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- Discussion and conclusions
1. There is increasing evidence to link major declines in skylark populations in Britain to agricultural intensification. However, whether causal mechanisms identified through localized studies can be generalized to the national scale remains unknown. The abundance of breeding skylarks was determined by surveying singing males in over 600 randomly selected 1-km squares throughout Britain, in which skylarks recorded were assigned to homogeneous habitat patches. A more intensive survey of skylarks was carried out on lowland farmland sites in England. Singing males were assigned to specific crop types, and data on crop height and field boundary features were recorded.
2. Skylark occupancy (presence/absence) and density where birds were present (i.e. omitting zero counts) were analysed in relation to habitat type, habitat diversity and time of year, using generalized linear modelling.
3. Set-aside, moorland and winter cereals had high rates of skylark occupancy at the national scale. Set-aside had consistently high rates of occupancy and high densities across the breeding season at different spatial scales. Apart from set-aside, there was little difference in density between habitats in the early half (March to mid-May) of the breeding season. In the later half of the breeding season (mid-May to July), density declined significantly on winter cereals, which showed significantly lower density than a number of habitats at this time, including spring cereals, legumes and moorland.
4. Within lowland farmland, there were significant effects of crop height on skylark occupancy, with crops of greater than 30 cm in height being occupied at relatively low rates. Winter cereals reached this height significantly earlier in the breeding season than a number of other crops, including spring cereals and legumes.
5. Skylark density increased with increasing habitat diversity across the whole sample of 1-km squares and in lowland 1-km squares in England. However, within the lowland farmland plots in England, skylark density showed a significant decrease with increasing habitat diversity. These conflicting results suggest that crop type rather than habitat diversity per se is important.
6. The effects of vegetation height on skylark abundance support the hypothesis that increases in winter cereal, and simultaneous loss of spring cereal, have had an adverse effect on skylark populations by reducing the number of breeding attempts made per year. These results support findings from smaller scale studies showing the generality of these habitat effects at different spatial scales. The extent of the British skylark population associated with agricultural land suggests that sympathetic changes in farming practice are likely to provide the best mechanism for improving the status of this species. The inclusion of options, such as spring cereal or fallow land (an equivalent to set-aside), in agri-environment schemes is likely to benefit skylarks breeding on farmland by providing suitable nesting habitat throughout the breeding season. In addition, reductions in the intensity with which cereals are managed, such as reduced pesticide and fertilizer input under approaches such as precision farming, and the creation of sparser patches of cereal sward, are also likely to increase the suitability of winter cereals for nesting skylarks.
- Top of page
- Discussion and conclusions
The skylark Alauda arvensis L. occurs commonly in most open habitats throughout northern Europe (Cramp 1988). Vegetation structure appears to be an important determinant of habitat preference; consequently, skylarks are sensitive to land management practices such as changes in crop type and grazing regime. Skylarks will not breed in particularly tall or dense vegetation, the optimum height range for nesting in arable crops being 20–60 cm (Wilson et al. 1997). Seasonal changes in the height of crops appear to cause shifts in nesting habitat during the course of the breeding season (Schläpfer 1988), and consequently skylarks seem to benefit from a mosaic of different crop types (Jenny 1990; Chamberlain & Gregory 1999). Skylarks will avoid areas with a high density of hedgerows and trees (Wilson et al. 1997) and intensively grazed areas (Wakeham-Dawson et al. 1998). Set-aside, where land is left fallow, is particularly preferred, supporting high breeding densities (Henderson, Cooper & Fuller 1998) and tending to have higher reproductive success than other crops (Poulsen, Sotherton & Aebischer 1998).
Skylark populations have declined in northern Europe over the past three decades (Tucker & Heath 1994; Fuller et al. 1995). While there is evidence of declines in a number of habitats in the UK, the farmland population has shown the steepest decline (Chamberlain & Crick 1999). Changes in agricultural management have been identified as the most probable cause (Fuller et al. 1995). Sowing regimes have changed, with winter cereals replacing spring-sown cereals as the predominant arable crop during the 1970s (Grigg 1989). The sward structure of winter cereals is too dense for skylarks, particularly late in the breeding season (Wilson et al. 1997). Other changes in farm management, including the increase of other unsuitable crops such as oilseed rape, the increased frequency of mowing silage grass, increases in grazing pressure (Fuller & Gough 1999) and a general decrease in habitat diversity within farmland (O’Connor & Shrubb 1986), may mean that there are fewer suitable alternative habitats. Consequently, breeding may be curtailed relatively early in the breeding season, leading to fewer breeding attempts and reduced reproductive output per season. A further potential impact of the increase in winter cereals is that cereal stubbles, an important feeding habitat outside the breeding season (Wilson, Taylor & Muirhead 1996), have been greatly reduced since the early 1970s. This may have had particular effects on seed-eating passerines, including the skylark which is largely granivorous in winter. Skylark reproductive success may also have been affected by increases in pesticide use (Campbell et al. 1997). However, there is no evidence to suggest that changes in reproductive success per nesting attempt underlie the population decline. Indeed, at a national scale, reproductive success of individual nesting attempts has increased over the past 30 years (Chamberlain & Crick 1999). Therefore, reductions in the number of nesting attempts per breeding season or changes in survival outside the breeding season seem to be the more probable potential mechanisms driving the population change.
Although skylark populations on farmland appear to have undergone the steepest declines, there is also evidence that upland populations are declining (Hancock & Avery 1998). The pattern of decline in this habitat is different from that in farmland and appears to have happened somewhat later (Chamberlain & Crick 1999), implying a different cause. There have been a number of changes in upland habitats that may have affected skylark populations adversely, including increasing grazing pressure (Fuller & Gough 1999), changes in moorland management and afforestation (Hancock & Avery 1998). As upland birds tend to move to lowlands in the winter, there is a possibility that agricultural changes are having consequences for upland populations as well. However, skylarks in uplands remain little studied and relatively little is known about habitat associations within upland landscapes.
Habitat associations of skylarks have previously been described at national (Chamberlain & Gregory 1999) and local (Schläpfer 1988; Jenny 1990; Wilson et al. 1997; Poulsen, Sotherton & Aebischer 1998; Wakeham-Dawson et al. 1998) scales. The national-scale study analysed skylark density in relation to broad classes of habitat in a large number of randomly selected 1-km squares throughout the UK, where sections of predominant habitat type were classified according to the methods of Crick (1992). Local-scale intensive studies have considered habitat associations and nesting success in relation to specific crop types, vegetation structure and other management features, but the intensity of these studies necessitated coverage of only a small number of study sites, and the extent to which results can be generalized throughout the UK remains unknown.
In this study, we use two data sets to provide the most detailed and extensive assessment of habitat use by skylarks throughout Britain to date. Most previous studies have focused on lowland farmland. This study gives data that are representative of the whole of Britain, allowing habitat associations on lowland farmland to be placed in a national context. Two separate surveys were carried out. The first, referred to as the extensive survey, was carried out at the level of the 1-km square and was designed to cover a wide range of habitats throughout Britain. Squares were randomly selected within broad-scale landscape types and the data were analysed to assess broad-scale habitat associations, and the generalities of these relationships at national and regional scales. The survey design permitted a more accurate assessment of habitat use than was possible with the data used by Chamberlain & Gregory (1999), because of more precise habitat data collection. The second survey, referred to as the intensive survey, was carried out on a relatively small number of lowland farmland study plots of variable size, and involved collecting more detailed data, both on bird distributions and habitat structure, including crop height and field boundary structure and size. Determination of detailed habitat associations in this survey enabled an assessment of the generalities of habitat relationships determined from previous farm-level studies with relatively small sample size, particularly that of Wilson et al. (1997). It is hoped that ultimately the findings of this paper will help to contribute to future conservation strategies for skylarks in Britain over a range of habitats and spatial scales.