I took 12 different approaches to calculate estimations (Fig. 2).
Published data estimating numbers of released birds
Aebischer (2019) calculated that a mean of 47 million pheasants and 10 million partridges were released in 2016. Estimates of mallard release were not made. A total of 0.94 million mallard were reported shot. If all of these were released birds and that these represent one-third of the mallard that were released (matching similar ratios for pheasant and partridges), then I estimate that 2.8 million mallard were released, giving a total of 59.8 million birds being released annually. If all the mallard shot were wild born, then 57 million birds were released.
Poultry register data reporting numbers of birds held for rearing
APHAPR2020 Records for 2019 show that 11,515,950 pheasants, 4,681,749 partridges and 201,308 duck (species unspecified) were reported as reared for shooting. This produces a total of 16.4 million birds (of all species) that were reared. If these are subject to the mortality rates reported by Đorđevic et al. (2018), then between 14.8 million and 15.9 million gamebirds might be released annually.
Poultry register data reporting numbers of birds held for release
APHAPR2020 Records for 2019 show that 10,448,166 pheasants and 3,807,263 partridges and 439,827 duck (species unspecified) were reported as held for release for shooting. This produces a total of 14.7 million birds (of all species) released annually.
Estimates based on numbers bred and reared for release
In addition to birds bred in the UK, gamebird eggs and chicks are commonly produced outside the UK where more clement conditions stimulate higher and earlier productivity. Therefore, to estimate the numbers of birds that may be being bred for release, it is necessary to consider numbers of eggs and chicks from UK and EU origins and account for various stages of productivity and mortality according to the species of interest. For pheasants (1,023,036 registered as held for breeding in APHAPR2020, the ratio of males to females maintained by breeders varies from one male to between seven and 10 females is reported (Butler and Davis 2014; Kontecka et al. 2014), so we might presume that 88–91% of the birds registered for breeding are females. Đorđevic et al. (2018) summarise data reporting pheasant productivity from various studies across Europe and state that females lay 40–45 eggs, of which 85–95% are classed as fertile and of these 55–70% hatch, with 3–10% of chicks dying before the age of release. For mallard (43,677 registered as held for breeding in APHAPR2020, the breeding ratio is similar to that of pheasants (Kontecka et al. 2014). Cheng et al. (1980) report that mallard reared in the USA closed-flock game farms produced mean clutches of 41 eggs with a fertility of 99.5% and a hatchability of 71%. I could find no data on survival post-hatching to release age, so use those reported for pheasants (Đorđevic et al. 2018). For red-legged partridge (313,945 registered as held for breeding in APHAPR2020, their naturally monogamous breeding system is replicated by breeders with partridges kept in pairs. Using clutch sizes and hatchability, values (accounting for fertilisation) derived from breeding pairs on a commercial game farm in Spain of 39.5 eggs/female and 58.6% respectively (Prieto et al. 2018) based on a mix of free choice and imposed pair breeders. Again, I could find no data on survival post-hatching to release age, so use those reported for pheasants (Đorđevic et al. 2018).
In addition to birds bred in the UK, records for 2019 show that 28,248,773 pheasant eggs were imported into the UK from Europe (Prentis 2020). I calculated the number of birds that might be released from these eggs by using the range of fertility, hatchability and survival figures for pheasants described above. Furthermore, 3,299,780 live pheasants and 1,673,165 live partridges were imported (Prentis 2020). I calculated the number of these birds that might survive rearing to be released by using the survival figures described above. I made 4096 calculations (Appendix 1) of how many birds might be bred and released based on all possible combinations of the extremes of the range of sex ratios, egg productivity, fertility, hatchability and survival values described above for each species and origin source (UK bred, EU eggs, EU chicks). This gave a mean estimate of 44.3 million birds (range 35.7–54.4 million) that could be released annually.
Estimates based on mean numbers of birds reported released on an average shoot
The GOPSOC (2017) reports a mean of 4307 birds being released on each of the 652 shoots that completed their survey. The GOPDB2019 reports a mean of 4666 birds being released across the 22 shoots reporting release sizes. The APHAPR2020 reports a mean of 4085 birds being held for release across the 3597 locations. The NGC2019 reports a mean of 6142 birds being released across the 431 shoots reporting release data. I multiplied these 4 figures by the six possible numbers of shoots in the UK described above to generate 24 estimates with a mean of 30.5 million (range = 14.7–55.9 million) birds released annually.
Estimates based on reported releases accounting for variations in the scale of different shoots
As described above, the size distribution of UK shoots is skewed with a large number of small shoots releasing relatively few birds and shooting relatively small bags on relatively few days, and a smaller number of large shoots releasing and shooting many birds. Therefore, I incorporated an assumption about the distribution of shoots in my estimations. According to the SSBS2017, small shoots reported a mean release of 1532 birds of all species, medium shoots reported a mean release of 6212 birds and large shoots reported a mean release of 26,241 birds. I multiplied each of the six estimates of shoot numbers in the UK by the mean proportions of shoots in each class (calculated above) and multiplied these values by the mean numbers of birds per class. This produced 6 estimates with a mean of 31.5 million (range = 17.8–45.1 million) birds released annually.
Estimates of birds released on shoots given the available keepers to manage them
The SSBS2017 reports that the mean number of birds released per full-time keeper is 10,204. Assuming a decline in numbers that can be managed with a decline in job intensity, then each full-time gamekeeper can and does manage 10,204 released birds, each part-time keeper can and does manage 5102 released birds, and each amateur keeper can and does manage 2551 birds. I calculate from the NGO figures that if the 6000 gamekeepers included amateurs at the ratios reported by Ewald and Gibbs (2019), then with 3000 FT, 1125 PT and 1875 amateur gamekeepers, a total of 41.1 million birds could be released and managed. If the NGO figures excluded amateur gamekeepers, the 3000 part-time keepers can all manage 5102 birds each, and an additional 2571 amateur keepers are included, then 52.5 million birds could be released and managed. A mean of these two estimates is 46.8 million birds that could be released and managed annually.
Estimates based on mean harvests reported from shoots
Harvest levels depend on the number of birds shot per day, the numbers of days shot per season and the harvest efficiency of the guns. I obtained four estimates across all shoots for daily bag size. These ranged from 98/day (GOPSOC 2017), through 112/day (GOPGSC 2017), 114.5/day (NGC 2019) to 165/day (GOPDB 2019). For shoot days per season, the GOPSOC2017 reported a mean of 13 days shooting/shoot while the mean NGC2019 data reports 13.7 days shooting/shoot. I could also use the PACEC (2014) provider data in these estimates. The 40,040 providers of driven and walked up shooting report a mean of 5 days shooting/provider, and by dividing the 18.4 million pheasant, partridge and duck shot between the 220,000 shooting days, the estimated bag size was 83.6/day. The harvest or bag size represents only a fraction of the birds that will have been released. The GOPSOC2017) reports 40%, SSBS2017 reports 38% and (for pheasants) Robertson et al. (2017) reports 33% return rates. I multiplied the various combinations of harvest levels, number of shoot days, harvest efficiencies and number of shoots in the UK to generate 147 estimates with a mean of 28.4 million (range = 11.5–61.7 million) birds being released annually.
Estimates based on reported harvests accounting for variations in the scale of different shoots
As described above, shoots in the UK differ greatly in scale, so I incorporated an assumption about the distribution of shoots in my estimations in which larger shoots offer more days and harvest larger bags. The SSBS2017 described small shoots as offering 9 days/season with a bag of 80 birds, medium shoots offering 16 days/season with a bag of 148 birds and large shoots offering 41 days/season with a bag of 232 birds. The NGC2019 described small shoots as offering 7.8 days/season with a bag of 63 birds, medium shoots offering 15.3 days/season with a bag of 153 birds and large shoots offering 30.3 days/season with a bag of 223.5 birds. By using the mean distribution of shoot classes calculated above (71.2% small, 18.3% medium, 10.4% large) and considering the range of numbers of shoots in the UK and the range of harvest efficiencies as above, I generated 36 estimates with a mean of 29.4 million (range = 13.3–52.8 million birds) being released annually.
Estimates based on harvests by individuals based on the total number of guns in the UK
According to PACEC (2014), 280,000 guns shoot driven game with 150,000 shooting walked up game. These two categories are not exclusive, but there is a maximum of 380,000 people shooting live quarry (including deer, wildfowl and avian and mammalian pests), so I considered the two extreme values of 280,000 and 380,000 people shooting. The numbers of birds shot per gun varies, and this variation is captured by the GOPGSC2017. The survey identified three classes of gun from 6510 individuals (of the 12,143 surveyed) who reported spend data. Like the shoots themselves, the number of birds shot per day typically increased with the number of days shot per year. Those described as ‘low spend’ comprised 2031 guns (17%) and shot a mean of 8 days/year with a mean bag size of 73 birds/shoot. Those described as ‘medium spend’ comprised 3759 guns (31%) and shot a mean of 12 days/year with a mean bag size of 123 birds/shoot. Those described as ‘high spend’ comprised 730 guns (6%) and shot a mean of 21 days/year with a mean bag size of 212 birds/shoot. A further 5623 guns (46%) did not report their annual spend, but they shot a mean of 9 days/year with a mean bag size of 99 birds. PACEC (2014) reports a mean of 10.4 guns/day across both walked up (7 guns) and driven (3 guns) days. I assume that a day’s bag was split equally between the 10.4 guns in attendance, meaning that over one shooting season, a mean low spend gun shot 56.2 birds, a medium spend gun shot 141.9 birds, a high spend gun shot 428.1 birds, and a no spend gun shot 85.7 birds. I assumed that the distribution of people reported as shooting birds in the PACEC survey matched the distribution of the behaviours of those in the GOPGSC2017. I multiplied all combinations of numbers of guns and harvest efficiencies by the distribution of gun classes and their harvest rates to generate 6 estimates with a mean value of 106.1 million (range = 83.1–135.3 million) birds being released annually.
Estimates based on harvests by individuals based on the number of shooting days
An alternative approach to relying on estimates of the total number of people shooting live quarry is to calculate the number of days that people shoot per season. Because the four different classes of guns described above shot different numbers of days/season as well as shooting different numbers of birds on each day, I calculated the percentage of the total number of shooting days that were taken by people of the four classes (low = 13%, medium = 35%, high = 12%, no spend data = 40%) and calculated the mean number of birds that the gun was likely to shoot on that day (low = 7.0, medium = 11.8, high = 20.4, no spend data = 9.5). The PACEC (2014) reports 1.6 million gun days/season targeting driven lowland game and a further 680,000 days/season targeting walked up lowland game which may include released birds. This gives a total of 2.28 million gun days/season with a conservative estimate of 1.6 million days if all driven days also include shooting at walked up birds. Therefore, I calculated estimates of harvests based on these reported gun days being shared among the gun classes as described above and accounting for harvest efficiency. This approach produced 6 estimates with a mean of 59.7 million (range = 45.1–77.9 million) birds being released annually.
Estimates assuming that BTO breeding bird survey data is a remnant of the release population
An alternative to calculating release numbers from birds being shot is to extrapolate backwards from the numbers of birds recorded as surviving following the shooting season and before the next year’s cohort has been released. Female pheasants (2.3 million) were calculated to be present in GB during the 2016 breeding season (Woodward et al. 2020). Assuming that each hen has a single partner (generous, because pheasants are a polygynous breeding species with single males holding harems of several females, although other males may be classed as non-reproductive satellites), there are 4.6 million pheasants present in the breeding season. If these are the all the remnant survivors of the released birds and 9% of released birds survive to the start of following breeding season (Hoodless et al. 1999), I estimate a release of 51.1 million pheasants. Seventy-two thousand and five hundred red-legged partridge territories were calculated to be present in GB during 2016 breeding season (Woodward et al. 2020). Assuming that each territory comprises a single male and female, and that survival of released partridges matches that of released pheasants to the same period (9%), I estimate a release of at least 1.6 million partridges. Wintering mallard in the UK include migrants so it is not possible to reliably attempt to calculate the size of mallard releases based on breeding populations. It was estimated that there were 665,000 mallard individuals present during winter 2012/2013–2016/2017 (Woodward et al. 2020), but these could include both wild and released birds. By combining values for pheasants and partridges (but excluding mallards due to unreliable data), I estimate that 52.7 million birds could be released annually.
What is the species composition of the released birds?
Four datasets report relative numbers of the three species of released gamebirds (Table 1). These mean values of released birds are similar to percentages reported as shot in PACEC (2014): pheasants 71%, partridges 24% and duck 5%.
Depending on the approach taken, I derived 4329 estimates of total numbers of birds being released annually with a mean of 43.6 million and range between 11.5 and 135.3 million. These estimates were derived by one of 12 different methods. The distribution of estimates across methods was unbalanced with 8 methods being based on < 10 estimates and one method based on 4096 estimates. When I weighted all 12 estimate methods equally, I obtained a mean of 43.2 million (95% CI 29.0–57.3 million, range = 14.7–106.1 million) birds released annually. These estimate methods fall into four rough groupings which approximately double in their mean value. The two estimation methods that depend on APHAPR2020 data on numbers of birds held for rearing or for release give mean values of ~ 15 million birds being released. The four estimation methods that depend on data reported by shoots about the numbers of birds that they release and harvest give mean values of ~ 30 million birds. A broad set of four different estimation methods including those based on BTO bird surveys, assumptions about gamekeeper numbers, breeding calculations from UK and EU sources and the estimated number of days that individual guns shoot produced estimates of ~ 45–60 million birds and the previous calculation by Aebischer (2019) falls within this range. Finally, the estimation method dependent on the number of guns involved in game shooting in the UK produced a notably higher mean estimate of 106 million birds.
By sharing the mean estimate for total gamebirds being released of 43.2 million birds between the mean and extreme proportions of each species reported in the datasets in Table 1, it is probable that 31.5 million pheasants (range 29.8–33.7 million), 9.1 million red-legged partridges (range 5.6–12.5 million) and 2.6 million mallard (range 0.9–6.0 million) are released annually in the UK.