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1. Over the past three decades changes in agricultural management have resulted in increased crop and grass production. This intensification has been accompanied by population declines among farmland bird species and a decline in farmland biodiversity. We have analysed trends in agricultural management in order to quantify the degree of intensification, and have considered how they match change in the farmland bird community.
2. Changes in agriculture through time (1962–95) were examined quantitatively for 31 variables representing crop areas, livestock numbers, fertilizer application, grass production and pesticide use. The majority were highly intercorrelated because factors facilitating intensification simultaneously affected many management activities.
3. Change in agriculture was measured using detrended correspondence analysis (DCA). The period 1970–88 saw most intensification, characterized by increases in the area of oilseed rape, autumn-sown cereals, and the use of pesticides and inorganic fertilizers. Spring-sown cereals, bare fallow and root crops declined.
4. Indices of relative population change between 1962 and 1996 were determined for 29 bird species using data from Common Birds Census (CBC) plots on farmland in England and Wales. Principal components analysis (PCA) described a gradient from species that had declined most to those that had increased.
5. The ordinations of agricultural change and bird population change were broadly matching but with a time lag in the response of birds. The most accurately measured agricultural variables for the period 1974–91 matched the changes in farmland birds more closely.
6. We conclude that large shifts in agricultural management are a plausible explanation for the declines in farmland bird populations. We propose a threshold model relating to critical amounts of high-quality habitat or food resources that may be relevant in explaining the lag in response of birds, and propose it should be taken into account in predicting the effects of future agri-environment schemes. Identifying individual factors responsible for bird declines is not possible without detailed experimental work because many components of intensification are interdependent. Birds may be responding to a suite of interacting factors rather individual aspects of farm management. Holistic conservation strategy that encourages general extensification of farming practices will be most likely to benefit farmland bird communities.
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There have been great changes in the management of farmland in England and Wales over the last few decades. These changes have affected all aspects of the farmed landscape. Crop management and the type and relative abundance of different crops has changed markedly; grassland management has shifted away from hay to silage systems; chemical inputs on farmland have increased substantially; the timing of farming operations has changed; non-crop habitats such as hedgerows and farm ponds have been reduced; and there has been a reduction in the diversity of different types of agriculture per individual farm, with farms tending to specialize in either arable or livestock (O'Connor & Shrubb 1986; Grigg 1989; Stoate 1996). This ‘intensification’ of farmland has led to greatly increased yields. For example, wheat yields increased by 66% between 1970 and 1990 [Ministry of Agriculture, Fisheries and Food (MAFF), unpublished data]. Other crops and grass have shown similar increases in yield over the same period. Developments in farming technology in terms of machinery, development of new crop strains and development of pesticides and fertilizers, have probably been the main underlying causes behind the increased yields, although both social and economic factors have also contributed (Grigg 1989).
The coincidence of this period of intensification of farm management and the decline of many farmland bird species has led to suggestions of a causal link between the two (Fuller et al. 1995). The temporal relationship between changes in farming practices and changes in bird populations has not, however, been examined previously in a quantitative manner for the majority of farmland species. The proposed mechanisms by which agricultural changes in management have affected bird populations are diverse, but generally concern diminished food supplies (Potts 1986; Campbell et al. 1997; Evans et al. 1997; Brickle et al. 2000), less suitable nesting habitat (Wilson et al. 1997; Chamberlain et al. 1999) or direct mortality of birds by farming operations (Crick et al. 1994; Green 1995). Several studies indicate that there is an association between agricultural management and changes in bird populations. A number of these studies have considered differences in regional and habitat-specific population trends. For example, population declines of skylarks Alauda arvensis L. have been steepest in agricultural compared with upland and coastal landscapes (Chamberlain & Crick 1999), granivorous birds have declined most in arable farmland (Marchant & Gregory 1994), but corvids have increased the most on pastoral and mixed farmland (Gregory & Marchant 1996). Also, Donald (1997) found change in the relative population size of corn buntings Miliaria calandra L. to be correlated significantly with annual variation in a number of agricultural variables which themselves were often intercorrelated.
Other potential causes of the population declines have been proposed, for example disease, climate and predation (Fuller et al. 1995). There is no evidence for disease causing long-term bird population change in the UK. Climate, however, has been shown to cause short-term changes in bird population size. For example, Baillie (1990) found that much variation in relative population change of a resident passerine, the song thrush Turdus philomelos L., could be explained by the number of freezing days in the preceding winter, although this effect was not sufficient to explain the most recent declines.
Although the timing of the onset of population declines of a number of bird species has been examined in detail (Siriwardena et al. 1998), there has been no comparable examination of the detailed changes in agricultural practices. In this paper, we present a review of the main changes in agricultural management that have occurred over the past four decades. We examine the evidence that changes in bird abundance have coincided with changes in agricultural management by using ordination techniques to identify patterns of change in both bird abundance and agricultural management data. These analyses demonstrate the pattern of temporal association between bird population change and agricultural intensification.