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Keywords:

  • agricultural intensification;
  • avian conservation;
  • distribution change;
  • PCA;
  • population change

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • 1
    Rapid agricultural change in western Europe has occurred in the last three decades, at cost to farmland biodiversity, particularly birds. This study reviewed agricultural change in Denmark from 1983 to 2001, to compare patterns of intensification and farmland bird abundance with the UK.
  • 2
    Changes in 26 agricultural variables summarized using principal components analysis (PCA) showed consistent changes throughout the period that were similar to the UK. Pig and sheep production, and the extent of winter cereals, rape and fodder maize, all increased. The area used to grow fodder beet and spring barley, the applications of agrochemicals and the numbers of cattle reared all declined. The greatest change in land area in Denmark was the switch from spring- to autumn-sown cereals in the 1980s, almost a decade later than in the UK.
  • 3
    PCA described changes in annual indices of bird abundance based on Danish point count surveys from 1983 to 2001, which were most marked during 1983–90, after which ordination values varied little despite continued agricultural change. Of 27 bird species associated with farmland habitat in Denmark, five declined, 10 showed stable trends and 12 increased, compared with 15, eight and four, respectively, among the same species in the UK.
  • 4
    Agricultural yields have been sustained or enhanced during the survey period, while most farmland bird species declining in the UK have remained stable or increased in Denmark. Of the five declining Danish species, only lapwing Vanellus vanellus and yellowhammer Emberiza citrinella are associated with predominantly farmland habitat. The timing of the declines suggests that the switch to autumn sowing in Denmark has had little effect on any species.
  • 5
    In contrast to the UK, pesticide and inorganic fertilizer use has declined and organic farming has expanded in Denmark since 1983, coinciding with the period of stability/increase in farmland bird abundance. It is not possible to establish any causality from this analysis.
  • 6
    The ability of species showing marked declines in Europe to maintain their number and distribution in the Danish landscape in the face of agricultural intensification gives some optimism for safeguarding farmland birds and biodiversity in the future. However, we need to understand the reasons behind contrasting population trends in Denmark and the UK.
  • 7
    Synthesis and applications. Marked differences between national patterns of agriculture and the contrasting nature of historical intensification offer the opportunity to contrast the effects of major changes in land-use practice on European farmland biodiversity. Appropriate comparative and individual studies of the effects of changes in specific agricultural management at greater spatial (i.e. supranational) scales are necessary in order to underpin the successful development of future European agricultural policies that will sustain and enhance agricultural yields whilst maintaining farmland biodiversity.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Marked population declines among many farmland birds in Europe have occurred during the last decades of the 20th century (Tucker & Heath 1994; Donald, Green & Heath 2001), most notably in the UK (Siriwardena et al. 1998). As agricultural production accounts for such a large proportion of the land cover in western Europe, these declines might reflect a major reduction in the overall biodiversity of terrestrial habitats in general (Krebs et al. 1999). The UK government has identified declines in farmland birds as a matter of national public, as well as scientific, concern (Gregory et al. 2000). The general increase in agricultural production per unit land area (broadly gathered under the term ‘intensification’) has been identified as a major cause of declines in farmland bird populations, although species respond differently according to their ecology (Fuller et al. 1995; Chamberlain et al. 2000). Three major factors associated with the intensification of arable agriculture affect bird populations. First, the increased use of pesticides has had indirect effects, through impacts on food resources (Rands 1985; Firbank et al. 1991), and direct effects on avian physiology and reproduction (Ratcliffe 1967, 1970). Secondly, the increased use of inorganic fertilizers has increased plant growth, creating tall, dense stands, reducing heterogeneity in plant diversity, crop structure and mosaics (Wilson et al. 1997). Finally, the switch from spring-sown to autumn-sown crops has radically changed the arable farming landscape. It has removed persistent stubble fields, which had provided important winter bird-feeding habitat (Evans & Smith 1994; Aebischer 1997). The spring crop density is higher in autumn-sown crops than those sown after the winter, making the crops unsuitable as habitat for breeding species such as lapwing and skylark (for scientific names and authorities see Table 1; Shrubb & Lack 1991; Wilson et al. 1997). As a further consequence, floral diversity is much reduced, thereby affecting the abundance and availability of wild seed stocks (Andreasen, Stryhn & Streibig 1996; Hald 1999).

Table 1.  Farmland bird species considered in the analysis. UK population trends between 1969 and 1995 are based on the British Trust for Ornithology Alert Limit system (Crick et al. 1998) for decreasing species and significance tests in Siriwardena et al. (1998) for increasing species. Trends are categorized as: +, increase; 0, stable; –, small decline; – –, severe decline; following the criteria of Crick et al. (1998). Danish population trends are based on Spearman rank correlation tests between index values and year for the period 1976–2001 inclusive (Jacobsen 2002). Also shown is the percentage change in occupied 5 × 5-km squares between the first (1971–74; Dybbro 1976) and second (1993–96; Grell 1998) Danish breeding bird atlas projects, based on breeding evidence obtained for each species in 2170 surveyed grid squares (for full details see Grell 1998). Habitats were determined by Fuller et al. (1995) for the UK, but are similar for Denmark, where F indicates a farmland ‘specialist’ species and W indicates a primarily woodland species that commonly uses farmland. Turtle dove Streptopelia turtur L. and yellow wagtail Motacilla flava L. (featured in Chamberlain et al. 2000) were not common enough in sampled point counts in Denmark to enable calculation of trends. The yellow wagtail showed a 32% reduction in occupied 5 × 5-km squares from 1971–74 to 1993–96 (Grell 1998)
SpeciesPopulation trend UKPopulation trend Denmark% change in occupied 5 × 5-km squares, Denmark 1971–74 to 1993–96Habitat
Kestrel Falco tinnunculus L.0 +19F
Grey partridge Perdix perdix L.––0   −2F
Lapwing Vanellus vanellus L.     0F
Stock dove Columba oenas L.+++311F
Skylark Alauda arvensis L.––0  +2F
Starling Sturnus vulgaris L.0  +2F
Jackdaw Corvus monedula L.++  +1F
Rook Corvus frugilegus L.++ +16F
Wren Troglodytes troglodytes L.0+ +11W
Dunnock Prunella modularis L.  +7W
Whitethroat Sylvia communis L.00  +7F
Lesser whitethroat Sylvia curruca Latham0  +9W
Robin Erithacus rubecula L.0+ +10W
Blackbird Turdus merula L.+  +2W
Song thrush Turdus philomelos L.––0  +2W
Blue tit Parus caeruleus L.0+  +9W
Great tit Parus major L.0  +3W
Long-tailed tit Aegithalos caudatus L.00 +19W
Tree sparrow Passer montanus L.––+  +3F
Chaffinch Fringilla coelebs L.++  +3W
Greenfinch Carduelis chloris L.+ +11F
Goldfinch Carduelis carduelis L.+ +24F
Linnet Carduelis cannabina L.––0  +4F
Bullfinch Pyrrhula pyrrhula L.––+ +33W
Corn bunting Miliaria calandra L.––0 −30F
Reed bunting Emberiza schoeniclus L.––0    0F
Yellowhammer Emberiza citrinella L.0    0F

To date, the majority of farmland bird studies have been carried out in the UK (Ormerod & Watkinson 2000). However, declines among farmland birds have been reported in Germany (Flade & Steiof 1990), the Netherlands (Saris et al. 1994) and the USA (Millenbah et al. 1996). On a European scale, the most dramatic declines and range contractions in farmland species have occurred in those countries with most intensive agriculture, which supports the hypothesis that rates of decline in farmland bird population are related to differences in agricultural intensification (Donald, Green & Heath 2001). For this reason, it is generally considered that the broad aim of maintaining farmland bird numbers is incompatible with current agricultural practice. Indeed, fears have been expressed about the future loss of avian biodiversity that may result from continued intensification, for example due to rapid changes in agriculture in new member states as the European Union (EU) expands (Donald, Green & Heath 2001; Donald et al. 2002). Increasingly, emphasis is placed upon balancing the ecological challenges of rapid agricultural change with the maintenance of environmental functions and biodiversity (Ormerod et al. 2003), so it is essential that we understand which elements of intensification impact most upon natural populations and farmland ecosystems.

Agriculture has dominated the Danish landscape for centuries, but recent policy and economic factors, together with great technological developments in farming, have accelerated changes in the last 30 years. Owner-occupancy is the highest in Europe, so the debt burden makes farming highly sensitive to economic change. Between 1970 and 2002, the number of farms fell from 140 200 to 50 500 as small, family farms were amalgamated into larger, more mechanized units. Mean farm size increased from 21 to 51 ha over the same period (all statistics courtesy of StatBank Denmark, see the Methods). The total farmed area has fallen from 2·94 million ha in 1970 to 2·67 million ha in 2002, with marginal land increasingly being converted to forestry. During this period, the use of agrochemicals, the introduction of novel crops such as rape, the decline in spring tillage, root and hay crops and fallow rotations all reflect the general patterns of intensification in lowland Britain (Chamberlain et al. 2000). Significantly, during the 1980s, legislative instruments were enacted regulating agriculture, imposing high rates of taxation on pesticides and limiting the use of fertilizers. The subsequent reduction in the application of pesticides and fertilizer during the 1980s and 1990s are in contrast to Britain, where agrochemical use continues to increase (Chamberlain et al. 2000; Robinson & Sutherland 2002). Organic agriculture in Denmark (formally initiated in the early 1980s) has also expanded from 5565 ha on 401 farms in 1989 to 165 000 ha (6% of the total farmed area) on 3466 farms (6% of the total) in 2000. At present more than 60% of the Danish land surface is cultivated, of which 60% is devoted to cereal production, and it is in this sector that most changes affecting farmland bird communities have occurred, particularly the shift from spring- to autumn-sown crops. However, despite all these changes, in general Danish farmland birds have shown neither the same contractions of range (Dybbro 1976; Grell 1998) nor the dramatic declines in abundance (Jacobsen 2002) that have occurred among the same species in the UK.

As Denmark is subject to the same EU Common Agricultural Policy (CAP) as the UK, experiences a similar climate, soils and agricultural practice to lowland Britain, this study reviewed the major changes in agriculture in Denmark over the past 30 years in relation to farmland bird abundance. In particular, it assessed the extent to which changes in bird abundance reflected these changes, and drew comparisons with the situation in the UK (Chamberlain et al. 2000).

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

agricultural data and analysis

Agricultural variables were obtained from StatBank Denmark, the on-line repository for Danish agricultural statistics published annually (http://www.dst.dk/). Variables were selected to reflect major changes at the gross land-use level (and therefore most likely to impact upon avian habitats) during the period for which reliable bird data were available. The annual national extent (i.e. total hectares) of the most important food and fodder crops, grassland and set-aside were used in the analysis, together with numbers of the most common livestock (Table 2). In addition, the annual applications of pesticides, growth regulators and fertilizers were also incorporated into the analysis. For the purposes of interpretation, many of these variables are presented over time periods longer than those specifically analysed below. These variables were then subjected to principal components analysis (PCA). Gaps in the time series for some of the agricultural variables and lack of bird data in some earlier years restricted analysis to the years 1983–2001 inclusive. In this approach, years were treated as samples, with associated agricultural variable and bird attributes. PCA was carried out on the correlation matrix of untransformed variables, enabling comparison of variables measured on differing scales, justified by the linear nature of the scaling (James & McCulloch 1990). The agricultural variables analysed were inevitably serially correlated, but the intention was to use PCA to exploit the redundancy in multivariate data to reduce the number of variables (rather than establish relationships between them). For this reason, this method has been used to summarize changes over time, following the same methods as Chamberlain et al. (2000) to ensure comparability.

Table 2.  List of the 26 agricultural variables used to summarize temporal changes in Danish agriculture during the period 1983–2001, using PCA. Variables are ranked according to their loadings on PCA axis 1
VariableUnits measuredPCA loading on axis 1
PigsTotal number−0·234
Winter wheatArea (ha)−0·234
Set-asideArea (ha)−0·208
Fodder maizeArea (ha)−0·199
SheepTotal number−0·185
Winter barleyArea (ha)−0·147
PotatoesArea (ha)−0·147
Winter rapeArea (ha)−0·121
LucerneArea (ha)−0·103
Growth regulatorskg active ingredient (per ha)   0·061
Spring wheatArea (ha)   0·092
Grass-clover rotationArea (ha)   0·115
RyeArea (ha)   0·167
Sugar beetArea (ha)   0·180
Permanent grasslandArea (ha)   0·213
Fungicideskg active ingredient (per ha)   0·216
Inorganic nitrogenkg (per ha)   0·220
Herbicideskg active ingredient (per ha)   0·220
Spring rapeArea (ha)   0·221
Insecticideskg active ingredient (per ha)   0·225
Inorganic potassiumkg (per ha)   0·225
Spring barleyArea (ha)   0·226
CattleTotal number   0·232
Total pesticideskg active ingredient (per ha)   0·235
Inorganic phosphoruskg (per ha)   0·240
Fodder beetArea (ha)   0·242

bird data

The Danish breeding bird monitoring scheme is based upon point counts undertaken since 1976 (Jacobsen 2002). This programme has involved sampling birds from more than 70 routes (> 300 since 1987) throughout the country. Most routes consisted of at least 20 (but always > 10) marked ‘points’ (which could be identified in subsequent years) at which all birds seen and heard (regardless of distance from observer) were registered and recorded in a 5-min observation period (Jacobsen 2002). In recent years, the majority (> 300) of routes were counted in at least two successive years by the same observer, at the same time of year (±7 days), same time of day (±30 min) and under comparable weather conditions. Although only c. 20% of points came from purely arable landscapes and c. 8% from permanent grassland plots, c. 50% of surveyed plots described as ‘mixed’ habitats included extensive areas of farmland (Jacobsen 2001). Hence, although these indices were generated from data collated from a variety of habitats, the predominance of agricultural land in the Danish landscape ensured good correlation between national trends and those from purely farmland habitat (Jacobsen 2001). Only those species registered in more than 20 repeated routes and from a minimum of 30 points in each successive year were included in the generation of a species index. The index for each species was set at 100 in the first year that a species fulfilled these criteria, and changes were tracked from year to year using the following formula:

  • It = It−1 × Nt/Nt−1

where It and Nt are the annual index value and number of registrations in year t, respectively. The index was not intended to generate an accurate estimate of the number of breeding pairs, but offered an index of change that potentially tracked changes in relative breeding abundance from year to year (Jacobsen 2002). Untransformed annual index values for each species were subjected to PCA on the correlation matrix of values for the years 1983–2001 inclusive.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

changes in danish agriculture

The proportion of Denmark's land area of 43 076 km2 cultivated as farmland fell from 71·8% in 1960 to 61·9% by 2002. The area of cereal production peaked at 1·85 million ha in the late 1970s, falling to 1·53 million ha in 2002, although the nature of this production had changed considerably. Barley is mainly spring-sown in Denmark and the area under barley cultivation more than halved between the 1970s and late 1990s (Fig. 1). There was an increase in the area under wheat, which is predominantly autumn-sown in Denmark (Fig. 1). In addition, Danish farmers experimented with rape from the late 1970s, initially as a spring-sown crop, latterly as a winter crop (Fig. 2). Taking these trends into account with cultivation of rye (mainly an autumn-sown cereal), there was a sudden shift amongst the four most widely grown tillage crops from 66% to 75% spring sowing in the mid-1980s to less than 50% in years since 1991. As in Britain, there have been other changes in the winter farm landscape, most notably decreases in the extent of land under fodder beet, as mixed farms have disappeared and the tradition of arable rotation has been abandoned with the development of pre-emergent fertilizers (Fig. 3).

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Figure 1. Changes in the annual extent of barley (filled squares) and wheat (filled triangles) grown in Denmark 1967–2001. The dashed line shows the extent of the barley crop sown in spring, and the solid line shows the extent of wheat sown in autumn.

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image

Figure 2. Changes in the annual extent of rape grown in Denmark 1975–2001, broken down by area sown in spring (filled squares) and autumn (filled triangles).

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Figure 3. Changes in the annual extent of winter green fields by crop in Denmark 1982–2001, showing sugar beet (filled diamonds), fodder beets (filled squares) and other crops (filled triangles).

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Managed rotational and permanent grassland declined from the 1960s until the mid-1980s in Denmark (Fig. 4), although with considerable regional variation. This trend has been reduced by the recent (early 1990s) advent of set-aside, of which 86–94% was classified as grassland.

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Figure 4. Changes in the annual extent of predominant grassland types in Denmark 1960–2001, showing rotational grassland (filled squares), permanent grassland (filled triangles) and total set-aside (> 86% grasslands in Denmark, open circles).

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The total numbers of livestock in Denmark have increased consistently, with the exception of cattle, due to reductions in the dairy herd (due to European Community quotas) and numbers of heifers during the period 1983–89 (Fig. 5). In contrast, pig numbers have increased consistently and substantially throughout recent years, with an increasing trend for free-range animals since the consumer pressure for this in the 1990s. In 1800, sheep grazing dominated most of the Danish landscape, but as heath and grassland were ploughed, so the tradition disappeared (Ejrnæs, Berthelsen & Fredshavn 1998). Numbers in recent years have increased again, although, after increases throughout the 1980s, numbers of sheep have stabilized and declined slightly to the present (Fig. 5). The better soils of the south and east have meant that mixed pastoral agriculture in general was converted to intensive arable farming earlier here than elsewhere, and this trend continues to the present, with most cattle and sheep in western parts, especially in Jutland (Ejrnæs, Berthelsen & Fredshavn 1998).

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Figure 5. Changes in the annual number of cattle (filled diamonds), pigs (filled squares) and sheep (filled triangles) in Denmark 1982–2001.

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Application of inorganic nitrogen fertilizer increased dramatically during the 1960s and 1970s, in contrast to stable application levels of potassium and phosphorus (Fig. 6). Efforts during the 1990s have brought about reductions in the levels of application of all inorganic supplements (Fig. 6). The amount of nutrient added in the form of slurry has changed little since 1983.

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Figure 6. Changes in the annual application of inorganic phosphorus (filled squares), potassium (filled triangles) and nitrogen (filled diamonds) fertilizer (expressed as kg ha−1) in Denmark 1960–2001.

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After increases in the 1980s, the application of herbicides, insecticides and fungicides has declined in Denmark, in terms of the amount of active ingredient applied per unit area, although there has been no change in the use of growth regulators (Fig. 7).

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Figure 7. Changes in the annual application of agrochemicals by category in Denmark 1981–2001, namely growth regulators (filled triangles), insecticides (open circles), fungicides (filled squares) and herbicides (open squares), all expressed as kg of active ingredient per ha.

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Despite these declines in artificial applications, the yields of all plant products have shown no trend over the period of most rapid change, with the exception of cereals cropped for green fodder, which has decreased significantly. Pig production increased by 50% between 1990 and 2002 from 1260 to 1892 million kg of slaughtered animal per annum, and cattle production declined by 22% over the same period (Danmarks Statistik 2002). These changes occurred despite a 4·5% overall reduction in the total cultivated area of Denmark and an increase in set-aside.

changes in bird abundance

Only five farmland bird species listed in Chamberlain et al. (2000) declined in Denmark and only one (the corn bunting) had contracted in range between the two breeding bird atlas projects (Table 1). Of the species declining in abundance, the lapwing and yellowhammer predominantly used farmland habitats and the great tit (primarily a woodland species) showed a less than 0·5% per annum decline in numbers. All showed consistent long-term declines in number (Jacobsen 2001). The majority of farmland species in Denmark were stable or increased, including several of the seed-eating birds showing the most serious declines in the UK (e.g. skylark, tree sparrow, linnet, bullfinch, corn bunting and reed bunting; Table 1).

ordination of agricultural data

The first PCA axis of agricultural variables for the period 1983–2001 accounted for 63% of the variation in the data. This axis effectively described the change from mixed farming in the early 1980s, characterized by the high positive loadings of cattle, fodder beet and spring tillage, with heavy fertilizer and pesticide use (Table 2). Annual scores thereafter consistently fell through the 1980s and 1990s (Fig. 8). The influence of pig abundance, winter tillage, set-aside and the upsurge in fodder maize all characterized the decrease in annual score through the 1990s (Table 2). The second axis explained a further 13% of variation in the data, but was more difficult to interpret, dominated by lucerne and fodder maize, which contributed relatively little to the total farmland land cover.

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Figure 8. Graph of annual habitat variable scores (based on 26 agricultural variables relating to Danish agriculture) and annual farmland birds scores (based on point count index values for 19 Danish species) generated by independent PCA. Note reversed axes for ease for chronological interpretation. The loadings of the variables on axis 1 of each analysis are listed in Tables 2 and 3.

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ordination of point count data

The PCA of annual bird index (based on 19 species with appropriate data for the years 1983–2001) showed a different pattern to that of the agricultural variables (Fig. 8). Change was most evident during the period 1984–90, after which values stabilized. The first axis represented species that had increased most (blackbird, greenfinch, bullfinch, rook, and linnet, with negative loadings) through to the declining species (yellowhammer, dunnock and lapwing; Table 3). This axis explained only 33% of the variation in the data, reflecting a lack of consistent pattern in bird abundance data. Subsequent axes had even lower explanatory power. Comparing changes in agricultural PCA scores with those of the bird point counts, it would seem that there was a strong correspondence during the years 1984–90. However, there was little change in bird abundance, nor great annual fluctuations, despite continued agricultural change since 1990 (Fig. 8).

Table 3.  List of breeding species used to summarize temporal changes in Danish farmland bird abundance during the period 1983–2001, using PCA of annual point count indices for each of the 19 species. Variables are ranked according to their loadings on PCA axis 1. See Table 1 for scientific names and authorities
SpeciesScores on axis 1
Blackbird−0·352
Greenfinch−0·304
Bullfinch−0·277
Rook−0·270
Linnet−0·268
Wren−0·239
Skylark−0·182
Robin−0·178
Chaffinch−0·147
Blue tit−0·109
Whitethroat−0·063
Reed bunting−0·051
Tree sparrow−0·039
Jackdaw−0·005
Corn bunting   0·126
Song thrush   0·136
Lesser whitethroat   0·138
Great tit   0·181
Yellowhammer   0·215
Dunnock   0·250
Starling   0·283
Lapwing   0·353

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

the nature of agricultural change in denmark

Denmark has shown the same level of agricultural change as much of western Europe, including the UK. Foremost has been the increased use of pesticides and growth regulators to reduce the competitive abilities of other organisms, and improved machinery to plough, sow, dress, spray and harvest more efficiently. So why, in the face of consistent intensification of agriculture in Denmark, especially the wide-scale shift from spring-sown crops to autumn-sown, have the same suite of bird species associated with farmland not declined as they have elsewhere? Heavily fertilized and chemically protected winter cereals grow too tall and dense for nesting skylarks, which favour spring-sown (or organically grown winter) cereals that have a sparser structure and slower growth rate (Wilson et al. 1997; Chamberlain, Wilson & Fuller 1999). The corn bunting has shown dramatic decline and contraction of breeding range in the UK and elsewhere in Europe (Hagermeijer & Blair 1997; Siriwardena et al. 1998), linked to agricultural intensification, including increased use of pesticides, which has reduced availability of chick-food invertebrates on lowland farmland (Brickle et al. 2000). However, improvements in nesting success during population decline (Crick 1997) suggest factors affecting survival or a decline in numbers of breeding attempts may also contribute. The specific disappearance of suitable over-wintering stubble field habitat as a consequence of the switch to autumn sowing (such as sparse barley stubble; Mason & Macdonald 2000; Moorcroft et al. 2002) has been implicated as a major factor in its decline. However, it is generally concluded that reduced winter food supplies resulting from the combination of loss of spring tillage, increased pesticide usage and improved harvesting and storage techniques may have been responsible (Donald & Forrest 1995). In Denmark, the corn bunting has shown no significant overall trend since 1980 (Fig. 9). In contrast to the UK, following declines in the 1970s, the skylark has been stable throughout the period of consistent agricultural change in Denmark (Figs 1 and 9). The decline in numbers of these two Danish species was most pronounced in the late 1970s and early 1980s, and it seems likely that present population levels are well below those, for example, of the middle of the last century. Nevertheless, the recent increase in Danish corn bunting abundance in the face of continued agricultural change gives cause for some optimism for developing successful recovery plans for the species elsewhere.

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Figure 9. Changes in the annual point count index values of skylark Alauda arvensis (filled squares) and corn bunting Miliaria calandra (filled triangles) in Denmark 1976–2001. Data from Jacobsen (2002).

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In Denmark, the vast majority of skylarks, linnets and goldfinches depart in winter, hence the loss of winter stubble is of no consequence for these species. All three species breeding in Denmark occur to a limited extent in the UK outside the breeding period and therefore populations might reflect winter habitat change there (Wernham et al. 2002) and elsewhere in Europe. However, other species of conservation concern in the UK that are stable or increasing in Denmark, such as the corn bunting, tree sparrow, bullfinch and reed bunting, utilize Danish farmland habitat to a greater or lesser extent during the non-breeding season. It seems that neither the switch from spring- to autumn-sown crops, nor any of the other features associated with agricultural intensification in the 1990s in Denmark, have affected population trends in these species.

Farmland bird abundance in the UK declined dramatically between 1976 and 1982 (Chamberlain et al. 2000), which predates the period with best-quality point count data from the Danish scheme. Species such as the grey partridge, linnet and skylark declined at that time and may already have been at reduced population densities prior to the study period (1983–2001). Nevertheless, the most rapid changes to the Danish agricultural landscape have been associated with intensification since 1983, involving factors that are thought to have been the causes of declines in UK farmland birds. These agricultural changes occurred slightly later in Denmark than the UK, but apparently without such widespread effects on the same avian species.

other potential factors affecting bird abundance

This analysis has not examined in detail changes in grassland management in recent decades in Denmark. However, as arable farming dominates the agricultural scene (comprising 60% of all agricultural land compared with 35% in Britain; Robinson & Sutherland 2002), these changes are likely to have had only local effects on bird populations. Loss of small mixed farms in Denmark has removed crop rotations including grass leys, which favour species such as lapwing and skylark (Galbraith 1988; Chamberlain & Gregory 1999). However, Robinson, Wilson & Crick (2001) argue that arable ‘pockets’ in grassland landscapes have proportionally more effect in helping restore numbers and range of some farmland birds than establishing similarly sized areas of grassland in already mixed or arable-dominated areas. Hence, reduction in grassland area in a predominantly arable system in Danish landscapes may have had proportionally little effect in the period under discussion.

Unfortunately, no data exist on changes in non-crop habitats, particularly field margins, headlands, wetlands, hedgerows and copses, over the period under review. The Danish agricultural landscape is relatively rich in such features, although they have been declining in the modern agricultural landscape (Agger & Brandt 1988). Their importance, especially as provisioning areas for breeding birds such as corn bunting and yellowhammer (Brickle et al. 2000; Perkins et al. 2002), suggests that their extent and availability might affect farmland bird populations. The Danish landscape is particularly rich in small woodlands, which support high bird densities (Vanhinsberg et al. 2002). Hedgerows and field margins are important for species, including dunnock, lesser whitethroat and yellowhammer, considered hedgerow ‘specialists’ (Fuller et al. 2001). Large-scale intensive farming inevitably decreases the length of crop margins by increasing field size, so the geometry of the landscape has a great effect on the relative availability of biotopes most exploited by farmland birds (Panek 2002).

Most farmland bird species in Denmark have been stable or increased in abundance during a period characterized by reductions in inorganic fertilizer and pesticide use. At the same time, all key crop species have maintained yields, while the production of many animal products (especially pigs and poultry) has increased substantially since the 1980s. This pattern has not been repeated elsewhere (Robinson & Sutherland 2002). Much of the European agricultural landscape has received more than 50 years of chemical applications, with possible cumulative effects on biodiversity. In contrast, organic agriculture has positive effects on species activity, abundance and species richness amongst many invertebrates and vertebrates (Wilson et al. 1997; Krebs et al. 1999; Beecher et al. 2002; Wickramasinghe et al. 2003). Hence, the increase in organic arable agriculture, coupled with a general reduction in use of pesticides in Denmark, may be significant in buffering farmland bird species against the effects of agricultural intensification.

Although not well recorded by the point count method, common birds of prey in the Danish farmland landscape (buzzard Buteo buteo L., marsh harrier Circus aeruginosus L., sparrowhawk Accipiter nisus L. and kestrel Falco tinnunculus L.) have generally showed stable trends over recent years (Jacobsen 2002). The same is true of members of the crow family, with hooded crow Corvus corone L., magpie Pica pica L., rook Corvus frugilegus L. and jackdaw Corvus monedula L. having all increased in the period 1976–2001 (Jacobsen 2002). This strongly suggests recent increases in farmland birds are not associated with declines in predatory species, confirming the results of studies elsewhere (Thomson et al. 1998).

The key messages from these results are that (i) the effects of agricultural intensification in Denmark (as in the UK) are multivariate and (ii) that no single factor, such as timing of cereal sowing, is likely to impact upon all farmland bird populations in the same way. The difference in the patterns of change in distribution and abundance of farmland birds in the UK and Denmark, however, enables a comparison between farming systems and the nature and timing of intensification in the two countries. More detailed investigation at the species level might therefore establish particular relationships between farmland bird declines and casual features of agricultural intensification. All the factors mentioned above differ in nature between the non-crop habitat and factors in the Danish landscape and those of the UK. Perhaps more importantly, these features differ radically between other regions and countries, reflecting differences in farming practice despite similarities in the pressures on modern European agriculture. It would seem sensible to widen this comparative international approach to better understand the temporal relationships that link the ecology of birds and changes in agricultural practices in the different regions (Benton et al. 2002).

implications for conservation

These data suggest that, despite CAP, intensification and other pressures on agriculture in Denmark, farmland bird populations have been maintained at similar or higher levels than those of the early 1980s. These trends contrast with those of the same species in the UK. Further investigation of the reasons for these differences is particularly urgent for those species (such as the skylark, tree sparrow, linnet, bullfinch, corn bunting and reed bunting) that show contrasting trends in the two countries and declines elsewhere in Europe. The introduction of set-aside in both countries has provided, on a very short time scale, habitats demonstrably preferred by many farmland bird species (Henderson, Vickery & Fuller 2000), although the impact at the population level is difficult to demonstrate (Henderson et al. 2000). The generally poor performance of agri-environment schemes has been blamed upon failure to target the specific ecological needs of farmland species and lagged response times (Kleijn et al. 2001; Bradbury & Allen 2003; Kleijn & Sutherland 2003). However, such schemes have been shown to be highly effective in cases where species’ needs have been identified (Peach et al. 2001; Ausden & Hirons 2002; Evans, Armstrong-Brown & Grice 2002). Other studies show that supplementary feeding or specific habitat provision can assist species recovery (Lock 1999). The ability to shape features of the landscape to benefit farmland birds through agri-environmental policy and specific single species’ measures is reliant upon a good scientific understanding of the mechanisms that regulate range and abundance of individual species. A better understanding of the mechanisms that enable Danish farmland birds to maintain population levels in the face of continued agricultural changes would give some optimism for modelling (Stephens et al. 2003) and for the development of management prescriptions for the successful future maintenance of farmland biodiversity elsewhere.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

My sincere thanks to Dan Chamberlain for his encouragement and suggestions for improvements to earlier drafts, and to Henning Noer for his indulgence. Thanks also to Jeremy Wilson, Ole Therkildson, Peter Odderskær and Preben Clausen for comments and suggestions on the text, help and advice, and Juliet Vickery and other staff of the British Trust for Ornithology for their comments and guidance with this analysis. Special thanks must go to the very many willing volunteers who contribute to the Danish point count system (listed in full in Jacobsen 2002), and to Erik Mandrup Jacobsen for his co-ordination and syntheses, without whom such analyses would be impossible.

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  6. Discussion
  7. Acknowledgements
  8. References
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