APEP4 (Woodward et al. 2020) provided breeding population size estimates for 226 species of British birds, and winter population size estimates for 104 species (including 27 species without breeding populations). All estimates were converted into number of individuals by doubling those presented in terms of number of breeding pairs, females, males or territories, following the methods used in Blackburn and Gaston (2018). Thus, we convert the APEP4 figures of 2.3 million breeding female Common Pheasants and 72,500 Red-legged Partridge territories into estimates of 4.6 million and 145,000 breeding birds, respectively. Unlike in this previous paper, we included seabirds in the biomass estimates. Some seabird species that breed in Britain and that have a large wintering biomass, pass the winter to a greater or lesser degree in coastal waters. There they feed alongside a range of species, such as ducks, cormorants, and grebes, that use inshore and offshore waters to different extents throughout the year. Including seabirds allows us more easily to track annual variation in biomass across the whole British bird assemblage, but excluding them would largely serve to increase the relative contributions of non-native galliforms to biomass in any given month.
We next derived rough estimates of how the population sizes of these species varied from month to month across the year. To produce these estimates, we collated information on adult mortality (survival) rates, mean date of first nesting, clutch size, number of clutches per year, time taken to fledge (nesting plus fledging periods), and average breeding success (proportion of eggs laid that successfully fledged), mainly from the BTO website (Robinson 2005), but with some data (principally for average breeding success) coming from Birds of the Western Palearctic (Cramp 1985, 1988, 1992; Cramp and Perrins 1993, 1994a, b; Cramp and Simmons 1977, 1980, 1983).
We began by assuming that the breeding population size provided by Woodward et al. (2020) was, for most species, the number of breeding birds present in April. For resident British species with minimal immigration or emigration (e.g. Crested Tit Parus major), we further assumed that the population size varied across the year because of the death of adults (as described below) and the fledging and subsequent death of juveniles. New birds were added to the population at a date (i.e. in a month of the year) that depended upon the mean date of first nesting, the time taken to fledge (nesting plus fledging periods), and the number of clutches per year. The number of new birds added in each month was calculated from the average clutch size and the average breeding success for a species (Fig. 1a).
We then assumed that annual adult survival, M (%), entailed the loss of individuals at a constant rate across the following 12 months, so that by the following April M% of breeding adults from the previous April were still alive. We also assumed that breeding populations are neither increasing nor decreasing. This assumption will not be true for many species in the British avifauna, but annual changes will in most cases be small, and even changing populations will fluctuate between good and bad years. Assuming a constant stable British bird population greatly simplifies the calculations, and means that the 100-M% of adults dying in each year were exactly replaced by recruits to the breeding population from reproduction. We assumed that the new recruits added to the population through reproduction died (and hence were removed from the population) at an exponential rate, subject to the assumptions about recruitment described below. Temporal variation in post-fledging mortality is not well known, but studies of U.K. passerines suggest that a high proportion (40–70%) of fledglings die in their first month or two out of the nest (Siriwardena et al. 2000).
Different bird species take different times to reach maturity. If time to maturity is t, we assumed that the recruits died at an exponential rate such that the number remaining to breed in April of year t was 100-M% of the original breeding population. For example, the Great Tit Parus major takes 1 year to reach maturity, and has an adult survival rate (based on data from the British Trust for Ornithology (BTO) website; see below) of 54.2%. For this species, birds fledged in year 0 were assumed to die at a rate that leaves a number equal to 45.8% of the breeding population alive in April of year 1. The total population size of this species varies from month to month depending on this pattern of birth and death, but returns to the breeding population size estimate from Woodward et al. (2020) in April. For species that take longer than one year to reach maturity, we assumed that the process of juvenile recruitment and death spans this period of maturation. Thus, for example, the Canada Goose Branta canadensis takes 3 years to reach maturity, and has an adult survival rate of 72.4%. For this species, birds fledged in year 0 die at a rate that leaves a number equal to 27.6% of the breeding population alive in April of year 3. However, in addition to the breeding population, in any given year there are also 1 and 2-year old geese in the population. The total population size in April is then the sum of these maturing birds and the breeding population size from Woodward et al. (2020). The total Canada Goose population size (and likewise also the Canada Goose biomass) in April is therefore larger than just the breeding population (Fig. 1b).
Many species in the British avifauna are present entirely or overwhelmingly as either summer or winter visitors. For summer migrants, we assumed that the entire breeding population arrived in April (or in a few cases May), and left in September. The imperative to breed means that these assumptions are probably reasonable—individuals will tend to arrive largely simultaneously, at least on the time-scale of months. We then assumed that the size of the population in Britain varied between the months of arrival and departure as described for resident species (adult mortality, fledgling recruitment and subsequent mortality, and no variation in breeding population size from year to year).
We used Balmer et al. (2013) and Wernham et al. (2002) to identify when populations of winter visitors to Britain generally arrived and departed. We typically assumed that the entire wintering population arrived together and early on, and departed together and late, in the normal wintering period. This is likely to overestimate numbers present in autumn and spring, as the arrival and departure of individuals in most wintering species is more flexible than for breeding species, may depend on weather in Britain and elsewhere, and spans a period of several weeks. During winter, we assumed that the population decreased following adult mortality rates, and from the wintering population size estimated by Woodward et al. (2020).
Some species breed and winter commonly in Britain, with the wintering population bolstered by immigrants from continental Europe, or with the wintering population reduced by emigrants. Estimating population changes over the year in these species is more difficult. Where Woodward et al. (2020) provided separate breeding and wintering population size estimates, we assumed that the breeding population varied across the year as described for species with minimal immigration or emigration, and then scaled the population size outside the breeding season (i.e. September to April) on the basis of the reported wintering population size, assuming this population size was reached in December and January (Fig. 1c). In some cases, this meant assuming emigration in autumn and immigration in March, and in other cases the reverse. These estimates are crude, but in fact there are relatively few species of this sort, and their biomass is generally small relative to primarily resident species.
Finally, for Common Pheasant and Red-legged Partridge, we assumed that annual variation in the breeding population followed the methods described above for resident species. For Common Pheasants, we used the figures in Robertson (1991) to produce an estimate of a single fledged juvenile per female, which is added to the population at the end of May, followed by exponential mortality of these first-year birds through to the following breeding season. Coupled with adult mortality, this suggests that the breeding population is not self-sustaining. However, the resident population is supplemented by the numbers released in summer, which we assumed were as recorded by GWCT (2018): 43 million Common Pheasants. These are slightly lower than the most recent figures (Aebischer 2019), but estimates of release sizes are likely to have high uncertainty, and these are closer to the centre of the current likely range (see below). We assumed that all the Pheasants were released in August, and that 25% of these died before the start of the shooting season on 1st October, as estimated by the Game and Wildlife Conservation Trust (GWCT). GWCT also estimates that 16% of released Pheasants survived until after the shooting season ends on 1st February (https://www.gwct.org.uk/research/species/birds/common-pheasant/fate-of-released-pheasants/). We therefore assumed, starting from October, that just over two-thirds of the releases alive in one month had died by the next month, and that this mortality rate continues through to the start of the next breeding season. This results in around 2 million birds surviving to contribute to the next year’s breeding population, bringing that back to 4.6 million birds. For Red-legged Partridges, we assumed that birds were released in July, and that 25% of these died before the start of the shooting season on 1st September. We then assumed that, starting from September, half of the releases alive in one month had died by the next month, with none of the birds left alive in March surviving to April. In fact, many Pheasants and Partridges are likely to have been released before August and July, respectively, and so our estimates of the overall contribution of these birds to annual British bird biomass is probably conservative.
The calculations described were used to produce estimates of the proportion of the British breeding population size (from APEP4) present in Britain in each month of the year. We then used these proportions to calculate the British population size for each species in each month. We used estimates of average body mass (from Gaston and Blackburn 2000; Dunning 2007) to calculate the biomass for each species in each month, assuming no annual variation in average mass. The mass for Common Pheasant in our data originated from Cramp and Simmons (1977), and at 850 g is low relative to estimates from other sources (female mass of 951 g from Storchova and Horak (2018), mean mass of 1.135 kg from Dunning (2007)). We may therefore underestimate Common Pheasant biomass, although probably not by much. Those birds added to the population through reproduction in spring will not be fully grown, while the same will be true of the reared birds released in summer (the vast majority of the population), which do not attain full adult body mass until the autumn.
We produced these intra-annual biomass estimates for 81 of the 253 species breeding or wintering in Britain as listed by APEP4 (Woodward et al. 2020). These 81 species represent 93.4% of the total breeding biomass, as many of the species breeding in Britain have small breeding population sizes, small body masses, or both, and so contribute negligibly to overall biomass (Fig. 2). Indeed, the top four species in our analysis account for more than 50% of British breeding biomass, while the species with the most biomass missing from our analysis (Common Buzzard Buteo buteo) accounts for just over 0.5%. The species considered also included those with large wintering biomasses (principally wildfowl and waders) to ensure that the winter avifauna of Britain was not under-represented: 82% of the biomass of species with winter population estimates given by Woodward et al. (2020) is included in our 81 species, although most winter biomass is actually in resident species without separate winter population size estimates (and > 90% of our estimate of total January biomass comes from resident species). The data used are given in an Online Appendix.
While there is undoubtedly uncertainty in all of the biomass estimates used in our analysis, the most significant is likely to relate to the number of Common Pheasants and Red-legged Partridges released in autumn. Madden and Sage (2020) highlight that the usually quoted figures (e.g. Aebischer 2019; Avery 2019) include high levels of uncertainty, because non-native galliform releases are not formally documented. They therefore explored a range of different methods of estimating release numbers, including the APHA poultry register and import records, in addition to the National Gamebag Census (on which the figures in Aebischer (2019) were calibrated). These multiple approaches suggest that numbers released could lie anywhere in the range 10–57 million Pheasants, and 1–13 million Partridges. We therefore re-ran our analyses with the lower and upper numbers in this range, to assess how the maximum likely uncertainty affects our conclusions.