The Common Buzzard in Britain: a new population estimate

Transcription

1 The Common Buzzard in Britain: a new population estimate Rob Clements Robin Prytherch ABSTRACT During recent years, the Common Buzzard Buteo buteo has spread eastwards from its former strongholds in western and northern Britain. At the same time, there is evidence that the species is now breeding at a much higher density than previously recorded, largely as a result of consolidation and in-filling in areas of prey-rich farmland habitat. An increase in the Rabbit Oryctolagus cuniculus population, and a reduction in persecution are both likely to be important contributing factors.this paper summarises recent data on Common Buzzard population densities from survey work in many parts of Britain. A new national population estimate, based on an analysis of the habitats found in its range, suggests that there were between 44,000 and 61,000 territorial pairs in Consequently, the Common Buzzard is probably now the most abundant diurnal raptor in Britain. Past population estimates The first post-war population estimate for the Common Buzzard Buteo buteo in Britain set the figure at 12,000 breeding pairs (Moore 1957). This was revised downwards, to 8,000-10,000 pairs, after the Atlas confirmed that the species continued to be restricted to western British Birds 95 August and northern Britain (Tubbs 1974; Sharrock 1976). During the 1980s, a survey by the BTO in 1983 estimated the population to be 12,000-17,000 pairs (Taylor et al. 1988). Writing in the New Atlas, Colin Tubbs saw no reason to increase this figure, noting that there had probably been little change since the pre- 377

2 David M. Cottridge/Windrush 240. Common Buzzard Buteo buteo,tregaron, Wales. vious Atlas (Gibbons et al. 1993). Now, the combined evidence of an increased density of territorial pairs in farmland habitats, together with an expansion of the breeding range eastwards, suggests that a new population estimate is appropriate. Towards a new population estimate A comparison between recent survey work on the Common Buzzard in Dorset (Kenward et al. 2000) and in Shropshire (Sim et al. 2001) illustrates the problems in estimating the size of raptor populations. In Dorset, where a population of individuals was estimated to occur within a 120-km 2 study area, only 31 confirmed breeding pairs were found (including an estimated six failed breeding attempts), suggesting that only 25% of the population attempted to breed. By contrast, in a hectad (10-km square) study area in Shropshire, 81 territorial pairs were found, of which 71 pairs bred. Since different studies use different methods, and generate data to answer particular questions, such as breeding productivity, prey species identification, population estimates etc., drawing overall conclusions about breeding density from a number of studies is fraught with difficulty. An estimate of pairs holding territory in suitable habitat in the breeding season will clearly provide a higher figure than the total number of pairs proven to breed successfully. Furthermore, data from one study area can be interpreted in different ways, leading to wide variations in population estimates. In the Dorset study referred to above, the data could be interpreted as showing only 31 breeding pairs within 120 km 2 or, alternatively, that perhaps as many as 120 pairs held territory within the same area. Does the Shropshire study indicate that a much greater proportion of the population bred each year, or that the total population was higher, with a similar percentage of non-breeders? In the latter study, which concentrated on breeding and prey-species data, no attempt was made to estimate the total number of individuals involved, so the reasons for the apparent difference must remain unknown. Perhaps the Shropshire study area was still in the process of colonisation, while in Dorset the Common Buzzard population had reached saturation point some time ago, in an area of lower prey resources. The lower percentage of two- and three-year-old buzzards breeding in Dorset compared with an experimentally released population in Sussex supports this theory (Kenward & Walls 2002). Owing to a radio-tracking programme of Common Buzzards in Dorset, it has been possible to estimate the number of birds which are present but not breeding in this study area. This is obviously much harder to do in areas where such tagging has not been carried out. It is possible that a 378 British Birds 95 August

3 similarly high percentage of non-breeders may exist in other well-established populations. A population estimate for any species must first define the unit of population to be used, whether breeding pairs, pairs attempting to breed, pairs holding territory, or individual birds. In this paper, the estimate refers to territorial pairs, since this unit of population is most easily compared between different study areas. To provide a more accurate population estimate than is possible here, survey work in other parts of the Common Buzzard s range will be required. Such work is most likely to provide data concerning territorial pairs, since proof of nesting or confirmed breeding is both time-consuming and may require access to private farmland and woodland. As the above studies show, the percentage of territorial pairs which fail to breed will vary between areas, and probably also between years, as food supply and other variables fluctuate. In a north Somerset study area (see below), the percentage of territorial pairs breeding successfully has shown marked variation, with 41.6% successful in 1998 compared with 67.5% in 2001 (R. Prytherch in litt). Common Buzzard breeding densities in Britain Table 1 illustrates the wide variation in breeding density of Common Buzzards across different habitats during the past 30 years. The first five study areas, grouped together as farmland, are comprised of predominantly mixed farmland, with 10-30% woodland. The north Somerset and SO37 densities far exceed those recorded in the past, when most surveys took place in traditional upland habitat. The reason for the lower density of breeding buzzards in the Dorset study area is uncertain, but the scarcity of mammal prey on heathland, especially Rabbits Oryctolagus cuniculus, and also a dearth of earthworms, are likely factors (Graham et al. 1995). In the nearby New Forest, Hampshire, the density of breeding buzzards has declined from pairs per hectad in the 1970s to 6-7 pairs per hectad in the early 1990s (Clark & Eyre 1993). Again, a deficiency of prey is perhaps the reason for such low densities. In general, the highest concentrations of Common Buzzards occur in areas of mixed farmland and pasture with a substantial proportion of deciduous or mixed woodland areas with a plentiful supply of prey. A longterm study in north Somerset has shown a steady increase in numbers from 13 pairs in 1982 to 26 pairs in 1991, in a 75-km 2 study area. By 1996, the total had reached 56 pairs, while in 2001, 84 pairs held territory, equivalent to 111 pairs per hectad (R. Prytherch, verbally). In Shropshire/Herefordshire (SO37), a density of Table 1. A summary of recorded breeding density in studies of the Common Buzzard Buteo buteo in Britain. Study Area Study Territorial Breeding Altitude Source period density density (m) (pairs/hectad) (pairs/hectad) Farmland recent North Somerset (75 km 2 ) Prytherch 1997/verbally Bath & northeast Somerset (60 km 2 ) J. Holmes (verbally) Shropshire/Herefordshire (SO37) Sim et al Worcestershire/Shropshire (SO77) Sim et al Dorset (120 km 2 ) 1996 * Kenward et al Farmland/Upland Postbridge, Devon (33 km 2 ) Dare 1998 Devon (2,620 km 2 ) Sitters 1988 Cambrian Mountains (475 km 2 ): Farmland <330 Newton et al Upland >330 Newton et al Snowdonia (926 km 2 ) ,085 Dare 1995/verbally ,085 Dare 1995/verbally Denbigh, Clwyd (440 km 2 ) Dare 1995 Upper Strathspey (94 km 2 ) Halley Halley 1993 * individuals British Birds 95 August

4 Peter Cairns/Windrush 241. Common Buzzard Buteo buteo, feeding on dead Roe Deer Capreolus capreolus buck, Strathspey, Highland. 81 pairs per 100 km 2 was found in similar habitat in (Sim et al. 2001). By contrast, in the SO77 study area (Worcestershire/Shropshire), only 22 territorial pairs per hectad were recorded. This area was close to the eastern edge of the Common Buzzard s range in 1991 (indeed, breeding was not confirmed during the New Atlas), and these relatively low numbers reflect the recent and probably incomplete colonisation of this area. It seems most probable that reduced persecution and a resurgence in the Rabbit population (Harris et al. 1995) are the most important factors underlying the Common Buzzard s success. The density of buzzards found in the Shropshire and north Somerset study areas may not be representative of the same habitat throughout Britain. For example, in Easter Ross, a density of territorial pairs per hectad was estimated for a farmland study area, where continuing persecution was thought to be a limiting factor (R. Swann, verbally). In west Moray, localised concentrations were found in Forestry Commission plantations, but fewer birds were found in adjacent farmland, again owing to persecution (Swann & Etheridge 1995). Nonetheless, there is mounting evidence that population density in mixed farmland is now far greater than that found in predominantly upland areas at the time the last population estimate for Britain was established. It is, of course, possible that the high densities recorded in recent studies are exceptional. In the largest study area, 2,620 km 2 of Devon farmland, there were territorial pairs per hectad in In the absence of more recent data, we can only speculate whether this area now boasts similar concentrations to those in Shropshire and north Somerset. Some study areas, such as those in Devon and the Cambrian Mountains, include both farmland and areas of upland sheepwalk and moorland. Observations in the Cambrian Mountains clearly illustrate the higher breeding density on farmland. In Devon, buzzard territories were smaller and more closely spaced in farmland, while larger, more widely spaced territories were characteristic of open moorland and conifer plantations, where food supply was more limited. In general, the decreased availability of prey, especially Rabbits, in upland areas leads to comparatively sparse concentrations of buzzards, and densities below 20 pairs per hectad are common. The increase in Common Buzzards since 1983 In both the farmland and upland components of the Common Buzzard s range, there is evidence of rising numbers since In the north Somerset study, there was an increase of over 500% between 1982 and 2001 (R. Pry- 380 British Birds 95 August

5 therch, verbally). Observations in the West Midlands, comparing original data from 1983 with sightings in 1996, found that soaring buzzards increased by 118% in the main part of the range, and by 348% in those tetrads at the edge of the range (Sim et al. 2000). In Snowdonia, the increase was more modest, being 40-50% over a similar period (Dare 1995). In north Breconshire, the buzzard population rose by 115% during the period , while the number of occupied tetrads grew by 51% during the same period (Shrubb 2000). At Postbridge, Devon, numbers increased by about 70% between and , reflecting a recovery from the cold winter of 1962/63 and the impact of myxomatosis, to reach levels similar to those recorded in In addition to higher densities in core parts of the range, careful assessment of the eastward spread and infilling which has occurred since the New Atlas is equally important when considering current population levels. Fig. 1 (based on Clements 2000) illustrates the Common Buzzard s range expansion since There are now substantial populations in Hampshire, Northamptonshire, Northumberland, Oxfordshire, Sussex, Warwickshire and Yorkshire. In Scotland, almost all suitable habitat is now occupied. Since 1997, small breeding populations have been established in the remaining eastern counties of Kent and Suffolk. While pairs are still widely scattered in some eastern counties, buzzards are probably approaching maximum breeding density in the west. For example, in west Oxfordshire, Common Buzzards are now estimated to occur at a level of around 20 pairs per hectad (R. Scroggs, verbally). Similarly, in SO77 (Worcestershire/Shropshire) 22 territorial pairs were found in 1996 in an area where there was no evidence of breeding during the New Atlas (Sim et al. 2001). In Essex, Hertfordshire and Kent, the population density remains far below the levels recorded in the west, although it is increasing rapidly. Much of this area contains prime buzzard habitat, which may eventually hold a population density of more than 50 territorial pairs per hectad. The Common Buzzard in Britain Fig.1. Map showing the spread of the Common Buzzard Buteo buteo in Britain up to The breeding range of Common Buzzard during the New Atlas fieldwork, (Gibbons et al. 1993). Approximate breeding range of Common Buzzard in 1997 (after Clements 2000). A population estimate of Common Buzzards for 2001 By analysing Ordnance Survey maps of the core part of the range, as identified by the New Atlas, the 1,425 hectads within this area were categorised into one of five broad habitat types, using the dominant habitat within each hectad (table 2). The number of hectads is lower than that in which Common Buzzards were recorded in the New Atlas period (1,544), since coastal hectads with less than 50% land area, together with squares in areas outside the main breeding range, have been excluded. As in the New Atlas, montane hectads were assumed to contain no buzzards, as were urban/suburban ones. The upland/conifer plantation category, which includes areas of unenclosed sheepwalk, as well as heather moorland, was assumed to support an average of 20 pairs per hectad. Parts of British Birds 95 August

6 Habitat No. hectads Density (pairs/hectad) Total (territorial pairs) Montane plateau Upland/conifer plantation ,140 Farmland, >5% woodland * ,760-37,140 Farmland, <5% woodland * ,520 Urban/suburban Total 39,420-51,800 Newly colonised, ,510-9,020 GRAND TOTAL 43,930-60,820 * approximate proportions Table 2. A population estimate of the Common Buzzard Buteo buteo in Britain. Peter Cairns/Windrush Britain with dominant heathland/woodland, such as the New Forest, are included in this category. Areas of farmland with more than 5% woodland are likely to be similar to the mixed farmland category where high densities of Common Buzzards have been recorded recently. For such squares, the average of pairs is a compromise between the higher levels found in north Somerset and Shropshire, and the lower levels recorded in Dorset and Easter Ross Common Buzzard Buteo buteo, Strathspey, Highland. Farmland with less than 5% woodland was allocated a lower mean density, since at least some of this habitat, even in northern and western Britain, is likely to consist of intensive arable farmland, where the availability of both nest sites and prey species will be lower than in mixed farmland or pasture. In addition, the level of persecution may also be greater here. In the absence of much data from this type of habitat, an average of 20 pairs per hectad was estimated. In those parts of Britain colonised since 1991, pairs per hectad were estimated, owing to the variation in density within such areas. Is the resulting estimate, of 43,930-60,820 territorial pairs of Common Buzzards in Britain, accurate? Until more surveys are carried out in other parts of the range, the answer must remain uncertain. Even if the highdensity farmland study areas described here prove to be exceptional, such that perhaps only 10% of such farmland would support more than 80 territorial pairs per hectad, the population would now be 35,000-40,000 pairs. If, however, such densities are commonplace in this habitat, the current population estimate would rise to 65,000-70,000 territorial pairs. Extrapolating further, these figures suggest a possible future population of more than 70,000 territorial pairs in Britain. Indeed, if the high concentrations found in some study areas recently are duplicated across similar habitat in the future, perhaps as a result of continuing growth in the Rabbit population and further reductions in persecution, then a figure of 100, British Birds 95 August

7 Richard Brooks/Windrush 243. Common Buzzard Buteo buteo, Wales, March territorial pairs may be realistic. Similarly, a resurgence of myxomatosis, or other diseases in the Rabbit population, would check further spread. The Common Kestrel Falco tinnunculus population at the time of the New Atlas was thought to be about 50,000 pairs (Gibbons et al. 1993). Since then, the population may have fallen; data from the BTO s Breeding Birds Survey suggest a reduction of 30% between 1994 and 1999 (Baillie et al. 2001). As such, the Common Buzzard is probably now the most abundant diurnal raptor in Britain. Acknowledgments Thanks are due to Peter Dare, Brian Etheridge, Kathy Hodder, Jeff Holmes, Robert Kenward, Robin Prytherch, Steve Roberts, Royston Scroggs, Innes Sim and Robert Swann for their assistance and advice. References Baillie, S. R., Crick, H. Q. P., Balmer, D. E., Bashford, R. I., Beaven, L. P., Freeman, S. N., Marchant, J. H., Noble, D. G., Raven, M. J., Siriwardena, G. M.,Thewlis, R., & Wernham, C.V Breeding Birds in the Wider Countryside: their conservation status 2000.BTO Research Report 252, BTO,Thetford. Clark, J. M., & Eyre, J. A Birds of Hampshire. Hampshire Ornithological Society. Clements, R Range expansion of the Common Buzzard in Britain. Brit. Birds 93: Dare, P. J Breeding success and territory of Buzzards Buteo buteo in Snowdonia and adjacent uplands of North Wales. Welsh Birds 1: A Buzzard population on Dartmoor, Devon Birds, no. 51. Gibbons, D. W., Reid, J. B., & Chapman, R. A The New Atlas of Breeding Birds in Britain and Ireland London. Graham, I. M., Redpath, S. M., & Thirgood, S. J The diet and breeding density of Common Buzzards Buteo buteo in relation to indices of prey abundance. Bird Study 42: Halley, D. J Population changes and territorial distribution of Common Buzzards Buteo buteo in the Central Highlands, Scotland. Bird Study 40: Harris, S., Morris, P., Wray, S., & Yalden, D A Review of British Mammals: population estimates of British mammals other than cetaceans. Peterborough. Kenward, R. E., & Walls, S. S Buzzard News CEH Newsletter.,, Hodder, K. H., Pakhala, M., Freeman, S. N., & Simpson,V. R The prevalence of non-breeders in raptor populations: evidence from rings, radio-tags and transect surveys. Oikos 91: Moore, N. W The past and present status of the Buzzard in the British Isles. Brit. Birds 50: Newton, I., Davis, P. E., & Davis, J. E Ravens and Buzzards in relation to sheep farming and forestry in Wales. J. Appl. Ecol. 19: Prytherch, R Buzzards. BBC Wildlife 15: Sharrock, J.T. R The Atlas of Breeding Birds in Britain and Ireland. Berkhamsted. Shrubb, M An increase in the Buzzard population in North Breconshire. Welsh Birds 2: Sim, I. M. W., Campbell, L., Pain, D. J., & Wilson, J. D Correlates of the population increase of Common Buzzard Buteo buteo in the West Midlands between 1983 and Bird Study 47: , Cross, A.V. Lammacraft, D. L., Pain, D. J., & Wilson, J. D Correlates of Common Buzzard Buteo buteo and breeding success in the West Midlands. Bird Study 48: Sitters, H. P. (ed.) Tetrad Atlas of the Breeding Birds of Devon. Devon Bird Watching and Preservation Society. Swann, R. L., & Etheridge, B A comparison of breeding success and prey of the Common Buzzard Buteo buteo in two areas of northern Scotland. Bird Study 42: Taylor, K., Hudson, R., & Horne, G Buzzard breeding distribution and abundance in Britain and Northern Ireland in Bird Study 35: Tubbs, C. R The Buzzard. Newton Abbot. Rob Clements, 8 Harrier Drive, Sittingbourne, Kent ME10 4UY British Birds 95 August