul-tural mosaic consisted of cereal fields (26% of the landuse) and fallows (73%), 27% of which were recent (re-sulting from the stubbles of the previous summer). Inspring, the landscape included ploughed fields (10%of the land cover), rapidly covered with vegetation andclassified as fallows in the following summer. In sum-mer, after harvest stubbles averaged 15% of the landcover. Several cereal fields were not harvested due tothe low seed production caused by adverse weatherconditions in that year.

Table 1 shows the seasonal variation of vegeta-tion height and cover in fallows and cereal fields.Most comparisons (only one exception) yielded sig-nificant differences between habitats and seasons(for each habitat). In fallows, vegetation height andcover decreased progressively from winter to sum-mer. For cereal fields, there was a peak in both these

Fig. 1. Average proportion of the land use of the Castro Verde study area occupied in each season by fallow land, cereal fields, stubblesand ploughed fields.

variables during spring, when the plants were fullydeveloped.

3.2. Bird assemblage structure

The 60 bird species observed during the field workare listed in Table 2. Of these, only 22 occurred in>5% of the transects.

3.2.1. WinterThe four commonest species wintering in both,

fallow land and cereal fields were skylark,Alaudaarvensis L., golden plover,Pluvialis apricaria L.,lapwing, Vanellus vanellusL., and meadow pipit,Anthus pratensisL., all reaching median densities>10 birds km−1 (Table 3). The results of the DCA(Fig. 2) suggest that there were differences in birdassemblage structure between these two componentsof the landscape, in spite of some overlap. Therewere significant differences between fallow and cerealtransect coordinates in both the first (Wilcoxon test,z= −4.06,p< 0.001) and the second axis (Wilcoxontest, z= −3.07, p< 0.01). The first axis separated agroup of species which were particularly abundant insome cereal fields, i.e. red-legged partridge,Alectorisrufa L., quail, Coturnix coturnixL., Galerida spp.and fan-tailed warbler,Cisticola juncidisRafinesque.The second axis separated two species associated


Table 1Seasonal variation in median (with 25% and 75% quartiles), vegetation height (cm) and cover (%) in fallow land and cereal fields inCastro Verde

Land use November January April July K–W testa

Vegetation heightFallows 19.0 (15–24) 15.0 (8–20) 12.0 (5–23) 10.0 (1–21) χ2 = 64.8***

nb = 200 n= 200 n= 200 n= 200Cereal fields 4.0 (0–8) 11.0 (8–15) 53.0 (41–63) 14.5 (0–35) χ2 = 630.8***

n= 280 n= 280 n= 290 n= 90Wilcoxon testa z= −17.3*** z= −3.8*** z= −17.3*** z= −2.3∗

Percentage coverFallows 81.2 (73–83) 87 (82–92) 77.5 (69–86) 77.0 (61–87) χ2 = 39.5***

n= 100 n= 100 n= 100 n= 100Cereal fields 20.0 (0–20) 20 (20–40) 80 (60–90) 70 (40–80) χ2 = 685.3***

n= 280 n= 280 n= 290 n= 90Wilcoxon testa z= −15.9*** z= −14.8*** z= −1.3nsc z= −2.8**

∗ p< 0.05, **p< 0.01, ***p< 0.001.a Results of the Kruskall–Wallis (K–W) test and Wilcoxon test are presented.b n= sample size.c p> 0.05.

with several fallow transects (stone curlew,BurhinusoedicnemusL., and Tetrax tetrax) and one species(Bubulcus ibis) occurring in high density in two ce-real transects. The remaining species were near to theorigin of the plot, showing no particular pattern ofassociation with any of the land-use types. A univari-ate comparison of species abundances between fallowland and cereal fields (Table 3) yielded significantdifferences forA. rufa (Wilcoxon test, z= −2.88,p< 0.01), C. juncidis (Wilcoxon test, z= −4.74,p< 0.001) and corn bunting,Miliaria calandra L.,(Wilcoxon test,z= −3.5, p< 0.001), all more abun-dant in cereal fields.

3.2.2. Breeding seasonEach of the habitats had a rather distinctive bird

assemblage (Table 4) as clearly shown in the DCAresults (Fig. 3), with significant differences betweencereal, fallow and ploughed land transect coordinatesin the three axes (Kruskal–Wallis test,p< 0.01). Thepatches of ploughed land had a high abundance ofshort-toed lark,Calandrella brachydactylaLeisler(24 birds km−1) and relatively high densities (com-pared to other habitats) of black-eared wheatear,Oenanthe hispanicaL., B. oedicnemus,and tawnypipit, Anthus campestrisL. Fallows were charac-terised by the abundance of calandra lark,Melano-corypha calandra(19 birds km−1) and Tetrax tetrax

(10 birds km−1), as well as a group of other speciesattaining higher densities in fallows than in otherhabitats:B. ibis, Ciconia ciconiaandOtis tarda.

Cereal fields had a high abundance ofMiliaria ca-landra(36 birds km−1), C. juncidis(12 birds km−1), T.tetrax(10 birds km−1), andC. coturnix(2.3 birds km−1).This habitat was the only one having exclusive species(A. pratensis, A. arvensis, A. rufa and southern greyshrike,Lanius meridionalisTemminck).

Comparison of species abundances across fallows,ploughed land and cereal fields (Table 4) yielded sig-nificant differences (Kruskal–Wallis test,p< 0.05) formost species.

3.2.3. Post-breedingThe structure of bird assemblages was more variable

after the breeding season (Fig. 4 and Table 5) in spiteof significant differences between cereal fields, fallowand stubbles transect coordinates (for the first axis,Kruskal–Wallis test,χ2 = 22.7,p< 0.001; for the sec-ond axis, Kruskal–Wallis test,χ2 = 11.8,p< 0.01).

Fallows were characterised by the abundance ofMelanocorypha calandra(4.0 birds km−1), C. ciconia(1.7 birds km−1) andL. meridionalis(1.4 birds km−1).Species more abundant in fallows included theblack bellied sandgrouse (Pterocles orientalisL.),B. oedicnemus, C. brachydactyla, O. tarda andT. tetrax.


Table 2Bird species observed in the study area from November 1996 to August 1997

Species When seena Species When seena

Tachybaptus rufficolis∗ R Athene noctua* RBubulcus ibis R Apus apus* BArdea cinerea* O Apus pallidus* OCiconia ciconia R Merops apiaster* BCiconia nigra* O Coracias garrulus* BAnas strepera* R Upupa epops* BAnas platyrhyncos* R Calandrella brachydactyla BMilvus milvus* W Melanocorypha calandra RMilvus migrans* B Galerida thecklae RCircus cyaneus* W Galerida cristata RCircus pygargus* B Alauda arvensis WButeo buteo* R Lullula arborea* RHieraaetus pennatus* O Hirundo daurica* BFalco naumanni* B Hirundo rustica* BFalco tinnunculus* R Anthus pratensis WAlectoris rufa R Anthus campestris BCoturnix coturnix B Motacilla alba WFulica atra* R Motacilla flava* BGrus grus* W Oenanthe hispanica BOtis tarda R Saxicola torquata* RTetrax tetrax R Lanius senator* BHimantopus himantopus* B Lanius meridionalis RBurhinus oedicnemus R Corvus corax* RGlareola pratincola* B Cyanopica cyanus* RVanellus vanellus W Sturnus unicolor* RPluvialis apricaria W Cisticola juncidis BCharadrius dubius* B Passer hispaniolensis* RTringa ochropus* O Passer domesticus* RPterocles orientalis R Carduelis carduelis* RStreptopelia turtur* B Miliaria calandra R

a O, occasional; W, exclusively during winter; B, breeding season; and R, all year round.∗ Species occurring in< 5% of the counts, and hence not included in the analyses.

The main feature of cereal fields was the abundanceof C. juncidis(6.1 birds km−1), along with the occur-rence ofC. ciconia(1.3 birds km−1), L. meridionalis(0.5 birds km−1) andC. coturnix(0.4 birds km−1).

Species typical of both, fallow land (Melanoco-rypha calandra) (2.4 birds km−1) and cereal fields(Miliaria calandra) (0.8 birds km−1) occurred instubbles.L. meridionalis (0.8 birds km−1) also at-tained high abundance in comparison with otherseasons.

A univariate comparison of species abundanceacross habitat types (Table 5) yielded significant differ-ences (Kruskal–Wallis test,p< 0.05) for Melano-corypha calandra, B. oedicnemusandC. ciconia, allmore abundant in fallows, and forC. juncidis andC. coturnix, more abundant in cereal fields.

4. Discussion

4.1. Assumptions and data limitations

Open landscapes with a flat topography werestudied, in which it was assumed that differencesin bird detectability across land-use types did nothave a significant influence on the results. The ex-ception could be the well-developed cereal fieldsduring the spring census, where detectability couldbe reduced. Nevertheless, the fact that during thisseason there was a clear relationship across habi-tats between the abundance index (birds km−1)and the densities (birds/10 ha) estimated using theJarvinen and Vaisanen method (thus corrected fordifferences in detectability between habitats) for


Table 3Winter relative densities (birds km−1) in fallow land (n= 20 tran-sects) and cereal fields (n= 29 transects) in Castro Verde, withmedian and inter-quartile distance

Species Fallows Cereal fields

Alauda arvensis 84.1 (26.6–111.5) 80.0 (28.0–131.0)Vanellus vanellus 20.2 (4.9–51.7) 13.3 (0.7–30.1)Pluvialis apricaria 18.8 (0.3–80.0) 17.1 (3.4–81.7)Anthus pratensis 11.1(9.6–20.9) 9.7 (6.8–14.4)Melanocorypha calandra 4.8 (0.1–15.5) 1.7 (0.5–4.9)Miliaria calandra∗ 1.6 (0–6.5) 6.9 (5.3–10.0)Motacilla alba 0.7 (0.1–2.1) 1.4 (0.6–2.7)Tetrax tetrax 0 (0–3.1) 0 (0–0)Alectoris rufa* NOa 0 (0–0.9)Bubulcus ibis 0 (0–0) 0 (0–0)Burhinus oedicnemus 0 (0–0) 0 (0–0)Ciconia ciconia 0 (0–0) 0 (0–0.5)Cisticola juncidis* NO 1.1 (0–2.8)Coturnix coturnix NO 0 (0–0)Galerida spp. NO 0 (0–0)Lanius meridionalis 0 (0–0) 0 (0–0)Otis tarda 0 (0–0) 0 (0–0.5)

∗ Species with significant differences between fallow and cerealfield densities (Wilcoxon test).

a Species not observed.

most species suggests that detectability was not aproblem.

Landscape-scale effects, which were not taken intoaccount, could influence the composition of bird as-semblages mainly through (1) patch size and edge ef-fects, and (2) a different scale of perception by speciesthat select landscape configurations rather than spe-cific patches (Forman, 1997; Farina, 1998). The for-mer problem was minimised by the fact that the plotssampled (with the exception of fallows, which werethe matrix of the landscape), had roughly the same sizeand shape and no distinct edges, such as hedgerows ortree lines. Also, some larger body sized species (e.g.B. ibis, O. tarda and C. ciconia) explored the land-scape at a larger scale, and caution should be takenwhen interpreting the results obtained.

Bird community structure varies in time, bothin the presence or absence of species and in theirrelative abundances (Wiens, 1989). The restrictedsampling period did not provide information on po-tential yearly variations in assemblage structure, butprevious data suggest that during the breeding seasonthe dominant species in each land-use type are fairlyconstant (Leitão and Moreira, 1996; Moreira, 1999).

Nevertheless, assemblage structure might be morevariable during winter (Wiens, 1989).

4.2. Seasonal variation in bird assemblages

The winter overlap in the composition of birdassemblages of fallow land and cereal fields wascaused by the widespread abundance of four win-tering species. The observed differences in habitatstructure between these two land uses seemed not toinfluence bird assemblage composition. It is possiblethat there was enough food available (mainly leavesand seeds forA. arvensisand A. pratensis, and soilinvertebrates forP. apricaria andV. vanellus(Crampand Simmons, 1983; Cramp, 1988) in both habitats.Diaz and Tellerıa (1994) have shown that food abun-dance was not a limiting factor for seed-eating birdswintering in cereal steppes of central Spain, and San-tos and Tellerıa (1985) argue that invertebrate activityduring winter, in the Iberian Peninsula, is sufficientfor providing the necessary food resources for birds.P. apricaria andV. vanelluscould respond to vegeta-tion structure, as they avoid dense and tall vegetation,but vegetation height during this time of year was be-low the described limits for these species (Cramp andSimmons, 1983). The main differences in winteringspecies density occurred for species which used ce-real fields as breeding habitat (A. rufa, C. juncidisandMiliaria calandra). These species started to definebreeding territories in cereal fields at the end of win-ter, at a time when vegetation height was increasing.

During the breeding season particular bird specieswere associated with specific habitats. This suggeststhat vegetation structure was the main determinantof habitat selection during the breeding season, asreported for many bird species (Cody, 1985; Wiens,1989). Species attached to small breeding territoriesprobably were not able to explore, e.g. for foraging,other landscape patches, in contrast with other seasonswhen birds were not attached to breeding territories.Ploughed fields were used as nesting habitat by agroup of species with a preference for bare ground andshort vegetation, particularlyC. brachydactyla(e.g.Cramp and Simmons, 1983; Cramp, 1988; Moreiraand Leitão, 1996). In fallow fields,Melanocrypha ca-landra andT. tetraxwere the more abundant species,as recorded in previous studies (Leitão and Moreira,1996; Moreira and Leitão, 1996; Moreira, 1999).


Fig. 2. Plot of the first two axes from detrended correspondence analysis based on the winter data. Transects with similar bird assemblagecomposition are placed together, as are species with similar pattern of abundance across transects. Eigenvalues are a measure of speciesspread in the axes. Axis 1, eigenvalue= 0.17 (20.7% variance); and Axis 2, eigenvalue= 0.12 (14.9% variance). Code for transect habitats:quadrats, fallow land; and triangles, cereal fields. Codes for species (dots): Aa,Alauda arvensis; Ap, Anthus pratensis; Ar, Alectoris rufa;Bi, Bubulcus ibis; Bo, Burhinus oedicnemus; Cc, Coturnix coturnix; Cic, Ciconia ciconia; Cj, Cisticola juncidis; Gal, Galerida spp.; Lm,Lanius meridionalis; Ma, Motacilla alba; Mc, Miliaria calandra; Mec, Melanocorypha calandra; Ot, Otis tarda; Pa,Pluvialis apricaria;Tt, Tetrax tetrax; and Vv, Vanellus vanellus.

Table 4Relative densities (birds km−1) during the breeding season in fallow land (n= 20 transects), cereal fields (n= 29 transects) and ploughedland (n= 10 transects) in Castro Verde, with median and inter-quartile distance

Species Fallows Cereal fields Ploughed land

Melanocorypha calandra∗ 19.7 (14.4–29.7) 1.3 (0–3.4) 1.2 (0–5.0)Tetrax tetrax* 10.1 (3.1–16.4) 10.0 (6.4–17.3) 2.8 (0.8–6.0)Bubulcus ibis 3.6 (0–8.0) 0 (0–1.9) 0.8 (0–3.1)Calandrella brachydactyla* 3.3 (0–5.5) 0 (0–0) 24.4 (20.1–26.8)Ciconia ciconia 2.7 (0.2–8.7) 0.8 (0.4–1.1) 0.7 (0–1.3)Miliaria calandra* 2.1 (0–8.0) 36.0 (26.7–43.3) 5.7 (3.7–8.9)Otis tarda 1.6 (0–6.1) 0.4 (0–2.9) 0 (0–0.2)Anthus campestris* 0 (0–0) 0 (0–0) 1.6 (0–4.4)Oenanthe hispanica* 0 (0–0) 0 (0–0) 7.0 (3.7–10.0)Pterocles orientalis 0 (0–2.0) 0 (0–0) 0 (0–3.9)Burhinus oedicnemus 0 (0–1.0) 0 (0–0) 1.0 (0–3.0)Cisticola juncidis* 0 (0–0) 12.2 (6.8–20.4) 0 (0–2.5)Coturnix coturnix* 0 (0–0) 2.3 (1.0–4.6) 0 (0–0)Galerida spp. 0 (0–0) 0 (0–0) 0 (0–0.3)Anthus pratensis* NOa 2.3 (1.1–4.2) NOAlauda arvensis NO 0 (0–0.2) NOAlectoris rufa* NO 0 (0–2.0) NOLanius meridionalis NO 0 (0–0) NO

∗ Species with significant diferences between habitats (Kruskal–Wallis test).a Species not observed in the transects.


Fig. 3. Plot of the first two axes from detrended correspondence analysis based on the breeding season data. Axis 1, eigenvalue= 0.56(19.4% variance); and Axis 2, eigenvalue= 0.25 (13.4% variance). Code for transect habitats: quadrats, fallow land; triangles, cereal fields;and diamonds, ploughed land. Codes for species (dots) similar to Fig. 2 with the additional species: Ac,Anthus campestris; Cb, Calandrellabrachydactyla; Oh, Oenanthe hispanica; and Po,Pterocles orientalis.

Fig. 4. Plot of the first two axes from detrended correspondence analysis based on the post-breeding season data. Axis 1, eigenvalue= 0.36(16.7% variance); and Axis 2, eigenvalue= 0.29 (13.3% variance). Code for transect habitats: quadrats, fallow land; triangles, cereal fields;and asterisks, stubbles. Codes for species (dots) similar to Figs. 2 and 3.

Cereal fields were used mainly by species preferringtall and dense vegetation, such asMiliaria calandra,C. juncidis and C. coturnix (Cramp and Simmons,1980; Cramp, 1992; Cramp and Perrins, 1994; Mor-eira, 1999; Borralho et al., 1998). Some individualswhich had not started to migrate were also recorded.High densities ofT. tetraxwere found in cereal fieldsand fallows. Most studies on habitat selection in this

species (Shulz, 1985; Martınez, 1994; Martınez andDe Juana, 1996; Moreira and Leitão, 1996; Moreira,1999) suggest that cereal is not a preferred habitat,but the result obtained might be explained by the factthat due to weather conditions cereal fields did notgrow as usual in that particular year.

The appearance of stubbles after the breedingseason offered access to a great abundance of seed


Table 5Post-breeding relative densities (birds km−1) in fallow land (n= 20 transects), cereal fields (n= 22 transects) and stubbles (n= 20 transects)in Castro Verde, with median and inter-quartile distance

Species Fallows Cereal fields Stubbles

Melanocorypha calandra∗ 4.0 (2.8–16.2) 0.2 (0–1.7) 2.4 (0–17.7)Ciconia ciconia* 1.7 (0.2–5.4) 1.3 (0–4.7) 0 (0–0.5)Calandrella brachydactyla 1.0 (0–3.4) 0 (0–2.2) 0 (0–2.4)Burhinus oedicnemus* 0.6 (0–2.8) 0 (0–0) 0 (0–0)Tetrax tetrax 0.5 (0–6.2) 0 (0–1.2) 0 (0–0)Otis tarda 0.5 (0–5.3) 0 (0–0.3) 0 (0–2.4)Miliaria calandra 0 (0–2.6) 0 (0–7.3) 0.8 (0–6.1)Lanius meridionalis 1.4 (0–2.3) 0.5 (0–1.9) 0.8 (0–1.1)Pterocles orientalis 0 (0–0.8) 0 (0–0) NOa

Bubulcus ibis 0 (0–0) 0 (0–2.2) 0 (0–6.5)Anthus campestris 0 (0–0) 0 (0–0) 0 (0–0.4)Oenanthe hispanica 0 (0–0) 0 (0–0) 0 (0–0)Cisticola juncidis* 0 (0–0) 6.1 (1.6–11.4) 0 (0–1.3)Coturnix coturnix* 0 (0–0) 0.4 (0–2.9) 0 (0–0.3)Galerida spp. 0 (0–0) 0 (0–0) 0 (0–0)Alectoris rufa 0 (0–0) 0 (0–0) 0 (0–0.9)

∗ Species with significant diferences between habitats (Kruskal–Wallis test).a Species not observed in the transects.

resource (Tellerıa et al., 1988a; Diaz and Telleria,1994), used especially byMiliaria calandra andMelanocrypha calandra. C. coturnixandC. juncidisremained associated to cereal fields, whereasC. ci-conia used fallows and non-harvested cereal fields asfeeding grounds.

Most species used several habitats of the agricul-tural landscape mosaic along the annual cycle. Themaintenance of the mosaic agricultural system is,therefore, essential for preserving the diversity of birdspecies in this landscape. Other authors have alsoshown the importance of habitat diversity either forspecific species (e.g. Martınez, 1994; Salamonardet al., 1996) and the whole bird community (Donazaret al., 1997).

4.3. Implications of agricultural changes

Cereal steppes are threatened by agriculturalchanges (Baldock, 1991; Diaz et al., 1993; Tuckerand Heath, 1994; Santos, 1996; Suárez et al., 1997).The results of this paper and other research madein the Castro Verde region can be used for a pre-liminary evaluation of the impact of several typesof agricultural changes on bird populations, with theassumption of non-adaptation of bird species to achanging environment.

Cereal fields and stubbles are likely to decrease, tobe replaced by pastures and fallow land. As a conse-quence bird species that rely on cereal would probablybe reduced, whereas bird species occurring mainly infallows could be promoted. An increase in stock den-sity would lead to changes in vegetation structure (de-creased height and higher cover by bare ground) due toovergrazing, along with increased trampling of nestsand eggs (Ausden and Treweek, 1995; Donazar et al.,1997). Thus, populations ofC. brachydactyla, O. his-panicaandA. campestriscould have an increased areaof available habitat, although with a potentially lowerreproductive success.

Homogenisation of the landscape, with a progres-sive increase in vegetation height and shrub cover(mainly Cistusspp.) would occur in abandoned fields.In a first stage, this would benefit species likeB. oedic-nemus, B. ibis and Galerida spp. (Moreira, 1999),but the increase in shrub cover would be equivalentto habitat loss for the steppe birds. Santos (1996)described the bird communities associated with suc-cessional stages of vegetation after land abandonmentin this region, and showed thatSylvia conspicillataTemminck was the only species with unfavourableconservation status found on scrubland.

The main impact of agricultural intensificationwould be a loss of fallow land, which would influence


negatively the populations ofT. tetrax and M. ca-landra. If intensification increased the frequency ofcereal cultivation, this could lead to a populationgrowth for species more dependent on cereal fields(such asC. juncidisandMiliaria calandra).

5. Conclusions

The bird diversity of the farmed landscape of CastroVerde depends on the maintenance of a diverse agri-cultural mosaic, as promoted by the extensive cultiva-tion of cereals in a rotational system. It is hypothesisedthat land-use modifications due to agricultural policieswill promote changes in bird assemblage structure,namely a loss in steppe bird richness and an increasein specific bird populations, depending on the type ofchanges. Scientific research in steppic landscapes withvarying level of agricultural changes or in sub-regionsof Castro Verde where agricultural changes are alreadytaking place would provide tests for these predictions.

Acknowledgements

This research was financed by the LPN project‘Conservation of steppe birds in Castro Verde region’,partially funded by LIFE programme (Contract4-3200/95/510) and Fundação Luso-Americana parao Desenvolvimento. We wish to thank Rui Morgadofor providing the data on land-use cover. Thanks alsodue to Pedro Beja, Rui Borralho, Mario Diaz and twoanonymous referees for useful comments that signifi-cantly improved the manuscript. This paper is a con-tribution to project PRAXIS XXI/C/AGR/11063/98“Factores determinates da biodiversidade em pousiosde estepes cerealiferas: implicações para definiçäo denormas de gestäo agro-ambiental”.

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Bird assemblages of an Iberian cereal steppe

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