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Agricultural intensification has been a major factor in the decline of farmland birds throughout Europe (O’Connor & Shrubb 1986; Marchant et al. 1990; Tucker & Heath 1994; Newton 2004) and the decline of the grey partridge Perdix perdix L. in the United Kingdom was one of the first to implicate it (Potts 1980). However, grey partridge is also affected by nest predation and shooting, which have been quantified, and by raptor predation, which has not (Potts 1980, 1986). Since Potts’ diagnosis of the causes of partridge decline, the abundance of sparrowhawks Accipiter nisus L. and common buzzards Buteo buteo L. has increased in the United Kingdom, while the manner in which shooting is conducted has also changed. The grey partridge decline has continued to a point where traditional shooting is not worthwhile (Aebischer 1997), and commercial lowland shooting has turned towards the release of large numbers of pheasants Phasianus colchicus L. and red-legged partridges Alectoris rufa L. Small numbers of grey partridges may also be released but red-legged partridges are much preferred for economic reasons (Tapper 1992). Against falling incomes from farming and forestry, commercial shooting has become an increasingly important land-use in the United Kingdom and management for game, if carried out sensitively, can benefit biodiversity (e.g. Aebischer 1997; Stoate & Szczur 2001). Commercial shooting often results in unintentional density-independent mortality of wild grey partridges because the number of shoot days depends on the number of gamebirds released, irrespective of wild grey partridge density. Considerable effort has been made to impose voluntary restrictions on shooting grey partridges at low densities (Tapper 2001).
A long-term study of the grey partridge in a 62-km2 area of Sussex showed that the observed decline in abundance was caused by reduced fecundity due to the effects of herbicides on chick insect food, compounded by a loss of hedgerow nesting habitat (Potts 1980; Rands 1985). In turn, there was an increase in incubating female and clutch losses through predation, resulting from reduced control of nest predators by gamekeepers as they switched to providing shooting from released birds instead of wild game (Potts 1980; Tapper, Potts & Brockless 1996). Recent analysis of Sussex monitoring data found that overwinter losses had increased with the estimated abundance of predatory mammals and raptors, and the population decline has continued (Fig. 1, Aebischer, Ewald & Potts 2002). This was the first time that raptors had been implicated in the partridge decline in the United Kingdom, although a similar process has been suggested in France (Bro et al. 2001; Bro, Arroyo & Migot 2006). Aebischer, Ewald & Potts (2002) emphasized the correlative nature of their analysis, noting that potential raptor predation was confounded with other factors, not least release of gamebirds for shooting.
Figure 1. Decline in grey partridge breeding density (line, spring pairs per km2, right axis) and increase in autumn raptor sighting rate (bars, sightings per km2, left axis) in the Sussex study area, 1970–2004.
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The present study investigated the relationship between partridge losses over winter, raptor dispersion and shooting pressure, and sought to quantify the relative magnitude of partridge losses to raptors, shooting and other factors.
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The data on the impact of overshooting on grey partridges were unequivocal. Our stock-yield model predicted extinction when the proportions of wild grey partridges shot exceeded 50%. Two sources of shooting bag data showed that when average annual shooting mortality exceeded this level (farm 1, area A and farm 2), extinction was the observed outcome. In neither case was the overshooting deliberate, but resulted from intensive commercial shooting regimes based on large-scale gamebird releasing.
How typical of UK lowland gamebird shoots is this pattern of unwittingly excessive wild grey partridge harvest? While the level of grey partridge releasing has declined nationally in the United Kingdom, that of red-legged partridges has doubled since 1990 (Aebischer 2002). Thus, it is likely that the situation in Sussex was repeated elsewhere in the United Kingdom. Aebischer & Ewald (2004) estimated that grey partridges are shot on a quarter of farms where they are at low densities, and that the average shooting mortality is 12%, suggesting that some managers moderated or abandoned shooting at low densities. In France, wild grey partridge shooting has been limited or stopped altogether in some areas where their density has fallen to low levels (Reitz 1996; Bro et al. 2001). However, 2 million grey partridges were released for shooting in France in 1995, and releasing is practised extensively in regions of greatest population decline (Reitz 2003; Bro, Arroyo & Migot 2006). Clearly, in France as well as the United Kingdom, the risk of overshooting exists as a factor in partridge population decline, and is complicated by landowners’ management responses to low stock levels. Most authors agree that releasing has negative consequences for biodiversity owing to increased risk of disease transmission from reared to wild stock, the risk of genetic dilution and the reduced need for habitat stewardship (Tapper 2001; Viñuela & Arroyo 2002; Bro, Arroyo & Migot 2006).
Traditional shooting of wild grey partridges in the United Kingdom was density-dependent, in that the proportion shot was calculated on the basis of annual counts (Potts 1980). With large gamebird releases, the number of shoot days is set from the number of birds released regardless of the wild stock. Thus the shooting pressure can be far higher than a small wild stock can withstand. Nevertheless, as the results from farm 1 demonstrate, even on commercial shoots it is possible to take precautionary measures to avoid shooting wild grey partridges. Adopting such measures reduced shooting pressure to below 20%, i.e. below the optimal sustainable yield for that stock density. It was followed by stabilization of numbers, indicating that commercial shooting can coexist with grey partridge conservation in the United Kingdom. Note that avoiding overshooting a wild stock of grey partridges became possible only once the red-legged partridge was adopted as the release species, as the two could then be differentiated by the hunters. Obviously, this management option is not available in situations where the vulnerable native species is the same as that released, for example, for red-legged partridges shot in large numbers in Spain and Portugal.
Our data on the effects of raptor predation were less robust. Carcass searching allowed us to identify the cause of death of only 10% of grey partridges that died during the winter. However, a separate multi-site study of mortality causes of 150 radio-tagged grey partridges found proportional allocations of predation losses (unadjusted for fox scavenging) of 41% to raptors and 59% to foxes (Watson 2004), remarkably similar to our estimates from carcass searches in Sussex (40% and 60%, respectively). This gives us confidence that, despite the low number of carcasses found in Sussex, the estimate of losses to raptors used in our model was realistic. Among radio-tagged partridges there was nil mortality to disease or starvation and a single case of mortality to fence collision, which again supported our assumption that all non-shooting mortality was due to predation.
An independent measure of maximum partridge losses to raptors was available from an empirical relationship in Watson's (2004) study: ln(raptor kills) = 0·48 ln(autumn density after deducting birds shot) – 1·04. Substituting the Sussex post-shooting partridge density in this equation estimated post-shooting raptor losses at 0·154, remarkably similar to our own estimate of 15% in the Sussex study. This is further evidence that the parameterization of the model was realistic.
The gamekeeper carcass survey indicated that two-thirds of all raptor kills, equal to 84% of kills outside the breeding season, were found in February–March. This strongly supports the hypothesis that losses to raptors occur primarily after the shooting season, implying that the true impact of raptors and of shooting is closer to the evaluations obtained on the basis of the 15% maximum estimate of partridge losses than to those derived using the minimum estimate of 9·5%. The accuracy of the gamekeeper survey is corroborated by data from Watson's (2004) multi-site study, whereby 72% (n = 29) of grey partridges found killed by raptors outside the breeding season died between end January and end March.
The model results confirmed that low-density grey partridge populations are very sensitive to overwinter loss, especially raptor loss that occurs in late winter. A reduction of up to 26% in spring equilibrium density caused by raptor predation is likely to be sufficient to at least hamper recovery of low-density populations. Traditional density-dependent driven wild partridge shooting carried out at low partridge densities had little impact on subsequent spring pair densities, but intensive shooting based on released gamebirds had a serious impact of at least 68% to 85% reduction. In combination the effect of raptors and intensive shooting was extremely severe, exceeding 85%.
In this study, we concentrated on modelling partridge losses to raptors outside the breeding season. In France, Bro et al. (2001) found that losses to raptors during the breeding season could be severe, but that the mortality was probably caused by harriers Circus spp. Harriers are present on the Sussex study area only in winter and even then at very low densities. The gamekeeper carcass survey (Fig. 4) indicated that only 18% of UK raptor kills were during the breeding season, when foxes and mustelids are the main predators of adult partridges (Potts 1980, 1986). In terms of model outcomes, this means that the impact of raptors is slightly underestimated, again suggesting that evaluations based on the maximum estimate of partridge losses to raptors are likely to be closest to the true picture.
A feature of the grey partridge is its high potential fecundity (Potts 1986). The causes of the declines that reduced partridge numbers to the levels examined in these model scenarios were reductions in brood and chick survival due to habitat degradation exacerbated by nest predation (Potts 1980, 1986). In large-scale, replicated and controlled experiments, nest predation reduced equilibrium spring pair densities by 62% (Tapper, Potts & Brockless 1996). In other words, nest predation was 3·6 times as important as raptor predation during the non-breeding season in determining breeding densities.
The last ‘ideal’ model runs suggest that habitat enhancement and legal predator control can have dramatic positive effects on partridge stocks. This has been demonstrated in practice in a partridge recovery experiment, where grey partridge spring density increased by an annual average of 66% over the 2 years following the start of management (Aebischer & Ewald 2004). In the model, with 1·5 times more nesting cover and low nest losses, raptor predation reduced the equilibrium spring pair density by 13%, at most. Although the pattern of density-dependent predation at high partridge densities is not clear, this result is suggestive of a reduced impact of raptor predation at high densities, due presumably to the lack of a numerical response. Few studies have demonstrated such a response to gamebird prey and these have been exclusively on grouse in northern latitudes killed by specialist predators when alternative prey availability was low (Valkama et al. 2005).
This study has looked at raptor predation using data from grey partridges as prey. A complementary approach would be to examine the functional response of the raptors (cf. Redpath & Thirgood 1999 and Thirgood et al. 2000b for hen harriers and red grouse). Although we were unable to distinguish reliably between the kills of different raptor species, during the 2 years of the study we saw sparrowhawks attacking or feeding on partridges 25 times and a buzzard once only. Previous studies of raptor predation on partridges have been conducted at different seasons, using different methods and at different densities of raptors, partridges and alternative prey, so may not be comparable. For example in northern France, predation caused 73% of mortality to female partridges during the breeding season (Bro et al. 2001). Of this, 29% was due to raptors and 64% to mammals. Most raptor predation was attributed to hen harriers and marsh harriers, as predation rates were correlated positively with harrier abundance. In earlier studies, partridge kills comprised 0·25 ± 0·03% of the sparrowhawk diet in Schleswig-Holstein (Uttendörfer 1939). Of 210 birds brought to buzzard nests in Moray, four were partridges, i.e. 2% of the birds killed (Swann & Etheridge 1995). In a recent study of prey brought to nests in southern England, Smart (2002) estimated that buzzards killed 2·3% of grey partridges available.
In this study, farms 1 and 2 held the highest density of raptors on the study area. In this particular case, the areas favoured by raptors coincided with areas of highest shooting pressure. This may have been because these areas had woodland that provided the only suitable places for raptors to nest, as well as cover crops and grain feeders that attracted concentrations of passerines that were the main prey of sparrowhawks. It is also possible that casualties among the released gamebirds provided a source of carrion that increased food availability for buzzards during the winter. This coincidence is possible support for Newton's (1986) suggestion that raptors benefit from landscapes managed for game. Certainly, raptors would have been absent if illegal raptor control was a corollary of intensive management for game on this study area. While our data provide evidence of a potential conflict of interests between grey partridge conservation and commercial shoot management, they imply that there is little conflict between raptor conservation and this type of shooting, as raptors were tolerated. The rapid re-colonization by buzzards west to east in lowland England supports this view (Clements 2000).
Our results are of wide relevance to the real or perceived conflict between gamebird management and raptor conservation (Kenward 1999; Thirgood et al. 2000a; Valkama et al. 2005). They suggest that on commercial shoots based on released gamebirds and in the absence of specialist gamebird predators, overshooting is a much greater risk to wild partridge stocks than raptor predation. Thus in addition to Valkama et al.'s (2005) recommendation for more studies that measure raptor predation rates and any compensation between mortality factors, there is a need to take into account the variety of gamebird management practices. Under releasing regimes where there are no wild gamebirds, raptor predation losses may usually be offset by releasing more birds (Kenward et al. 2001). Ultimately, shooting of released non-native species such as pheasants or red-legged partridges (in the United Kingdom) should not threaten the conservation status of the native grey partridge.
While some might suggest from these findings that partridges of both species should be removed from the quarry list, we argue that this would be counterproductive in the United Kingdom as grey partridge density is highest on estates managed specifically as wild partridge shoots: shooting provides the incentive (and private funding) for the management package that allows grey partridges to thrive in these places (Aebischer 1997). Thus, a third of the estates participating in the Game Conservancy Trust's Partridge Count Scheme have autumn densities that exceed 20 wild grey partridges per km2, greater than that required to achieve the UK Species Action Plan target (Aebischer & Ewald 2004). However, it is imperative to reduce the accidental kill rate of grey partridges at low densities on intensive commercial shoots based on released gamebirds. Training the shooters to identify grey partridges and have a warning system (whistle) to alert them when birds of the non-target species are approaching the gun line are all that is required. We believe that such voluntary precautionary measures can be effective when implemented, and they need to be implemented nationally as a matter of routine. The threshold below which precautionary measures should be adopted is 20 birds per km2 in autumn and this has been included as part of recommendations for the Game Conservancy Trust's Partridge Recovery Programme (Tapper 2001). Together with the agricultural reforms launched in the United Kingdom in March 2005, which effectively reduce the cost of habitat management, there is now the opportunity to reverse the fortunes of this charismatic farmland species.