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Seasonal distribution of meadow birds in relation to in-field heterogeneity and management
Agriculture, Ecosystems and Environment 142 (2011) 161–166
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Seasonal distribution of meadow birds in relation to in-field heterogeneity and management Jort Verhulst a,∗ , David Kleijn a,b , Willem Loonen c , Frank Berendse a , Christian Smit d a
Nature Conservation and Plant Ecology Group, Wageningen University, Droevendaalsesteeg 3a, P.O. Box 47, 6700 AA Wageningen, The Netherlands Alterra, Centre for Ecosystem Studies, Droevendaalsesteeg 3, P.O. Box 47, 6700 AA Wageningen, The Netherlands Netherlands Environmental Assessment Agency, Antonie van Leeuwenhoeklaan 9, 3721 MA Bilthoven, The Netherlands d Community Ecology and Conservation Ecology Group, University of Groningen, Nijenborg 7, P.O. Box 11103, 9700 CC Groningen, The Netherlands b c
a r t i c l e
i n f o
Article history: Received 1 December 2010 Received in revised form 22 April 2011 Accepted 26 April 2011 Available online 16 June 2011 Keywords: Black-tailed godwit Farmland birds Habitat selection Lapwing Sward Wet grassland
a b s t r a c t Effectiveness of European initiatives to restore populations of meadow breeding waders is heavily debated. We studied field preference of meadow birds throughout the breeding season in four areas of over 100 ha each and related observed patterns of individual birds to in-field heterogeneity, sward height and management. Over the four areas, most waders were observed in the more heterogeneous fields at both the period of nest site selection and incubation. Additionally, fields grazed at relatively low-intensity for longer consecutive periods (on average 6 cows/ha for 30 d instead of 20 cows/ha for 2 d) were hosting high densities of lapwings but also black-tailed godwits. Our results suggest that in-field heterogeneity may be important for meadow breeding waders at the nest site selection and incubation stages. Conservation initiatives aimed at meadow breeding waders might improve their effectiveness when they increase the heterogeneity of fields. Grazing for longer consecutive periods at relatively low stocking rates might be a way to achieve this, if carried out at stocking rates low enough to allow waders to reproduce successfully. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Agricultural intensification over the last 50 years has resulted in substantial changes in Western European farming practices (e.g. Donald et al., 2002). For wet grasslands, this meant increases in stocking levels and fertiliser applications and the frequent reseeding of the fields (e.g. Beintema et al., 1997). Improvements in field drainage allowed farmers to access their fields earlier in spring and thus to advance their activities, and to replace hay crops with silage crops (Beintema et al., 1985; Vickery et al., 2001). At the field scale, these factors greatly reduced variation in micro-topographical features of grasslands and resulted in more homogeneous and denser swards (Vickery et al., 2001; Wilson et al., 2005). However, agricultural intensification also reduced farmland heterogeneity at the larger spatial scales (Benton et al., 2003). These changes in agricultural practices and reductions in farmland heterogeneity coincided with large declines in European farmland bird populations (e.g. Siriwardena et al., 1998; Donald et al., 2001), including grassland breeding waders (e.g. BirdLife International, 2004).
∗ Corresponding author. Tel.: +31 6 44140931; fax: +31 527 245501. E-mail address: [email protected] (J. Verhulst). 0167-8809/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2011.04.016
In the Netherlands, agri-environment schemes were designed to halt declines in meadow breeding waders. The Netherlands have an international responsibility for this species group, as the country harbours over 40% and 30% of the European breeding populations of black-tailed godwits Limosa limosa and oystercatchers Haematopus ostralegus (BirdLife International, 2004). The key tool was, and is, to postpone mowing or grazing of grassland to allow birds to safely hatch their eggs (Beintema and Müskens, 1987). Additionally, Schekkerman and Müskens (2000) and Schekkerman and Beintema (2007) found black-tailed godwits with chicks to select fields with tall swards. Fields with the postponed mowing scheme however, do not have higher settlement densities of waders (Kleijn et al., 2001) and neither do additional schemes implemented at larger scales (Verhulst et al., 2007). The most recent initiative designed to maintain especially the black-tailed godwit populations on farmland is the so-called ‘mosaic management’. It aims to provide at least one ha with tall swards for food and shelter per black-tailed godwit family and to create a spatial mixture of differently managed fields at the polder scale (200–400 ha). However, in a large scale study, the breeding success of black-tailed godwits was found not to differ from control areas without these measures and reproductive success was insufficient to maintain stable populations (Schekkerman et al., 2008). Consequently, despite the fact that over 15% of all Dutch grassland is managed under some sort of meadow bird scheme, meadow birds are still declining rapidly both in range
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and in population size (SOVON Vogelonderzoek Nederland, 2002; Teunissen and Soldaat, 2006; Landschapsbeheer Nederland, 2007). Kleijn et al. (2004) hypothesize that the reductions in farming intensity on postponed mowing scheme fields are not sufficient to deliver benefits to breeding waders. Schekkerman et al. (2008) suggest that the poor variation in vegetation structure of (late mowed) grassland might be responsible for the observed low reproductive success of black-tailed godwits. Preferences of different species of farmland breeding waders concerning sward structure have been determined in several studies but most of these studies were carried out in natural habitats (coastal marshes) or mixed farmland and few have done so in intensively farmed wet meadows (e.g. Galbraith, 1989; Berg, 1992; Norris et al., 1997; Johansson, 2001, but see Schekkerman et al., 1998). This study aims to explore the effects of field characteristics on meadow breeding waders in the Netherlands. We specifically test the effects of management type, sward height and heterogeneity on the physical position of individual black-tailed godwits, lapwings Vanellus vanellus, redshanks Tringa totanus and oystercatchers throughout the breeding season in four farmed areas of over 100 ha each. Each area contained fields that differed in heterogeneity, ranging from uniform fields that were recently reseeded to fields that contained many flowering forbs or areas with retarded grass growth. 2. Methods In 2005, meadow breeding waders were surveyed in four areas in the Eempolders (N52◦ 15 , E5◦ 19 ) in the centre of the Netherlands. Each study area covered over 100 ha. To make sure that at least some fields with tall swards were to be present during the entire breeding season, each study area contained several fields under the postponed mowing scheme. 2.1. Surveys From the end of March until mid-June, the positions of the four meadow breeding waders were mapped at least two days per week from roads intersecting the study areas using telescopes and binoculars. Flocks of perceived non-breeding black-tailed godwits and lapwings that occurred from mid-May onwards were excluded from analyses. On a survey day, a study area was usually surveyed twice but at the end of the breeding season three surveys could be completed because of the low number of birds. On average, each study area was mapped 54 times on 26 days divided over the breeding season. Nest sites were not searched for. Four times during the breeding season (end of March, mid April, mid May and beginning of June) the heterogeneity of each field was estimated. Five classes were distinguished, ranging from low heterogeneity (I) to high (V). This classification was based on the cover of heterogeneity features in the field, such as shallow pools, tussocks (i.e. small patches of taller vegetation), molehills, flowering forbs (predominantly Ranunculus, Rumex or Taraxacum spp.), grasses other than Lolium perenne and a high diversity of sward height (see Table 1 for definition of classes). Fields with signs of recent reseeding (grass clearly growing in rows instead of a continuous sward) got a very low score in this classification (class I). Pools and molehills were only present at the onset of the breeding season (end of March). On each survey day, the sward height of each field was visually estimated to 5-cm classes (0–5, 5–10, 10–15, 15–20, 20–25, >25 cm). When a field had differences in sward height (e.g. tussocks), the height of the vegetation covering more than 50% of the field was recorded. On each survey day we also recorded the type of management, either no management, mowing or grazing (includ-
Table 1 Definition of the heterogeneity classes, with percentage field cover of shallow pools, tussocks, molehills, flowering forbs, divergent grass species (other than Lolium perenne) and a high diversity in sward height over the field as scored heterogeneity features. Class
Definition
I
Highly uniform grassland (e.g. recently reseeded), <1% heterogeneous features Uniform grassland with 1–5% heterogeneous features Grassland with 5–10% heterogeneous features Grassland with 10–25% heterogeneous features Heterogeneous grassland, >25% heterogeneous features
II III IV V
ing the number and type of grazers (cattle, sheep or horses)). Table 2 presents an overview of the characteristics of the four study areas.
2.2. Analyses Prior to analyses, the breeding season was divided in seven 10-day periods. For each of these periods, the average density of individuals of each meadow bird species per field was calculated. Also, the dominant heterogeneity class, management type and sward height class for each field were determined. Several management classes were distinguished. Fields were classified as ‘high-intensity grazing’ when they were subjected to high densities of cattle for a short time (on average density 20 cows/ha for 2 successive days) or ‘low-intensity grazing’ when they had lower densities of cattle for a longer period (on average 6 cows/ha for over 30 successive days). Few fields were grazed by horses or sheep. These generally were small fields with high densities of grazers that were present for over five weeks. Therefore, these fields were classified as ‘high-intensity grazing’, similar to those grazed by high densities of cattle. Fields without grazing or mowing were classified as “unmanaged”. Each management type affects the field characteristics in a different manner (e.g. mowing removes all vegetation at the same height, grazing leaves more or less variation in height). So when a field was mowed in period three, it was classified as ‘recently mowed’ in period four. In period five it would be classified as ‘regrown’, as we expected the effects of the management type to disappear after some time. Summarizing, the management types were: ‘mowing’ and ‘recent mowing’, ‘high-intensity grazing’ and ‘recent high-intensity grazing’, ‘lowintensity grazing’ and ‘recent low-intensity grazing’, ‘unmanaged’ and ‘regrown’. Generalized Linear Models with a Poisson distribution and a log-link function were applied (Type III), to determine whether meadow birds showed preferences for fields with specific characteristics. Models included study area, field size (as covariate, continuous variables), management type, heterogeneity and sward height as explanatory variables (categorical variables). Analyses were carried out for each of the four wader species for the first observation period (25 March–10 April) when lapwings started nesting while black-tailed godwits and redshank were selecting nest sites (Snow and Perrins, 1998) and for the third observation period (21–30 April) when the first lapwing nests had hatched and the other three species were nesting (Snow and Perrins, 1998). After these observation periods, birds were observed to be less territorial, and therefore their distribution became more dependent of good foraging sites for adults, which is of less interest to this study. Management and heterogeneity categories present on less than five fields over all study areas were omitted from analyses. Our explanatory variables were not independent. When a field is mowed or grazed, obviously the sward height is reduced. Therefore, Spearman’s correlation was used to test for correlations between management type, heterogeneity and sward height.
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Table 2 Characteristics of the four different study areas in the nest site selection phase (period 1; 25 March–10 April) and the nesting phase (period 3; 21–30 April). The management types ‘mowing’ and ‘recent mowing’ did not occur during the periods in the table and are therefore not included. The numbers in the columns of ‘Management’, ‘Heterogeneity’ and ‘Sward height’ refer to the number of fields under the given management type, heterogeneity class or sward height. Period
Study area
Total area (ha)
Number of fields
Management Unmanaged
1
3
High-intensity grazing
1 2 3 4
117 114 105 106
56 51 52 61
56 49 50 48
1 2 9
1 2 3 4
117 114 105 106
56 51 52 61
45 43 47 36
4 5 4 17
2
Number of fields
I (low)
II
III
1
1 2 3 4
117 114 105 106
56 51 52 61
25 20 1 16
18 24 17 29
8 4 24 16
5 2 10
1 2 3 4
117 114 105 106
56 51 52 61
33 31
19 18 20 37
2 1 25 7
2 1 4
Density per 100 ha
Heterogeneity
Black-tailed Godwit Lapwing
80
Oystercatcher
60
Redshank
40 20 0 25 March
11 April
21 April
1 May
11 May
21 May
1 June
Fig. 1. Average number of observations per 10-day period of different wader species throughout the breeding season.
3. Results Fig. 1 shows the densities of all four species during the breeding season. Numbers of black-tailed godwits and lapwings declined throughout the breeding season while redshank and oystercatcher remained more or less stable throughout the breeding season. Both at the end of March and in April, black-tailed godwits were predominantly found in the more heterogeneous fields (Fig. 2a). In May, differences between the different heterogeneity classes were smaller. In the first part of the breeding season, unmanaged fields and fields with relatively low-intensity grazing held high densities of black-tailed godwits (Fig. 2b). As the season proceeded, more management classes could be distinguished and differences between classes became smaller. However, fields with low-intensity grazing became more important later in the season. Similar to black-tailed godwits, most lapwings were found on the more heterogeneous fields from the end of March until the beginning of May (Fig. 2c). The highest densities of lapwings were observed on fields with low-intensity grazing throughout the breeding season (Fig. 2d). Surprisingly, fields with high-intensity grazing – which had short swards – had low densities in March and April but gradually became more important.
3 2 1 4
1
Total area (ha)
100
Recent low-intensity grazing
Regrown
4
Study area
17
Lowintensity grazing 1
Period
3
Recent highintensity grazing
4
2
Sward height (cm) IV
V (high) 1
3
0–5
5–10
33 9 37 5
23 41 15 56
1 2 2
1 16 33 13
10–15
15–20
20–25
1
27 34 12 42
28 4 4
1
homogeneous fields, as can be derived from the significant heterogeneity effect in Table 3. Management significantly affected black-tailed godwit densities with the highest densities being observed in unmanaged fields and fields with low-intensity grazing. These categories were significantly preferred over intensively grazed fields (Fig. 2b, Table 3). As hardly any differences in sward height occurred, no differences in black-tailed godwit densities were observed between fields with different sward heights (Table 3). At the end of April when black-tailed godwits started incubating, fields with intermediate heterogeneity were preferred over the homogeneous ones (preference for class III; Table 3). Again, management type no longer significantly affected black-tailed godwit density (Table 3). At the incubation period, black-tailed godwits were found in significantly higher densities on fields with the lower sward heights, i.e. those with swards of 5–10 cm (Table 3). Similar to black-tailed godwits, the more heterogeneous fields held significantly higher densities of lapwings than the homogeneous fields, both in early and late April (Fig. 2c, Table 3). Management type had a significant effect on lapwings with highest densities observed on fields with low-intensity grazing in both the nest site selection and the incubation phases (Fig. 2d, Table 3). During both phases, fields with the shortest swards hosted the highest densities (Table 3). At the end of March, both redshank and oystercatcher showed a significant preference for the more heterogeneous fields while neither of the species showed a significant preference for any of the management types or the sward heights (Table 3). By the end of April, redshanks were found in the significantly highest densities at fields with intermediate heterogeneity (class III). During this period, densities of both species were significantly affected by the type of management. Oystercatchers showed a preference for the lowest sward heights, while redshanks preferred fields with lower sward heights, i.e. those with swards of 5–10 cm (Table 3).
3.1. Effects of explanatory variables on bird densities
3.2. Mutual correlation of explanatory variables
At the nest site selection phase, black-tailed godwits significantly preferred the higher heterogeneity classes over the more
At the end of March, sward height was positively correlated with heterogeneity (Spearman’s rho = 0.145, p < 0.05). By late April,
J. Verhulst et al. / Agriculture, Ecosystems and Environment 142 (2011) 161–166
Heterogeneity class 200
4 3
160
2
120
1
80 40 0 25 March
Lapwing density per 100 ha
c
b
11 April
1 May
Management type
80 60 40 20 0
21 May
25 March
d
200
high-intensity grazing regrown low-intensity grazing unmanaged mowing recent high-intensity grazing
100
11 April
200
3 2
120
1
80 40
1 May
21 May
high-intensity grazing regrown low-intensity grazing unmanaged mowing recent high-intensity grazing
250
4
160
Lapwing density per 100 ha
Black-tailed godwit density per 100 ha
a
Black-tailed godwit density per 100 ha
164
150 100 50
0
0
25 March
11 April
1 May
21 May
25 March
11 April
1 May
21 May
Fig. 2. (a)–(d) Average densities of black-tailed godwits (a, b) and lapwings (c, d) on fields with different heterogeneity classes (a, c) and management types (b, d) over the four study areas throughout the breeding season in number of individuals per 100 ha. The management type ‘recent mowing’ is not depicted because it was only present from 21 May onwards. Bird densities represent the averages of two time periods. Dates in the figure indicate the beginning of the time period (e.g. 11 April: average of periods 11–20 April and 21–30 April).
sward height was both positively correlated with management (Spearman’s rho = 0.165, p < 0.05) and heterogeneity (Spearman’s rho = 0.403, p < 0.01). Heterogeneity and management were not correlated during either of the periods. See Appendix A for the relative covers of the different heterogeneity classes, management types and sward heights over the different observation periods.
4. Discussion Black-tailed godwits selected the more heterogeneous fields in both periods (nest site selection and nesting phases;
Fig. 2a, Table 3). These heterogeneous fields were predominantly unmanaged (data not shown). However, fields with relatively low-intensity grazing in general also were intermediately heterogeneous and contained high densities of black-tailed godwits. Several Dutch studies (e.g. Buker and Groen, 1989; Beintema et al., 1995) found nesting black-tailed godwits to prefer mowed fields over grazed fields while the latter are generally more heterogeneous. The high densities of individual black-tailed godwits on fields with relatively low-intensity grazing in our study seem to contradict those studies. However, Johansson (2001) found that Swedish black-tailed godwits preferably nest in tussocks with low
Table 3 Test statistics of the effects of management type, heterogeneity and sward height on the distribution of meadow birds at the onset of the breeding season (period 1; 25 March–10 April) and when most species were incubating (period 3; 21–30 April). Analyses were carried out on the density per period (thus not divided by field size). Models (GLM, Poisson distribution, log-link function) included study area, field size (covariate), management type, heterogeneity and sward height. Shown are the likelihood ratios. The trends of the overall effects of sward height and heterogeneity are indicated in brackets (not tested in a post hoc way). Period
Black-tailed Godwit
Lapwing
Redshank
Oystercatcher
Study area Field size Management Heterogeneity Height
35.33*** 27.27*** 6.88* 19.58*** (+) 0.92
37.38*** 45.50*** 15.10** 11.33* (+) 8.54** (−)
40.18*** 18.65*** 2.50 15.60** (+) 1.60
40.37*** 4.43* 0.61 8.28* (+) 2.16
Study area Field size Management Heterogeneity Height
8.31 38.2*** 3.33 15.27** , a 6.03* , b
11.31* 46.29*** 47.84*** 13.69** , a 9.91** (−)
29.88*** 14.91*** 16.04*** 18.47*** , a 6.22* , b
6.35 0.59 16.00*** 4.63 6.47* (−)
1
3
a b * ** ***
Intermediate heterogeneity (class III) preferred. Lower sward height (5–10 cm) preferred. Significance level of the main effect: p < 0.05. Significance level of the main effect: p < 0.01. Significance level of the main effect: p < 0.001.
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surrounding vegetation. Tussocks were more common in the more heterogeneous fields. Another option is that the black-tailed godwits in the heterogeneous grazed fields might have been nesting on mowed fields. Finally, comparing our results with previous Dutch studies should be done cautiously, because those studies were based on data collected in the 1980s. Kleijn et al. (2010) found that vegetation growth in spring increased significantly since the 1980s making unmanaged swards less suitable for grassland breeding waders. Grazing opens up the swards, which may explain why in this study black-tailed godwits were also observed in high densities in fields with relatively low-intensity grazing. Similar to black-tailed godwits, the highest densities of lapwings were observed during both periods (nesting and hatching phases) at the more heterogeneous fields (Fig. 2d, Table 3). Lapwings are known to nest in a wide variety of field types, from homogeneous dry to wet tilled fields and to heterogeneous rough grazing grasslands (e.g. Galbraith, 1988; Berg, 1993). Galbraith (1988) and Baines (1990) both found that unimproved pastures were the preferred grassland type. In our study, high lapwing densities were found in fields with relatively low-intensity grazing (low cattle load over longer period). Early in the season, the grazers probably kept swards short and created some small-scale heterogeneity which might have contributed to their attractiveness to lapwings. Fields with high-intensive grazing (high cattle load during 1 or 2 d) however, similarly had reduced sward heights but were avoided. These fields tended to be more homogeneous which suggests that these grazing regimes with (extremely) high stocking densities are too intensive for lapwings. Redshanks were observed to significantly select heterogeneous fields over the more homogeneous fields by early and late April (Table 3). Norris et al. (1997) found this species to preferably nest in structure-rich swards in coastal grazing marshes. Even though oystercatchers occurred in low densities throughout the breeding season (Fig. 1), their density in the heterogeneous fields significantly exceeded those in the homogeneous fields by both the beginning and the end of April (Table 3). Oystercatchers usually start nesting later than the other wader species (Beintema et al., 1995; Snow and Perrins, 1998), so probably the end of April coincided with their nest site selection phase. The explanatory variables in our study were not independent of each other. Sward height correlated in both periods with heterogeneity, and by the end of April it also was significantly correlated with management. As heterogeneity and management were not correlated to each other during either of the periods, we focused at these two variables. 4.1. Implications for management Several recent publications have stressed the importance of infield heterogeneity for farmland birds (e.g. Vickery et al., 2001; Benton et al., 2003; McCracken and Tallowin, 2004; Wilson et al., 2005; Kleijn et al., 2010). Heterogeneous fields are likely to provide optimal foraging opportunities for meadow birds (chicks), because the variation in vegetation structures both increases the range of potential prey species in these swards and ensures their availability (e.g. Morris, 2000; Vickery et al., 2001; McCracken and Tallowin, 2004). In our study, fields with relatively low-intensity grazing – grazed at longer consecutive periods with lower cattle densities – were much more heterogeneous than those with conventional high-intensity grazing regimes (with 80 cattle in fields of 2 ha for 2 d). Also, these fields with relatively low-intensity grazing contained high densities of lapwings but also of black-tailed godwits. We draw two main conclusions from our results. First, stocking densities of grazers have rarely been distinguished in recent Dutch meadow bird studies. In most studies relating breeding waders and grazing regimes, stocking rates did not exceed 1 cow/ha. However,
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we observed on average 20 cows/ha on the intensively grazed fields. The low-intensity grazing fields in this study usually had 6 cows/ha, so the risk of trampling is very high for both grazing categories. Maximum stocking rates in nearby meadow bird reserves supporting stable wader populations are 1.5 livestock unit/ha. However, our results suggest that distinguishing between high and lowintensity grazing may be important for breeding waders. Second, our results suggest that initiatives aimed at breeding waders might improve their effectiveness when incorporating infield heterogeneity. We observed higher densities of all four species on the more heterogeneous fields. Grazing fields for longer consecutive periods at relatively low-intensity might be an effective way to improve the in-field heterogeneity, as shown by our results. An alternative way to improve field heterogeneity is raising groundwater tables. Also, reseeding (parts of) fields with slowly growing grass species might be an option. With the latter two methods, more open swards are created which are attractive to both adult and juvenile meadow birds (see Kleijn et al., 2010). Both options will more seriously impact agricultural practices than low-intensity grazing, and would require high financial compensation for land managers. However, schemes that include rewetting whole fields have been applied in several European countries (see Ausden and Hirons, 2002; Vickery et al., 2004; Kahlert et al., 2007), and found to be cost-effective (Ausden and Hirons, 2002; Wilson et al., 2007). Acknowledgements We thank Bas van de Meulengraaf and Idde Lijnse for assistance with the field work. Comments of Jinze Noordijk improved the manuscript. This work was funded by the EU Project QLK5CT-2002-1495 Evaluating current European Agri-environment Schemes to quantify and improve Nature Conservation efforts in agricultural landscapes (EASY). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.agee.2011.04.016. References Ausden, M., Hirons, G.J.M., 2002. Grassland nature reserves for breeding wading birds in England and the implications for the ESA agri-environment scheme. Biol. Conserv. 106, 279–291. Baines, D., 1990. The roles of predation, food and agricultural practice in determining the breeding success of the lapwing (Vanellus vanellus) on upland grasslands. J. Anim. Ecol. 59, 915–929. Beintema, A.J., Beintema-Hietbrink, R.J., Müskens, G.J.D.M., 1985. A shift in the timing of breeding of meadow birds. Ardea 73, 83–89. Beintema, A.J., Dunn, E., Stroud, D.A., 1997. Birds and wet grasslands. In: Pain, D.J., Pienkowski, M.W. (Eds.), Farming and Birds in Europe. Academic Press, London, pp. 269–296. Beintema, A.J., Moedt, O., Ellinger, D., 1995. Ecologische Atlas van de Nederlandse Weidevogels. Schuyt & Co., Haarlem. Beintema, A.J., Müskens, G.J.D.M., 1987. Nesting success of birds breeding in Dutch agricultural grasslands. J. Appl. Ecol. 24, 743–758. Benton, T.G., Vickery, J.A., Wilson, J.D., 2003. Farmland biodiversity: is habitat heterogeneity the key? Trends Ecol. Evol. 18, 182–188. Berg, Å., 1992. Factors affecting nest-site choice and reproductive success of Curlews Numenius arquata on farmland. Ibis 134, 44–51. Berg, Å., 1993. Habitat selection by monogamous and polygamous lapwings on farmland—the importance of foraging habitats and suitable nest sites. Ardea 81, 99–105. BirdLife International, 2004. Birds in Europe: Populations Estimates, Trends and Conservation Status. Birdlife International, Cambridge. Buker, J.B., Groen, N.M., 1989. Verspreiding van Grutto’s Limosa limosa over verschillende typen grasland in het broedseizoen. Limosa 62, 183–190. Donald, P.F., Green, R.E., Heath, M.F., 2001. Agricultural intensification and the collapse of Europe’s farmland bird populations. Proc. R. Soc. Lond. B 268, 25–29. Donald, P.F., Pisano, G., Rayment, M.D., Pain, D.J., 2002. The Common Agricultural Policy. EU enlargement and the conservation of Europe’s farmland birds. Agric. Ecosyst. Environ. 89, 167–182.
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Schekkerman, H., Teunissen, W.A., Müskens, G.J.D.M., 1998. Terreingebruik, mobiliteit en metingen van broedsucces van Grutto’s in de jongenperiode. IBN-rapport 403. Instituut voor Bos- en Natuuronderzoek (IBN-DLO), Wageningen. Schekkerman, H., Teunissen, W., Oosterveld, E., 2008. The effect of ‘Mosaic management’ on the demography of black-tailed godwit Limosa limosa on farmland. J. Appl. Ecol. 45, 1067–1075. Siriwardena, G.M., Baillie, S.R., Buckland, S.T., Fewster, R.M., Marchant, J.H., Wilson, J.D., 1998. Trends in the abundance of farmland birds: a quantitative comparison of smoothed Common Birds Census indices. J. Appl. Ecol. 35, 24–43. Snow, D.W., Perrins, C.M., 1998. The Birds of the Western Palaearctic. Concise edition, vol. 1, Non-passerines. Oxford University Press, Oxford, NY. SOVON Vogelonderzoek Nederland, 2002. Atlas van de Nederlandse broedvogels 1998–2000. Nederlandse Fauna 5. Nationaal Natuurhistorisch Museum Naturalis, KNNV Uitgeverij and European Invertebrate Survey, Leiden, The Netherlands. Teunissen, W.A., Soldaat, L.L., 2006. Recente aantalontwikkeling van weidevogels in Nederland. De Levende Natuur 107, 70–74. Verhulst, J., Kleijn, D., Berendse, F., 2007. Direct and indirect effects of the most widely implemented Dutch agri-environment schemes on breeding waders. J. Appl. Ecol. 44, 70–80. Vickery, J.A., Bradbury, R.B., Henderson, I.G., Eaton, M.A., Grice, P.V., 2004. The role of agri-environment schemes and farm management practices in reversing the decline of farmland birds in England. Biol. Conserv. 119, 19–39. Vickery, J.A., Tallowin, J.R., Feber, R.E., Asteraki, E.J., Atkinson, P.W., Fuller, R.J., Brown, V.K., 2001. The management of lowland neutral grasslands in Britain: effects of agricultural practices on birds and their food resources. J. Appl. Ecol. 38, 647– 664. Wilson, A., Vickery, J., Pendlebury, C., 2007. Agri-environment schemes as a tool for reversing declining populations of grassland waders: mixed benefits from environmentally sensitive areas in England. Biol. Conserv. 136, 128–135. Wilson, J.D., Whittingham, M.J., Bradbury, R.B., 2005. The management of crop structure: a general approach to reversing the impacts of agricultural intensification on birds? Ibis 147, 453–463.
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Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee
Seasonal distribution of meadow birds in relation to in-field heterogeneity and management Jort Verhulst a,∗ , David Kleijn a,b , Willem Loonen c , Frank Berendse a , Christian Smit d a
Nature Conservation and Plant Ecology Group, Wageningen University, Droevendaalsesteeg 3a, P.O. Box 47, 6700 AA Wageningen, The Netherlands Alterra, Centre for Ecosystem Studies, Droevendaalsesteeg 3, P.O. Box 47, 6700 AA Wageningen, The Netherlands Netherlands Environmental Assessment Agency, Antonie van Leeuwenhoeklaan 9, 3721 MA Bilthoven, The Netherlands d Community Ecology and Conservation Ecology Group, University of Groningen, Nijenborg 7, P.O. Box 11103, 9700 CC Groningen, The Netherlands b c
a r t i c l e
i n f o
Article history: Received 1 December 2010 Received in revised form 22 April 2011 Accepted 26 April 2011 Available online 16 June 2011 Keywords: Black-tailed godwit Farmland birds Habitat selection Lapwing Sward Wet grassland
a b s t r a c t Effectiveness of European initiatives to restore populations of meadow breeding waders is heavily debated. We studied field preference of meadow birds throughout the breeding season in four areas of over 100 ha each and related observed patterns of individual birds to in-field heterogeneity, sward height and management. Over the four areas, most waders were observed in the more heterogeneous fields at both the period of nest site selection and incubation. Additionally, fields grazed at relatively low-intensity for longer consecutive periods (on average 6 cows/ha for 30 d instead of 20 cows/ha for 2 d) were hosting high densities of lapwings but also black-tailed godwits. Our results suggest that in-field heterogeneity may be important for meadow breeding waders at the nest site selection and incubation stages. Conservation initiatives aimed at meadow breeding waders might improve their effectiveness when they increase the heterogeneity of fields. Grazing for longer consecutive periods at relatively low stocking rates might be a way to achieve this, if carried out at stocking rates low enough to allow waders to reproduce successfully. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Agricultural intensification over the last 50 years has resulted in substantial changes in Western European farming practices (e.g. Donald et al., 2002). For wet grasslands, this meant increases in stocking levels and fertiliser applications and the frequent reseeding of the fields (e.g. Beintema et al., 1997). Improvements in field drainage allowed farmers to access their fields earlier in spring and thus to advance their activities, and to replace hay crops with silage crops (Beintema et al., 1985; Vickery et al., 2001). At the field scale, these factors greatly reduced variation in micro-topographical features of grasslands and resulted in more homogeneous and denser swards (Vickery et al., 2001; Wilson et al., 2005). However, agricultural intensification also reduced farmland heterogeneity at the larger spatial scales (Benton et al., 2003). These changes in agricultural practices and reductions in farmland heterogeneity coincided with large declines in European farmland bird populations (e.g. Siriwardena et al., 1998; Donald et al., 2001), including grassland breeding waders (e.g. BirdLife International, 2004).
∗ Corresponding author. Tel.: +31 6 44140931; fax: +31 527 245501. E-mail address: [email protected] (J. Verhulst). 0167-8809/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2011.04.016
In the Netherlands, agri-environment schemes were designed to halt declines in meadow breeding waders. The Netherlands have an international responsibility for this species group, as the country harbours over 40% and 30% of the European breeding populations of black-tailed godwits Limosa limosa and oystercatchers Haematopus ostralegus (BirdLife International, 2004). The key tool was, and is, to postpone mowing or grazing of grassland to allow birds to safely hatch their eggs (Beintema and Müskens, 1987). Additionally, Schekkerman and Müskens (2000) and Schekkerman and Beintema (2007) found black-tailed godwits with chicks to select fields with tall swards. Fields with the postponed mowing scheme however, do not have higher settlement densities of waders (Kleijn et al., 2001) and neither do additional schemes implemented at larger scales (Verhulst et al., 2007). The most recent initiative designed to maintain especially the black-tailed godwit populations on farmland is the so-called ‘mosaic management’. It aims to provide at least one ha with tall swards for food and shelter per black-tailed godwit family and to create a spatial mixture of differently managed fields at the polder scale (200–400 ha). However, in a large scale study, the breeding success of black-tailed godwits was found not to differ from control areas without these measures and reproductive success was insufficient to maintain stable populations (Schekkerman et al., 2008). Consequently, despite the fact that over 15% of all Dutch grassland is managed under some sort of meadow bird scheme, meadow birds are still declining rapidly both in range
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and in population size (SOVON Vogelonderzoek Nederland, 2002; Teunissen and Soldaat, 2006; Landschapsbeheer Nederland, 2007). Kleijn et al. (2004) hypothesize that the reductions in farming intensity on postponed mowing scheme fields are not sufficient to deliver benefits to breeding waders. Schekkerman et al. (2008) suggest that the poor variation in vegetation structure of (late mowed) grassland might be responsible for the observed low reproductive success of black-tailed godwits. Preferences of different species of farmland breeding waders concerning sward structure have been determined in several studies but most of these studies were carried out in natural habitats (coastal marshes) or mixed farmland and few have done so in intensively farmed wet meadows (e.g. Galbraith, 1989; Berg, 1992; Norris et al., 1997; Johansson, 2001, but see Schekkerman et al., 1998). This study aims to explore the effects of field characteristics on meadow breeding waders in the Netherlands. We specifically test the effects of management type, sward height and heterogeneity on the physical position of individual black-tailed godwits, lapwings Vanellus vanellus, redshanks Tringa totanus and oystercatchers throughout the breeding season in four farmed areas of over 100 ha each. Each area contained fields that differed in heterogeneity, ranging from uniform fields that were recently reseeded to fields that contained many flowering forbs or areas with retarded grass growth. 2. Methods In 2005, meadow breeding waders were surveyed in four areas in the Eempolders (N52◦ 15 , E5◦ 19 ) in the centre of the Netherlands. Each study area covered over 100 ha. To make sure that at least some fields with tall swards were to be present during the entire breeding season, each study area contained several fields under the postponed mowing scheme. 2.1. Surveys From the end of March until mid-June, the positions of the four meadow breeding waders were mapped at least two days per week from roads intersecting the study areas using telescopes and binoculars. Flocks of perceived non-breeding black-tailed godwits and lapwings that occurred from mid-May onwards were excluded from analyses. On a survey day, a study area was usually surveyed twice but at the end of the breeding season three surveys could be completed because of the low number of birds. On average, each study area was mapped 54 times on 26 days divided over the breeding season. Nest sites were not searched for. Four times during the breeding season (end of March, mid April, mid May and beginning of June) the heterogeneity of each field was estimated. Five classes were distinguished, ranging from low heterogeneity (I) to high (V). This classification was based on the cover of heterogeneity features in the field, such as shallow pools, tussocks (i.e. small patches of taller vegetation), molehills, flowering forbs (predominantly Ranunculus, Rumex or Taraxacum spp.), grasses other than Lolium perenne and a high diversity of sward height (see Table 1 for definition of classes). Fields with signs of recent reseeding (grass clearly growing in rows instead of a continuous sward) got a very low score in this classification (class I). Pools and molehills were only present at the onset of the breeding season (end of March). On each survey day, the sward height of each field was visually estimated to 5-cm classes (0–5, 5–10, 10–15, 15–20, 20–25, >25 cm). When a field had differences in sward height (e.g. tussocks), the height of the vegetation covering more than 50% of the field was recorded. On each survey day we also recorded the type of management, either no management, mowing or grazing (includ-
Table 1 Definition of the heterogeneity classes, with percentage field cover of shallow pools, tussocks, molehills, flowering forbs, divergent grass species (other than Lolium perenne) and a high diversity in sward height over the field as scored heterogeneity features. Class
Definition
I
Highly uniform grassland (e.g. recently reseeded), <1% heterogeneous features Uniform grassland with 1–5% heterogeneous features Grassland with 5–10% heterogeneous features Grassland with 10–25% heterogeneous features Heterogeneous grassland, >25% heterogeneous features
II III IV V
ing the number and type of grazers (cattle, sheep or horses)). Table 2 presents an overview of the characteristics of the four study areas.
2.2. Analyses Prior to analyses, the breeding season was divided in seven 10-day periods. For each of these periods, the average density of individuals of each meadow bird species per field was calculated. Also, the dominant heterogeneity class, management type and sward height class for each field were determined. Several management classes were distinguished. Fields were classified as ‘high-intensity grazing’ when they were subjected to high densities of cattle for a short time (on average density 20 cows/ha for 2 successive days) or ‘low-intensity grazing’ when they had lower densities of cattle for a longer period (on average 6 cows/ha for over 30 successive days). Few fields were grazed by horses or sheep. These generally were small fields with high densities of grazers that were present for over five weeks. Therefore, these fields were classified as ‘high-intensity grazing’, similar to those grazed by high densities of cattle. Fields without grazing or mowing were classified as “unmanaged”. Each management type affects the field characteristics in a different manner (e.g. mowing removes all vegetation at the same height, grazing leaves more or less variation in height). So when a field was mowed in period three, it was classified as ‘recently mowed’ in period four. In period five it would be classified as ‘regrown’, as we expected the effects of the management type to disappear after some time. Summarizing, the management types were: ‘mowing’ and ‘recent mowing’, ‘high-intensity grazing’ and ‘recent high-intensity grazing’, ‘lowintensity grazing’ and ‘recent low-intensity grazing’, ‘unmanaged’ and ‘regrown’. Generalized Linear Models with a Poisson distribution and a log-link function were applied (Type III), to determine whether meadow birds showed preferences for fields with specific characteristics. Models included study area, field size (as covariate, continuous variables), management type, heterogeneity and sward height as explanatory variables (categorical variables). Analyses were carried out for each of the four wader species for the first observation period (25 March–10 April) when lapwings started nesting while black-tailed godwits and redshank were selecting nest sites (Snow and Perrins, 1998) and for the third observation period (21–30 April) when the first lapwing nests had hatched and the other three species were nesting (Snow and Perrins, 1998). After these observation periods, birds were observed to be less territorial, and therefore their distribution became more dependent of good foraging sites for adults, which is of less interest to this study. Management and heterogeneity categories present on less than five fields over all study areas were omitted from analyses. Our explanatory variables were not independent. When a field is mowed or grazed, obviously the sward height is reduced. Therefore, Spearman’s correlation was used to test for correlations between management type, heterogeneity and sward height.
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Table 2 Characteristics of the four different study areas in the nest site selection phase (period 1; 25 March–10 April) and the nesting phase (period 3; 21–30 April). The management types ‘mowing’ and ‘recent mowing’ did not occur during the periods in the table and are therefore not included. The numbers in the columns of ‘Management’, ‘Heterogeneity’ and ‘Sward height’ refer to the number of fields under the given management type, heterogeneity class or sward height. Period
Study area
Total area (ha)
Number of fields
Management Unmanaged
1
3
High-intensity grazing
1 2 3 4
117 114 105 106
56 51 52 61
56 49 50 48
1 2 9
1 2 3 4
117 114 105 106
56 51 52 61
45 43 47 36
4 5 4 17
2
Number of fields
I (low)
II
III
1
1 2 3 4
117 114 105 106
56 51 52 61
25 20 1 16
18 24 17 29
8 4 24 16
5 2 10
1 2 3 4
117 114 105 106
56 51 52 61
33 31
19 18 20 37
2 1 25 7
2 1 4
Density per 100 ha
Heterogeneity
Black-tailed Godwit Lapwing
80
Oystercatcher
60
Redshank
40 20 0 25 March
11 April
21 April
1 May
11 May
21 May
1 June
Fig. 1. Average number of observations per 10-day period of different wader species throughout the breeding season.
3. Results Fig. 1 shows the densities of all four species during the breeding season. Numbers of black-tailed godwits and lapwings declined throughout the breeding season while redshank and oystercatcher remained more or less stable throughout the breeding season. Both at the end of March and in April, black-tailed godwits were predominantly found in the more heterogeneous fields (Fig. 2a). In May, differences between the different heterogeneity classes were smaller. In the first part of the breeding season, unmanaged fields and fields with relatively low-intensity grazing held high densities of black-tailed godwits (Fig. 2b). As the season proceeded, more management classes could be distinguished and differences between classes became smaller. However, fields with low-intensity grazing became more important later in the season. Similar to black-tailed godwits, most lapwings were found on the more heterogeneous fields from the end of March until the beginning of May (Fig. 2c). The highest densities of lapwings were observed on fields with low-intensity grazing throughout the breeding season (Fig. 2d). Surprisingly, fields with high-intensity grazing – which had short swards – had low densities in March and April but gradually became more important.
3 2 1 4
1
Total area (ha)
100
Recent low-intensity grazing
Regrown
4
Study area
17
Lowintensity grazing 1
Period
3
Recent highintensity grazing
4
2
Sward height (cm) IV
V (high) 1
3
0–5
5–10
33 9 37 5
23 41 15 56
1 2 2
1 16 33 13
10–15
15–20
20–25
1
27 34 12 42
28 4 4
1
homogeneous fields, as can be derived from the significant heterogeneity effect in Table 3. Management significantly affected black-tailed godwit densities with the highest densities being observed in unmanaged fields and fields with low-intensity grazing. These categories were significantly preferred over intensively grazed fields (Fig. 2b, Table 3). As hardly any differences in sward height occurred, no differences in black-tailed godwit densities were observed between fields with different sward heights (Table 3). At the end of April when black-tailed godwits started incubating, fields with intermediate heterogeneity were preferred over the homogeneous ones (preference for class III; Table 3). Again, management type no longer significantly affected black-tailed godwit density (Table 3). At the incubation period, black-tailed godwits were found in significantly higher densities on fields with the lower sward heights, i.e. those with swards of 5–10 cm (Table 3). Similar to black-tailed godwits, the more heterogeneous fields held significantly higher densities of lapwings than the homogeneous fields, both in early and late April (Fig. 2c, Table 3). Management type had a significant effect on lapwings with highest densities observed on fields with low-intensity grazing in both the nest site selection and the incubation phases (Fig. 2d, Table 3). During both phases, fields with the shortest swards hosted the highest densities (Table 3). At the end of March, both redshank and oystercatcher showed a significant preference for the more heterogeneous fields while neither of the species showed a significant preference for any of the management types or the sward heights (Table 3). By the end of April, redshanks were found in the significantly highest densities at fields with intermediate heterogeneity (class III). During this period, densities of both species were significantly affected by the type of management. Oystercatchers showed a preference for the lowest sward heights, while redshanks preferred fields with lower sward heights, i.e. those with swards of 5–10 cm (Table 3).
3.1. Effects of explanatory variables on bird densities
3.2. Mutual correlation of explanatory variables
At the nest site selection phase, black-tailed godwits significantly preferred the higher heterogeneity classes over the more
At the end of March, sward height was positively correlated with heterogeneity (Spearman’s rho = 0.145, p < 0.05). By late April,
J. Verhulst et al. / Agriculture, Ecosystems and Environment 142 (2011) 161–166
Heterogeneity class 200
4 3
160
2
120
1
80 40 0 25 March
Lapwing density per 100 ha
c
b
11 April
1 May
Management type
80 60 40 20 0
21 May
25 March
d
200
high-intensity grazing regrown low-intensity grazing unmanaged mowing recent high-intensity grazing
100
11 April
200
3 2
120
1
80 40
1 May
21 May
high-intensity grazing regrown low-intensity grazing unmanaged mowing recent high-intensity grazing
250
4
160
Lapwing density per 100 ha
Black-tailed godwit density per 100 ha
a
Black-tailed godwit density per 100 ha
164
150 100 50
0
0
25 March
11 April
1 May
21 May
25 March
11 April
1 May
21 May
Fig. 2. (a)–(d) Average densities of black-tailed godwits (a, b) and lapwings (c, d) on fields with different heterogeneity classes (a, c) and management types (b, d) over the four study areas throughout the breeding season in number of individuals per 100 ha. The management type ‘recent mowing’ is not depicted because it was only present from 21 May onwards. Bird densities represent the averages of two time periods. Dates in the figure indicate the beginning of the time period (e.g. 11 April: average of periods 11–20 April and 21–30 April).
sward height was both positively correlated with management (Spearman’s rho = 0.165, p < 0.05) and heterogeneity (Spearman’s rho = 0.403, p < 0.01). Heterogeneity and management were not correlated during either of the periods. See Appendix A for the relative covers of the different heterogeneity classes, management types and sward heights over the different observation periods.
4. Discussion Black-tailed godwits selected the more heterogeneous fields in both periods (nest site selection and nesting phases;
Fig. 2a, Table 3). These heterogeneous fields were predominantly unmanaged (data not shown). However, fields with relatively low-intensity grazing in general also were intermediately heterogeneous and contained high densities of black-tailed godwits. Several Dutch studies (e.g. Buker and Groen, 1989; Beintema et al., 1995) found nesting black-tailed godwits to prefer mowed fields over grazed fields while the latter are generally more heterogeneous. The high densities of individual black-tailed godwits on fields with relatively low-intensity grazing in our study seem to contradict those studies. However, Johansson (2001) found that Swedish black-tailed godwits preferably nest in tussocks with low
Table 3 Test statistics of the effects of management type, heterogeneity and sward height on the distribution of meadow birds at the onset of the breeding season (period 1; 25 March–10 April) and when most species were incubating (period 3; 21–30 April). Analyses were carried out on the density per period (thus not divided by field size). Models (GLM, Poisson distribution, log-link function) included study area, field size (covariate), management type, heterogeneity and sward height. Shown are the likelihood ratios. The trends of the overall effects of sward height and heterogeneity are indicated in brackets (not tested in a post hoc way). Period
Black-tailed Godwit
Lapwing
Redshank
Oystercatcher
Study area Field size Management Heterogeneity Height
35.33*** 27.27*** 6.88* 19.58*** (+) 0.92
37.38*** 45.50*** 15.10** 11.33* (+) 8.54** (−)
40.18*** 18.65*** 2.50 15.60** (+) 1.60
40.37*** 4.43* 0.61 8.28* (+) 2.16
Study area Field size Management Heterogeneity Height
8.31 38.2*** 3.33 15.27** , a 6.03* , b
11.31* 46.29*** 47.84*** 13.69** , a 9.91** (−)
29.88*** 14.91*** 16.04*** 18.47*** , a 6.22* , b
6.35 0.59 16.00*** 4.63 6.47* (−)
1
3
a b * ** ***
Intermediate heterogeneity (class III) preferred. Lower sward height (5–10 cm) preferred. Significance level of the main effect: p < 0.05. Significance level of the main effect: p < 0.01. Significance level of the main effect: p < 0.001.
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surrounding vegetation. Tussocks were more common in the more heterogeneous fields. Another option is that the black-tailed godwits in the heterogeneous grazed fields might have been nesting on mowed fields. Finally, comparing our results with previous Dutch studies should be done cautiously, because those studies were based on data collected in the 1980s. Kleijn et al. (2010) found that vegetation growth in spring increased significantly since the 1980s making unmanaged swards less suitable for grassland breeding waders. Grazing opens up the swards, which may explain why in this study black-tailed godwits were also observed in high densities in fields with relatively low-intensity grazing. Similar to black-tailed godwits, the highest densities of lapwings were observed during both periods (nesting and hatching phases) at the more heterogeneous fields (Fig. 2d, Table 3). Lapwings are known to nest in a wide variety of field types, from homogeneous dry to wet tilled fields and to heterogeneous rough grazing grasslands (e.g. Galbraith, 1988; Berg, 1993). Galbraith (1988) and Baines (1990) both found that unimproved pastures were the preferred grassland type. In our study, high lapwing densities were found in fields with relatively low-intensity grazing (low cattle load over longer period). Early in the season, the grazers probably kept swards short and created some small-scale heterogeneity which might have contributed to their attractiveness to lapwings. Fields with high-intensive grazing (high cattle load during 1 or 2 d) however, similarly had reduced sward heights but were avoided. These fields tended to be more homogeneous which suggests that these grazing regimes with (extremely) high stocking densities are too intensive for lapwings. Redshanks were observed to significantly select heterogeneous fields over the more homogeneous fields by early and late April (Table 3). Norris et al. (1997) found this species to preferably nest in structure-rich swards in coastal grazing marshes. Even though oystercatchers occurred in low densities throughout the breeding season (Fig. 1), their density in the heterogeneous fields significantly exceeded those in the homogeneous fields by both the beginning and the end of April (Table 3). Oystercatchers usually start nesting later than the other wader species (Beintema et al., 1995; Snow and Perrins, 1998), so probably the end of April coincided with their nest site selection phase. The explanatory variables in our study were not independent of each other. Sward height correlated in both periods with heterogeneity, and by the end of April it also was significantly correlated with management. As heterogeneity and management were not correlated to each other during either of the periods, we focused at these two variables. 4.1. Implications for management Several recent publications have stressed the importance of infield heterogeneity for farmland birds (e.g. Vickery et al., 2001; Benton et al., 2003; McCracken and Tallowin, 2004; Wilson et al., 2005; Kleijn et al., 2010). Heterogeneous fields are likely to provide optimal foraging opportunities for meadow birds (chicks), because the variation in vegetation structures both increases the range of potential prey species in these swards and ensures their availability (e.g. Morris, 2000; Vickery et al., 2001; McCracken and Tallowin, 2004). In our study, fields with relatively low-intensity grazing – grazed at longer consecutive periods with lower cattle densities – were much more heterogeneous than those with conventional high-intensity grazing regimes (with 80 cattle in fields of 2 ha for 2 d). Also, these fields with relatively low-intensity grazing contained high densities of lapwings but also of black-tailed godwits. We draw two main conclusions from our results. First, stocking densities of grazers have rarely been distinguished in recent Dutch meadow bird studies. In most studies relating breeding waders and grazing regimes, stocking rates did not exceed 1 cow/ha. However,
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we observed on average 20 cows/ha on the intensively grazed fields. The low-intensity grazing fields in this study usually had 6 cows/ha, so the risk of trampling is very high for both grazing categories. Maximum stocking rates in nearby meadow bird reserves supporting stable wader populations are 1.5 livestock unit/ha. However, our results suggest that distinguishing between high and lowintensity grazing may be important for breeding waders. Second, our results suggest that initiatives aimed at breeding waders might improve their effectiveness when incorporating infield heterogeneity. We observed higher densities of all four species on the more heterogeneous fields. Grazing fields for longer consecutive periods at relatively low-intensity might be an effective way to improve the in-field heterogeneity, as shown by our results. An alternative way to improve field heterogeneity is raising groundwater tables. Also, reseeding (parts of) fields with slowly growing grass species might be an option. With the latter two methods, more open swards are created which are attractive to both adult and juvenile meadow birds (see Kleijn et al., 2010). Both options will more seriously impact agricultural practices than low-intensity grazing, and would require high financial compensation for land managers. However, schemes that include rewetting whole fields have been applied in several European countries (see Ausden and Hirons, 2002; Vickery et al., 2004; Kahlert et al., 2007), and found to be cost-effective (Ausden and Hirons, 2002; Wilson et al., 2007). Acknowledgements We thank Bas van de Meulengraaf and Idde Lijnse for assistance with the field work. Comments of Jinze Noordijk improved the manuscript. This work was funded by the EU Project QLK5CT-2002-1495 Evaluating current European Agri-environment Schemes to quantify and improve Nature Conservation efforts in agricultural landscapes (EASY). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.agee.2011.04.016. References Ausden, M., Hirons, G.J.M., 2002. Grassland nature reserves for breeding wading birds in England and the implications for the ESA agri-environment scheme. Biol. Conserv. 106, 279–291. Baines, D., 1990. The roles of predation, food and agricultural practice in determining the breeding success of the lapwing (Vanellus vanellus) on upland grasslands. J. Anim. Ecol. 59, 915–929. Beintema, A.J., Beintema-Hietbrink, R.J., Müskens, G.J.D.M., 1985. A shift in the timing of breeding of meadow birds. Ardea 73, 83–89. Beintema, A.J., Dunn, E., Stroud, D.A., 1997. Birds and wet grasslands. In: Pain, D.J., Pienkowski, M.W. (Eds.), Farming and Birds in Europe. Academic Press, London, pp. 269–296. Beintema, A.J., Moedt, O., Ellinger, D., 1995. Ecologische Atlas van de Nederlandse Weidevogels. Schuyt & Co., Haarlem. Beintema, A.J., Müskens, G.J.D.M., 1987. Nesting success of birds breeding in Dutch agricultural grasslands. J. Appl. Ecol. 24, 743–758. Benton, T.G., Vickery, J.A., Wilson, J.D., 2003. Farmland biodiversity: is habitat heterogeneity the key? Trends Ecol. Evol. 18, 182–188. Berg, Å., 1992. Factors affecting nest-site choice and reproductive success of Curlews Numenius arquata on farmland. Ibis 134, 44–51. Berg, Å., 1993. Habitat selection by monogamous and polygamous lapwings on farmland—the importance of foraging habitats and suitable nest sites. Ardea 81, 99–105. BirdLife International, 2004. Birds in Europe: Populations Estimates, Trends and Conservation Status. Birdlife International, Cambridge. Buker, J.B., Groen, N.M., 1989. Verspreiding van Grutto’s Limosa limosa over verschillende typen grasland in het broedseizoen. Limosa 62, 183–190. Donald, P.F., Green, R.E., Heath, M.F., 2001. Agricultural intensification and the collapse of Europe’s farmland bird populations. Proc. R. Soc. Lond. B 268, 25–29. Donald, P.F., Pisano, G., Rayment, M.D., Pain, D.J., 2002. The Common Agricultural Policy. EU enlargement and the conservation of Europe’s farmland birds. Agric. Ecosyst. Environ. 89, 167–182.
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