SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

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.

  • 1
    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;
  • 2
    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).
  • 3
    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.

METHODS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

Sources of data

Partridge Count Scheme

The GCT's Partridge Count Scheme (PCS) collects information on the annual abundance and breeding success of Grey Partridges based on counts of pairs in spring and counts of young and old birds in autumn. The area counted is usually known, and information on numbers shot has been collected in recent years. A measure of breeding success, young:old ratio, is derived from the autumn counts. From 1933 to 1998, the scheme mainly involved up to 100 Partridge estates in southern and eastern England (Potts 1980, Potts & Aebischer 1995). In October 1998 the GCT relaunched the scheme with Green Globe Consultancy under the banner ‘Every one counts’. Since 1999 the scheme has been extended nationally; there are currently around 1350 registered participants (Aebischer in press). Not all provide consistent returns, and there are many information gaps that reduce the sample sizes available for analysis.

Annual chick survival rates derived from past PCS autumn brood counts over the period 1933–93 have been published in Potts (1980) and Potts and Aebischer (1995). Their calculation has since been discontinued in favour of young:old ratio, because of the steady decrease in numbers of broods caused by the ongoing decline.

Norfolk estates

Since 1991, members of the Norfolk Partridge Group have been encouraging one another to undertake habitat management and predator control for Grey Partridges. From 1992 to 2003, consistent annual spring count data are available for five estates that did so, and five that did not (Aebischer 1997). In 1996, the success of the former led two of the latter estates to start management (Aebischer et al. 2000).

National Gamebag Census

The GCT's National Gamebag Census (NGC), in operation since 1961, collects bag statistics from over 600 UK shooting estates annually (Tapper 1992). Participation is voluntary, and data are submitted via an annual survey form sent out at the end of the shooting season. Participants are asked to provide information on numbers of gamebirds released, numbers shot and estate area.

Sussex Study

Since 1968, the GCT has monitored autumn numbers of Grey Partridges on 62 km2 of arable chalk land on the South Downs, Sussex. By recording sexes and ages, it has been possible to estimate spring pair density and annual rates of nest loss, female loss during reproduction, chick survival during the first 6 weeks of life and female overwinter loss (Potts 1986). The rates were averaged over the period 1990–2000 to give mean demographic parameters thought to be representative of the current agricultural environment of the Grey Partridge; during this period there was no trend over time in young:old ratio or overwinter loss (Aebischer et al. 2002). Numbers of Grey Partridges shot were obtained in most years.

Grey Partridge Restoration Project

The GCT's Grey Partridge Restoration Project provides a demonstration site where visitors may see for themselves the management techniques needed to restore Grey Partridges. A 10-km2 area of arable chalk land near Royston, Hertfordshire, has been managed since January 2002 deliberately to increase wild Grey Partridge densities, using predation control, habitat management and supplementary feeding. Grey Partridges are monitored by counts in spring (starting March 2002) and autumn (starting September 2001).

Common Birds Census

An annual index of national Grey Partridge abundance from 1962 to 2000 is given by the BTO's Common Birds Census (CBC) (Marchant et al. 1990). Annual measures of year-on-year change are published in the BTO quarterly magazine BTO News, and the complete index is given in Baillie et al. (2002).

Population change and management implications

In combination, the PCS and the CBC yield data from 1962 to 1994 whereby annual changes in abundance (CBC) can be related to chick-survival rate (PCS) at the national scale using independent sources of information.

We then sought to model annual change via the Potts model of Grey Partridge population dynamics (Potts 1980, 1986) in a way that would tie in with habitat management. Rands (1987) showed how the amount and quality of nesting habitat, primarily non-cropped linear features such as hedgerows and grass banks, determined an area's attractiveness to Grey Partridges. The Potts model already incorporated the density (km/km2) of nesting cover as an environmental variable influencing density-dependent female losses. Originally, it also took as input observed annual chick survival rates. Given that these vary in relation to the availability of chick-food invertebrates in cereals, itself dependent on the pesticide regime (Potts 1986), we replaced them by a rate recalibrated in terms of the amount of insect-rich brood-rearing habitat in the landscape. The recalibration assumed a linear increase in chick survival rate between no brood-rearing habitat (the fully sprayed situation) and the amount provided by the use of Conservation Headlands, an ideal brood-rearing habitat (Sotherton et al. 1993). Where necessary, brood sizes at 6 weeks were converted to chick survival rates following Potts (1986).

We used landscape data (percentages of arable and cereal-growing land, field size) collated by Oxford University (Aebischer et al. 2003) to express Conservation Headland use as a percentage of arable area providing insect-rich brood-rearing habitat in a typical arable situation. Oxford data on hedgerow density provided a measure of nesting cover for the Potts model. The equilibrium density that the model produced with a constant chick survival rate set to the 1990–2000 Sussex average was adopted as a standard reference level, against which the equilibrium levels for all combinations of nesting cover (from 2 to 10 km/km2) and amounts of insect-rich arable area (from 0% to 6%) were expressed as percentage differences. The model runs included density-dependent shooting, and were performed separately with and without predation control.

Statistical analyses

Annual indices of spring density were obtained by fitting a generalized linear model with Poisson error, logarithmic link function and ln(area) as offset to the number of spring pairs recorded in the PCS from 1990 to 2003, using site and year as factors. Models were fitted separately to data from long-term contributors and from sites newly recruited since 1999, and compared using likelihood ratio tests adjusted for overdispersion. Average young:old ratios, also from the PCS, were calculated as annual weighted means using the number of old birds as weights, and analysed by weighted analysis of variance.

We also used the PCS to investigate the extent of Grey Partridge shooting and its relationship with Partridge density using the 1999–2002 data. In each year, we classified sites as below the GCT's recommended threshold for not shooting Grey Partridges (< 20 birds/km2 in the autumn), up to twice the threshold (20–39 birds/km2), and beyond (≥ 40 birds/km2). For each category, we calculated the percentage of sites where Grey Partridges were shot, the percentage of total site area that was shot over, the average autumn density and the offtake rate (percentage of total autumn birds that were shot). The PCS did not record Grey Partridges released for shooting, which could distort the bags. Releasing was suspected in four instances (more Grey Partridges shot than counted in the autumn), so these cases were omitted. We carried out analysis of covariance (or in the case of the first variable, the generalized linear equivalent with binomial error and logistic link) with year as factor and autumn density as covariate, weighted by area and autumn birds in the cases of percentage area and offtake rate, respectively.

The numbers of Partridges shot per unit area, collated by the NGC, were analysed in the same way as PCS spring densities to give annual indices of bag density from 1990 to 2002. NGC estates were grouped into three categories according to the type of shoot: those based on wild birds only (no releasing of other gamebirds), those that released Pheasants only and those that released Pheasants and Red-legged Partridges. Some wild estates occasionally release small numbers of birds for restocking. We interpreted estates where releases occurred fewer than once every five years as being based on wild birds only. Estates that released more often than once every two years were deemed to be releasing for shooting. Estates that released at intermediate rates could not safely be classified so were excluded from analyses, as were those that released Grey Partridges for shooting (bags meaningless in terms of wild Partridges). For wild estates, any years when releasing for restocking occurred were omitted. Mean annual rates of change in bag density over time in the different categories of estate were calculated and compared by a generalized linear mixed model with estate as the random component.

Analyses were carried out using Genstat 7.1 (Numerical Algorithms Group, Oxford). All means reprted below are expressed with their standard error unless otherwise stated.

RESULTS

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

Population change and breeding success

Based on the 1990–2000 Sussex data, mean (± se) demographic parameters for Grey Partridge were 0.619 ± 0.035, 0.703 ± 0.014, 0.294 ± 0.020 and 0.593 ± 0.042 for survival rates of nests, breeding females, chicks and overwintering females, respectively. Assuming an average of 14 eggs hatching per clutch (Potts 1986), these values gave an annual rate of population change of 0.929, which was very close to the value of 0.926 estimated from BTO CBC data for the Grey Partridge over the same period.

Combining data from the CBC and the PCS, we found a close relationship between chick survival rate in one year and the change in abundance from that year to the next (r30 = 0.826, P < 0.001; Fig. 1). Population change was negative at low chick survival rates, increased linearly as chick survival rate increased and was stable at a chick survival rate of 0.35.

image

Figure 1. Annual changes in UK Grey Partridge abundance from the BTO's Common Birds Census in relation to annual chick survival rates from the GCT's national Partridge Count Scheme (y = 2x − 70, r30 = 0.826, P < 0.001).

Download figure to PowerPoint

Population change in relation to habitat management

Calibrating chick survival rate

Sotherton et al. (1993) found that, over 8 years, chick survival rate averaged 0.23 on fully sprayed areas and 0.39 on areas with Conservation Headlands within the same East Anglian farms. In Hampshire and eastern England, they also reported mean brood sizes of 4.8 chicks on fully sprayed land and 7.3 chicks on land with Conservation Headlands, equivalent to chick survival rates of 0.19 and 0.37, respectively. In Sussex, chick survival rate averaged 0.22 over 6 years of intensive herbicide and insecticide use (Aebischer & Potts 1998). Under an intensive spraying regime (no brood-rearing habitat), chick survival rate was therefore close to 0.20. With Conservation Headlands, chick survival rate was c. 0.38.

In a typical arable situation, 54% of land is arable, of which 69% is in cereals, with an average field size of 8.24 ha (Oxford data). Assuming square cereal fields with Conservation Headlands along three sides of each field, the amount of insect-rich brood-rearing habitat would be 6.3% of cereal area, or 4.3% of arable area. Thus a 1% increase in such habitat on the arable area was equivalent to an increase in chick survival rate of around 0.04. To maintain stability (cf. Fig. 1), 4% of arable area would be needed as insect-rich brood-rearing habitat. By extrapolation, 6% of such habitat would yield a chick survival rate of around 0.44, close to that recorded during the pre-pesticide era (Potts 1986).

Modelling population change

Average hedgerow density in a typical arable situation was 4 km/km2 (Oxford data). Using this measure of nesting cover in the Potts model, together with the average Sussex chick survival rate of 29.4%, gave a reference equilibrium density of 5.56 pairs/km2. The pattern of changes in abundance relative to this density (Fig. 2) was broadly similar with and without predation control, although responses were much slower without it (wider contours).

image

Figure 2. Contour maps of relative differences (%) in equilibrium levels according to the amount of nesting cover and the percentage of arable area made up of insect-rich brood-rearing habitat (left: no predation control, right: with predation control). The baseline (contour 0) passes through the density predicted on an ‘average’ arable farm over the last decade.

Download figure to PowerPoint

In the absence of predation control, the model predicted that a 1% increase in insect-rich brood-rearing habitat on the arable area, from 2.35% (equivalent to the average chick survival rate of 0.294) to 3.35%, would yield a 11.9% increase in equilibrium density when nesting cover is kept at its average of 4 km/km2. Conversely, keeping insect-rich habitat constant at 2.35% of arable area, a 1% increase in nesting cover (from 4.00 to 4.04 km/km2) would produce a population increase of 1.1%. Predator control alone would produce a 166% increase.

Considering that the average annual rate of population change over the last decade was 0.926, stabilizing the population (BAP Objective 4.2, Anon. 1995) requires a relative increase of 8% (100/0.926–100). The Potts model implies that this can be achieved without predation control by having 3% of arable area as insect-rich brood-rearing habitat, or by increasing nesting cover from 4 to 4.3 km/km2.

The average annual rate of population change over the last decade of 0.926 amounts to a change of 0.58 (42% decline) between 1995, when the BAP targets were set, and 2002. Recovery of the population to 150 000 pairs by 2010 (BAP target 4.3, Anon. 1995) demands the reversal of a 54% decline since 1990, i.e. a relative increase of 117% (100/0.46–100), equivalent to an average annual increase of 10% from 2002 to 2010. Without predation control, it would require a combination of increased brood-rearing habitat and increased nesting habitat, for instance 5% of arable land being insect-rich together with 6.9 km/km2 of nesting cover. With predation control on, say, 10% of land, the requirements become 5% and 6.2 km/km2, respectively.

Effectiveness of management

We review below the evidence that management has an effect upon Grey Partridge densities, at three different scales: the GCT's demonstration project, ten estates in Norfolk monitored since 1992 and recent evidence from the PCS.

Grey Partridge Restoration Project

The habitat management now in place on the demonstration area makes use of set-aside and the Countryside Stewardship Scheme (CSS) to restore nesting and brood-rearing cover. In 2003, the CSS areas comprised 6.65 ha of wildlife mixtures, 41.38 ha of low-input or spring cereals, 10.31 ha of fallow, 23.40 ha of grass margins, 0.77 ha of beetle banks and 0.34 ha of new hedgerows. The set-aside comprised 13.34 ha of wild bird cover and 144.40 ha of natural regeneration. There were an additional 1.90 ha of game cover crops not on set-aside. This translates approximately into 6% of the area as brood-rearing cover and 16% as nesting cover, which compensates for a low hedgerow density of 1.6 km/km2.

The 2001 autumn counts and 2002 spring pair counts reflect the densities of Grey Partridges before management began. The autumn counts were very low, producing spring densities under the 4.5 pairs/km2 expected on unmanaged ground (Table 1). The autumn 2003 density was double the threshold value of 20 birds/km2, and the spring 2004 density was 8.0 pairs/km2, a 176% increase in 2 years. Chick survival rate in 2001 was close to 20%, the value expected in a fully sprayed situation (Table 1). In the two managed years, it averaged 54%.

Table 1.  Grey Partridge density and breeding success on the demonstration area of the GCT's Grey Partridge Restoration Project from autumn 2001 to spring 2004. Management began at the start of 2002.
Year2001200220032004Change (%)
  • *

    Average of 2002/03 and 2003/04 annual changes.

  • Average of 2001/02 and 2002/03 annual changes.

  • Average of 2002 and 2003 (managed) relative to 2001 (unmanaged).

Spring density (pairs/km2) 2.90 5.108.0 +66*
Autumn density (birds/km2)7.628.8039.20+158
Young:old ratio0.61 3.04 2.90+387
Chick survival rate0.17 0.55 0.53+218
Norfolk estates

On unmanaged estates, spring pair density fell throughout the period; on managed estates, numbers all increased relative to the year when management began (Fig. 3). The effects of poor weather from 1997 to 2000 was reflected in a halving in numbers from 1998 to 2001. Despite the fall, pair densities remained at least four times higher on managed estates than on unmanaged estates, and the subsequent recovery in 2002 and 2003 was quicker.

image

Figure 3. Average annual spring density (pairs/km2) of Grey Partridges on five estates in Norfolk where Partridge management began in 1992 (solid line), and on five unmanaged estates from the same area (dashed line), 1992–2003. In 1996, two of the formerly unmanaged estates started management.

Download figure to PowerPoint

Partridge Count Scheme

Counts from the long-term contributors to the PCS provide a background level of spring pair density and breeding success against which to assess the performance of new recruits to the scheme since its expansion after 1998 (Fig. 4). The annual pattern of change in spring pair density did not differ significantly between new recruits and long-term contributors (F10,618 = 0.62, P = 0.798), although the spring density of the former was higher than that of the latter (F1,213 = 9.17, P = 0.003).

image

Figure 4. Average annual spring density (pairs/km2) and breeding success (young:old (Y:O) ratio) of Grey Partridges on sites contributing to the GCT's Partridge Count Scheme, 1990–2003. Data from long-term contributors and new recruits since the scheme was relaunched in 1999 are presented separately. Error bars represent ± 1 se.

Download figure to PowerPoint

Looking at breeding success, again the annual pattern of change in young:old ratio from the autumn counts did not differ significantly between new recruits and long-term contributors (F6,673 = 0.58, P = 0.748). By contrast, for the three most recent years 2000, 2001 and 2002, the average young:old ratio was significantly higher for new recruits (1999: t64 = −0.27, P = 0.792; 2000: t82 = 2.98, P = 0.004; 2001: t97 = 2.24, P = 0.027; 2002: t182 = 3.59, P < 0.001).

Influence of shooting

Sussex Study

The fall in average numbers of wild Grey Partridge pairs on four Sussex farms from 1995–97 to 1998–2000 closely reflected the losses to shooting (Table 2). Particularly high losses occurred where there was intensive driven shooting based on released Pheasants and Red-legged Partridges, the wild Grey Partridges inadvertently becoming caught up in the drives of released gamebirds. Because it happened repeatedly within the same season, the Grey Partridges suffered unsustainably heavy losses. On a farm where no shooting took place, the number of wild Grey Partridge pairs remained stable during the same period.

Table 2.  Changes in average numbers of wild Grey Partridge pairs in relation to the percentage of the autumn stock shot during organized shoots of released Red-legged Partridges: data from four farms in Sussex, 1995–2000.
LocationSpring pairs (3-year average)Change between periods (%)Autumn stock shot (%) (3-year average) 1997/98–1999/2000
1995–971998–2000
Farm A18.018.0    0 0
Farm B10.3 8.7−1637
Farm C 7.3 4.0−4570
Farm D21.3 5.0−7791

Since 2000, farms B and D implemented and monitored precautionary measures to avoid shooting Grey Partridges during the 2000/01, 2001/02 and 2002/03 shooting seasons. The percentage shot of the autumn stock of wild Grey Partridges dropped from an average of 64% over 3 years to one of 16% over 3 years.

National Gamebag Census

Average annual Grey Partridge bag density declined from 1990 to 2002 (Fig. 5), but the rates of decline differed according to the nature of gamebird releasing (χ22 = 11.70, P = 0.003). On 18 ‘wild’ estates (no gamebird releasing), the rate averaged −7.9 ± 2.8%. On 39 estates that released Pheasants only, it was significantly higher, at −16.0 ± 1.5% (z = 2.62, P = 0.008). On 85 estates that released Pheasants and Red-legged Partridges, the rate was −9.8 ± 1.1%, significantly lower than on the previous type of estate (z = 3.26, P = 0.001), but not significantly different from estates that released no gamebirds (z = 0.63, P = 0.528).

image

Figure 5. Average annual bag density (number shot/km2 ± 1 se) of Grey Partridges on estates contributing to the GCT's National Gamebag Census, for the shooting seasons 1990/91 to 2002/03. Wild estates, estates releasing Pheasants only, and estates releasing Pheasants and Red-legged Partridges are presented separately.

Download figure to PowerPoint

Partridge Count Scheme

Within years, the percentage of sites on which Grey Partridges were shot increased with autumn density (inline image = 3.90, P = 0.048; Table 3). At levels below 20 birds/km2, three-quarters of sites did not shoot Grey Partridges, compared with nearly half of the sites with the highest densities. On an area basis, the percentage area shot over was just over one-quarter for the low-density sites, and increased rapidly to nearly three-quarters at the highest densities (F1,7 = 17.32, P = 0.004). By contrast, the proportion of the autumn stock that was shot did not differ according to density (F1,7 = 1.61, P = 0.245), and averaged 11%.

Table 3.  Extent of shooting and offtake of Grey Partridges 1999–2002, in relation to three categories of autumn density, from sites that contributed to the GCT's Partridge Count Scheme.
Autumn density categoryTotal no. of sitesSites where Grey Partridges were shot (%)Total area (km2)Area shot over (%)Autumn density (birds/km2)Autumn stock shot (%)
1999
 < 20 2317.4143.48 9.2 8.914.9
 20–39  616.7 27.9332.931.1 3.2
 ≥ 40  580.0 14.7295.963.913.3
2000
 < 20 1936.8131.5626.9 9.021.8
 20–39  850.0 26.2570.327.1 9.9
 ≥ 40  250.0 11.3280.448.813.2
2001
 < 20 3420.6184.9227.6 7.4 5.6
 20–39 1233.3 65.3041.528.2 6.5
 ≥ 40  742.9 17.6081.064.0 5.4
2002
 < 20 9027.8404.8438.7 6.811.9
 20–39 2524.0102.9737.728.2 4.0
 ≥ 40 2133.3 60.2762.164.316.7
Overall
 < 2016625.9864.8029.7 7.612.4
 20–39 5129.4222.4542.128.4 5.6
 ≥ 40 3542.9103.9172.162.513.7

DISCUSSION

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

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.

Acknowledgments

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

We thank in particular all the contributors to the Partridge Count Scheme and the National Gamebag Census, who by voluntarily providing data over the years have made the assessments presented here possible. We are grateful also to the Sussex farmers and landowners who have so kindly granted the GCT access over 35 years. Many thanks also to all the people, organizations and companies who have provided money, time and effort to the cause of the Grey Partridge. The Department for Food and Rural Affairs funded the quantification of habitat requirements relative to population change under project BD1618. As part of this project, the BTO kindly provided the 1990–2000 CBC rate of change for Grey Partridge, and Oxford University the land-use statistics.

REFERENCES

  1. Top of page
  2. Abstract
  3. METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES
  • Aebischer, N.J. 1991. Sustainable yields: gamebirds as a harvestable resource. Gibier Faune Sauvage 8: 335351.
  • Aebischer, N.J. 1997. Gamebirds: management of the Grey Partridge in Britain. In Bolton, M. (ed.) Conservation and the Use of Wildlife Resources: 131151. London: Chapman & Hall.
  • Aebischer, N.J. in press. Restoring the fortunes of the Grey Partridge in the UK: the GCT recovery programme. In Perco, F. & Potts, G.R. (eds) Managing Partridges and Other Game in the Agricultural Landscape. Udine: Provincia di Udine.
  • Aebischer, N.J. & Potts, G.R. 1998. Spatial changes in Grey Partridge (Perdix perdix) distribution in relation to 25 years of changing agriculture in Sussex, U.K. Gibier Faune Sauvage 15: 293308.
  • Aebischer, N.J., Green, R.E. & Evans, A.D. 2000. From science to recovery: four case studies of how research has been translated into conservation action in the UK. In Aebischer, N.J., Evans, A.D., Grice, P.V. & Vickery, J.A. (eds) Ecology and Conservation of Lowland Farmland Birds: 4354. Tring: British Ornithologists’ Union.
  • Aebischer, N.J., Ewald, J.A. & Potts, G.R. 2002. Preliminary results from using GIS to examine spatial variation in Grey Partridge demography over 30 years in Sussex, UK. Proc. IUGB Congr. 24: 2333.
  • Aebischer, N.J., Bradbury, R.B., Eaton, M., Henderson, I.G., Siriwardena, G.M. & Vickery, J.A. 2003. Predicting the Response of Farmland Birds to Agricultural Change. BTO Research Report 289. Thetford: British Trust for Ornithology.
  • Anon. 1995. Biodiversity: the UK Steering Group Report, Vol. 2: Action Plans. London: HMSO.
  • Anon. 2003. Entry Level Agri-Environment Scheme Pilot: Scheme. Guidance Booklet. London: Defra.
  • Baillie, S.R., Crick, H.Q.P., Balmer, D.E., Beaven, L.P., Downie, I.S., Freeman, S.N., Leech, D.I., Marchant, J.H., Noble, D.G., Raven, M.J., Simpkin, A.P., Thewlis, R.M. & Wernham, C.V. 2002. Breeding Birds in the Wider Countryside: Their Conservation Status 2001. BTO Research Report no. 278. Thetford: British Trust for Ornithology.
  • Bradbury, R.B., Browne, S.J., Stevens, D.K. & Aebischer, N.J. 2004. Five-year evaluation of the impact of the Arable Stewardship Pilot Scheme on birds. In Ecology and Conservation of Lowland Farmland Birds II: The Road to Recovery. Ibis 146 (Suppl. 2): 171180.
  • Curry, D. 2002. Farming and Food: a Sustainable Future. Policy Commission on the Future of Farming and Food. London: HMSO.
  • Gibbons, D.W., Reid, J.B. & Chapman, R.A. 1993. The New Atlas of Breeding Birds in Britain and Ireland: 1988–1991. London: T. & A.D. Poyser.
  • Gregory, R.D., Noble, D.G. & Custance, J. 2004. The state of play of farmland birds: population trends and conservation status of lowland farmland birds in the United Kingdom. In Ecology and Conservation of Lowland Farmland Birds II: The Road to Recovery. Ibis 146 (Suppl. 2): 113.
  • Hill, D.A. & Robertson, P.A. 1988. The Pheasant: Ecology, Management and Conservation. Oxford: Blackwell Scientific Publications.
  • Marchant, J.H., Hudson, R., Carter, S.P. & Whittington, P. 1990. Population Trends in British Breeding Birds. Tring: British Trust for Ornithology.
  • Potts, G.R. 1978. The effects on a partridge population of predator control, insect shortages, different shooting pressures and releasing reared birds. Game Conserv. Ann. Rev. 1977 9: 7583.
  • Potts, G.R. 1980. The effects of modern agriculture, nest predation and game management on the population ecology of partridges (Perdix perdix and Alectoris rufa). Adv. Ecol. Res. 11: 179.
  • Potts, G.R. 1986. The Partridge: Pesticides, Predation and Conservation. London: Collins.
  • Potts, G.R. & Aebischer, N.J. 1995. Population dynamics of the Grey Partridge Perdix perdix 1793–1993: monitoring, modelling and management. Ibis 137 (Suppl. 1): 2937.
  • Rands, M.R.W. 1987. Hedgerow management for the conservation of partridges Perdix perdix and Alectoris rufa. Biol. Conserv. 40: 127139.
  • Raven, M.J., Noble, D.G. & Baillie, S.R. 2003. The Breeding Bird Survey 2002. BTO Research Report no. 334. Thetford: British Trust for Ornithology.
  • Robertson, P.A. 1991. Wise use and conservation. Gibier Faune Sauvage 8: 379388.
  • Sotherton, N.W., Robertson, P.A. & Dowell, S.D. 1993. Manipulating pesticide use to increase the production of wild game birds in Britain. In Church, K.E. & Dailey, T.V. (eds) Quail III: National Quail Symposium: 92–101. Pratt: Kansas Department of Wildlife and Parks.
  • Tapper, S.C. 1992. Game Heritage: an Ecological Review from Shooting and Gamekeeping Records. Fordingbridge: Game Conservancy Ltd.
  • Tapper, S.C. 1999. A Question of Balance: Game Animals and Their Role in the British Countryside. Fordingbridge: The Game Conservancy Trust.
  • Tapper, S.C. 2001. Conserving the Grey Partridge. Fordingbridge: The Game Conservancy Trust.
  • Varley, G.C. & Gradwell, G.R. 1963. The interpretation of insect population changes. Proc. Ceylon Assoc. Adv. Sci. 18(D): 142156.