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In 1995, the UK Government designated the Grey Partridge Perdix perdix as a priority species under its Biodiversity Action Plan (BAP). The Game Conservancy Trust (GCT), nominated as lead partner for the species, launched a programme to help Grey Partridge recovery by (1) raising awareness among the shooting and farming communities, (2) encouraging land managers and local BAP groups to conserve Partridges by setting numerical targets and offering feedback from counts on how to increase numbers, and (3) motivating by example by setting up a site demonstrating management techniques and the increase in Partridge abundance that ensues. Building on past research, we quantify the amount of habitat management required to achieve the BAP targets of halting the decline and achieving population recovery. Using three major GCT databases, (1) the National Gamebag Census, which collates data on shooting bags and gamebird releasing in the UK, (2) the Partridge Count Scheme, which monitors national Partridge density and reproduction, and (3) the Sussex Study, which monitors local Partridge population dynamics over 37 years, we emphasize the importance of good breeding success for increasing densities, review the evidence that management can be successful and evaluate the impact of shooting on Grey Partridge conservation.
A hundred years ago the Grey Partridge Perdix perdix was common in the UK, with around two million birds shot annually between 1870 and 1930 (Tapper 1992). After the Second World War, the Grey Partridge bag declined by over 80% between 1950 and 1990 (Aebischer 1997). More recently, the Common Birds Census of the British Trust for Ornithology (BTO) showed that Grey Partridge abundance in the UK has continued to decline (Baillie et al. 2002, Gregory et al. 2004), dropping by 84% from 1974 to 1999. In 1995, the UK Government designated the Grey Partridge as a priority species under its Biodiversity Action Plan (BAP). It defined three targets for recovery (Anon. 1995): halt the decline by 2005, ensure that the population is above 150 000 pairs by 2010, and maintain and where possible enhance the current range.
Much of what is known about the ecology and population dynamics of the Grey Partridge in the UK stems from The Game Conservancy Trust (GCT)'s ‘Sussex Study’ (Potts 1980, 1986). Potts identified three main causes for the decline: a herbicide-induced fall in cereal invertebrate abundance leading to reduced chick survival, increased predation during nesting leading to lower brood production, and the disappearance of nesting cover as field boundaries were removed to improve farming efficiency.
As Grey Partridges declined, many estates switched from managing wild Partridge stocks to shoots based on releasing artificially reared Pheasants Phasianus colchicus and Red-legged Partridges Alectoris rufa (Potts 1986, Tapper 1992). Others abandoned shooting altogether in favour of intensive farming. The deterioration of farm incomes since 1995 has driven farms to diversify, leading to intensification of shoot management based on released gamebirds. This has led to concern that shooting is damaging the remaining stocks of wild Grey Partridges. The issue is not new: for many years the GCT has warned its members of the dangers of overshooting wild Grey Partridges when releasing Red-legged Partridges (e.g. Potts 1978). Typically, a small Grey Partridge bag is interpreted as evidence of low shooting pressure. However, when densities of wild Grey Partridges are low, a small number shot can represent an unsustainably high proportion of the stock.
Against this background, in 1996 the UK Government nominated the GCT as lead partner to meet the BAP conservation targets for Grey Partridge. Since then, the GCT has launched a major programme for Partridge recovery (Aebischer in press), with three main strands: raising awareness, setting targets and motivating by example.
To raise awareness, the GCT mounted a major information campaign centred around a new Grey Partridge conservation leaflet (Tapper 2001
), which describes management appropriate for the nesting, chick-rearing and overwinter periods. It alerts shoots and shooters to the dangers of over-shooting wild Grey Partridges, and underlines the need for precautionary measures. In particular, it recommends not shooting Grey Partridges unless their autumn density exceeds 20/km2
. This is the density required for sustainably maintaining stocks at 4.5 pairs/km2
, the average predicted from the Potts (1980
) simulation model for unmanaged land (Aebischer 1991
) assuming natural annual losses to be 55% (Potts 1986
The GCT also expanded its Partridge Count Scheme (see Methods
), using counts by farmers and landowners to provide them with feedback on management needed to increase Grey Partridge numbers, while simultaneously monitoring progress towards the BAP targets;
To set targets for realistic Grey Partridge densities, the GCT classified 1 × 1-km squares across Great Britain as optimum, suboptimum or unsuitable on the basis of their land-cover characteristics (Tapper 1999
). The approach gave a reasonable approximation to the bird's post-War distribution (Gibbons et al. 1993
). Under modern agriculture, around 4 pairs/km2
would be expected on optimum ground and 2 pairs/km2
on suboptimum ground, with higher densities expected if suitable management is undertaken (Potts 1986, Aebischer 1991
). The approach has been used to estimate target numbers of Grey Partridges at the national, regional, county and estate levels (Aebischer in press
To motivate by example, in 2001 the GCT set up a Grey Partridge Restoration Project (see Methods
), so that visitors may see for themselves the practicalities of management needed to restore Partridge numbers.
This paper brings together information on Grey Partridge trends and breeding success. Building on past research, it attempts to quantify the amount and nature of habitat management needed to achieve the BAP targets, and compares the result against recent observations from GCT datasets. The BAP target of 150 000 pairs by 2010 is the number estimated from the 1988–91 Atlas survey (Gibbons et al. 1993), so in our assessment of trends we considered data from 1990 (roughly the survey midpoint) to the present day. The paper also brings together what information is available on the national extent and impact of shooting on Grey Partridges. The overall aim is to provide an assessment of progress under the BAP, with a view to informing and guiding progress in the future.
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The causal factors underlying the decline of Grey Partridges in the UK are well understood, as is the management needed to reverse the situation (Potts 1986, Aebischer et al. 2002). The ultimate factor behind the decline is the deterioration of the bird's agricultural habitat. Potts (1980, 1986) identified chick survival rate as the key factor (sensuVarley & Gradwell 1963) determining population change in the Sussex Study. We found that the Sussex population parameters reproduced the national rate of change from the CBC since 1990, suggesting that Sussex was typical of the national picture. The importance of chick survival rate at the national level was confirmed by Figure 1, which shows a close linkage between chick survival rate estimated from the PCS and population change measured by the CBC. Chick survival rate is independent of Partridge density, and depends crucially on the availability of invertebrate food (Potts 1980, 1986). To be successful, management must above all ensure a ready supply of chick food through the restoration of brood-rearing habitat.
How much brood-rearing habitat is required to aid Partridge recovery? We estimated that stabilizing the population required 3% of arable area to serve as brood-rearing cover. The BAP target of population recovery to 150 000 pairs requires increases in both brood-rearing and nesting habitat, for instance 5% of arable land as brood-rearing habitat and 6.9 km/km2 of nesting habitat. The latter is an increase of 2.9 km/km2 over what is typical. It requires 5.8 ha/km2 of 20-m set-aside strips, for example, which could be managed as adjacent bands of cereal mixtures for brood-rearing and tussocky grass for nesting, distributed over the farm. Such management is permitted under existing prescriptions and the quantities are just feasible (Oxford data: 11% set-aside on 54% arable gives 5.9 ha/km2 of set-aside). It would, however, need to be done UK-wide, which is unrealistic. The new Entry Level Agri-environment Scheme, due to be rolled out early in 2005 and piloted in four areas of England (Anon, 2003, Bradbury et al. 2004), will be open to all farmers following the recommendation of Curry (2002). By providing financial incentives to farmers for environmentally friendly management, this could complement set-aside and provide a major boost to national Partridge habitat availability.
The above estimates relate to a situation without predation control, typical of the modern agricultural environment. Nesting losses limit the potential for increase, which is why population response to habitat management is much faster with predation control (cf. Fig. 2). For financial and also ethical reasons, the practice is unlikely to become widespread despite its effectiveness. If predators were controlled on 10% of Partridge ground, the extra nesting habitat requirement for recovery would be reduced by 28%, from 2.9 to 2.1 km/km2.
The Grey Partridge Restoration Project offers quantitative corroboration of the modelling outputs described above. There are practical problems of interpretation, because certain habitat types, such as uncropped wildlife strips, grass margins and natural regeneration set-aside, can play a dual or partial role, and set-aside may be used for nesting even though it does not enter into measurements of km/km2 of hedgerow. Nevertheless, the habitat in place approximates to 6% of arable area as brood-rearing cover and 16% as nesting cover, which satisfies the requirements calculated above. It has resulted in chick survival rates averaging 54% and, in conjunction with predation control, a change in density of +176% over 2 years, in line with the data in Figure 1.
Even in the absence of a quantitative assessment, the qualitative assessment of management vs. no management in Norfolk provides ongoing evidence that the ‘Grey Partridge management package’ (Aebischer 1997, Tapper 2001) works in the long as well as in the short term, with densities on the managed estates at least four times higher than on the unmanaged estates. It is still too early to tell whether the encouragement offered under the relaunched PCS has resulted in improved Partridge densities nationally. The BTO's Breeding Bird Survey showed no significant change in national Grey Partridge numbers for 2000–02 (Raven et al. 2003), nor does the PCS. Newly recruited sites have, however, demonstrated an improvement in breeding success relative to long-term contributors. This indicates perhaps that new recruits have indeed been sufficiently motivated to carry out some management. Although breeding success has been measured as young:old ratio rather than as chick survival, it would be surprising if the young:old ratio improved without a positive change in chick survival rates, which should eventually feed through into higher densities.
To attain 150 000 pairs by 2010, the UK Grey Partridge population needs to grow by 10% annually from 2002 – is this a realistic prospect? From 2002 to 2003, Partridge numbers on PCS sites increased by around 20% and on the Norfolk estates by about 40% (Figs 3 and 4). This demonstrates that 10% growth is feasible, and if representative of the national picture, it holds great promise.
As regards the impact of shooting, the Sussex Study provided field evidence that over-shooting can have disastrous effects on wild Grey Partridges when they are caught up in drives of released Red-legged Partridges and shot inadvertently. At the same time, the Sussex Study also holds out considerable hope. Once the shoot managers realized the damage caused by the shoot to the wild Grey Partridges, they took measures (Tapper 2001) that reduced the average loss rate from 64% to 16%.
How common is such over-shooting in the wider UK countryside? NGC data showed that on ‘wild’ estates (no gamebird releasing), the annual rate of change in Grey Partridge bag density since 1990 was remarkably similar to the national BTO trend. It was twice as high on estates releasing Pheasants only, but not detectably different on estates that released Pheasants and Red-legged Partridges. Nationally therefore there is no evidence to suggest that shoots based on released Red-legged Partridges have an impact on Grey Partridges that goes beyond that of agricultural intensification. Clearly that does not hold for shoots that release Pheasants, but is the mechanism the same as in Sussex? Whereas Grey and Red-legged Partridges share the same habitats, Pheasants are associated much more with woodlands, as is their releasing and shooting (Hill & Robertson 1988). Heavily wooded land is suboptimal for Grey Partridges (Tapper 1999). Moreover, Robertson (1991) describes how a sporting interest in Pheasants is a major motivator for the planting of new woodlands. It seems likely that the increased decline rate of Grey Partridges on Pheasant estates results from the nature of land management rather than from shooting mortality. The PCS provides further evidence to show that the over-shooting observed in Sussex is not widely prevalent. At the low level of Grey Partridge density that is of concern, three-quarters of sites did not shoot Grey Partridges. In terms of area, these sites were not ones too small to run a shoot as their accumulated area amounted again to around three-quarters of the total.
That over-shooting can lead to extinction comes as no surprise, as the stock–yield relationships in Potts (1986) demonstrate. Potts (1986) found from modelling that at low densities, Grey Partridges could not tolerate more than 40% shooting losses without going extinct. Compared with no shooting losses, the equilibrium density halved with 20% shooting losses. Nevertheless, at this shooting pressure, Aebischer (1991) showed that the impact of habitat deterioration had much worse consequences in the long term than the shooting. With habitat improvement rather than deterioration, it was paradoxically possible to have a higher Partridge density with shooting than on deteriorated ground without shooting. On this basis, the average offtake rate of 11% recorded on PCS sites that shot Grey Partridges was sustainable, as was the level of 16% achieved in Sussex with precautionary measures.
In summary, the main issue of concern for the Grey Partridge is still to redress the deterioration that has taken place in its farmland habitat. There must be no relaxation in the ongoing information campaign to farmers, land owners and shoot managers, who are the people best placed to help restore the fortunes of the Grey Partridge. Although it is still at an early stage, the different strands of the GCT recovery programme are starting to produce results that are encouraging. The recovery of the Grey Partridge should be boosted considerably when the Entry-Level Agri-Environment Scheme is rolled out. We believe that the future of the Grey Partridge is looking better now than it has done for many years.