MONITORING BREEDING BIRDS IN THE UNITED KINGDOM

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1 R. GREGORY, S. BAILLIE & R. BASHFORD, Monitoring breeding birds in the United Kingdom. In: Anselin, A. (ed.) Bird Numbers 1995, Proceedings of the International Conference and 13 th Meeting of the European Bird Census Council, Pärnu, Estonia. Bird Census News 13 (2000): MONITORING BREEDING BIRDS IN THE UNITED KINGDOM R. D. Gregory 1,2, S. R. Baillie 1 & R.I. Bashford 1,2 ABSTRACT. A new annual survey of terrestrial breeding birds was introduced in the United Kingdom (UK) in The aim of the scheme, known as the Breeding Bird Survey (BBS), is to monitor population levels of a broad spectrum of common and widespread species across a representative sample of sites and habitats in the UK. Population changes will be interpreted in the light of habitat and other environmental data sets. A primary objective of the scheme is to identify species of conservation concern and, in combination with other data from the British Trust for Ornithology's Integrated Population Monitoring Programme, to provide pointers to the causes of population changes. Survey squares, 1 1 km squares of the UK National Grid, are chosen as a random sample stratified by potential observer density. Birds are counted twice during the breeding season using standardised line transects. Birds are recorded in three distance categories from the transect line or as in flight. Habitat data are recorded annually for each 200 m section of transect. In the first year of the scheme, 1994, over squares were surveyed and 190 species recorded. A total of 76 species were recorded from over 100 separate squares. We aim for the sample size to rise to squares over the next few years. Here we describe the background to the introduction of the BBS, details of survey and methods, and preliminary results from British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, United Kingdom. 2. Current address: The Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire, SG19 2DL United Kingdom. INTRODUCTION The need to monitor wildlife populations has arguably never been so great with rapid development, urbanisation and industrialisation across the globe. Effective bird conservation is not possible without monitoring information to tell us how population levels are changing, and ideally, providing pointers as to why these changes are taking place. The monitoring of bird populations has the added advantage that birds can act as valuable barometers or indicators of the general health of the countryside (Furness & Greenwood 1993). Large-scale surveillance of birds in the UK is made possible by the number of skilled and dedicated ornithologists who are willing to participate in survey work. Compared with other taxa, birds are relatively easy to detect and identify, particularly in the breeding season. The British Trust for Ornithology (BTO) has a long history of involvement in monitoring and associated research. It is the unique partnership between skilled volunteers and the professional staff that sets their research programme apart from other organisations in the UK. The BTO works closely with other organisations concerned with the conservation of birds, particularly by the statutory conservation bodies for the UK, the Joint Nature Conerservation Committee (on behalf of the country agensies), and the Royal Society for the Protection of Birds (RSPB). The increasing collaboration among organisations reflects mutual interest and concern for the fate of bird populations in the countryside. The statutory requirement to monitor birds arises in law from the Wildlife and Countryside Act 1981, the European Union Birds Directive, and the Ramsar, Bonn and Berne Conventions

2 The scope of monitoring A total of 215 bird species breed in the UK on a regular basis (Gibbons et al. 1993). Over 80 % of those species are censused or surveyed on an annual basis through a variety of surveys supported by government and non-government conservation organisations (Fig. 1: Greenwood et al. 1995). Specific surveys cover population changes of the Heron, wetland birds (10 species), seabirds (22 species), and rare breeding birds (56 species: being defined as having populations of fewer than 300 pairs). We envisage that a small number of birds (27 species) can be covered by improved collation of data collected by bird clubs, reserve wardens and raptor monitoring groups (Fig. 1). There is a group of ten species for which there is no specific survey at present. Figure 1. Monitoring of terrestrial breeding birds in the United Kingdom. The graph shows the approximate number of species monitored by various annual surveys and recording groups. The proposed monitoring of species covered by raptor and owls monitoring, and improved data collation, have yet to be implemented. Data from autumn Wetland Bird Survey counts would be used to monitor a small number of breeding waterfowl which are not covered by other schemes. The remaining breeding birds (89 species) represent the common and widerspread species of the countryside and these form the focus of BTO's annual monitoring programme. At this scale, it is not feasible nor efficient to count or estimate total population sizes as it is for the rarer species, and monitoring must be based on sample surveys. For the information to be of greatest use, it is crucial for the sample of sites to be representative of the countryside as a whole. The primary objective of bird monitoring, or more strictly surveillance, is to provide an index of population changes from year to year. Although the terms monitoring and surveillance tend to be interchanged in the literature, monitoring implies (1) the measurement of changes against predefined standards of targets, (2) the collection of supplementary data to help intepret and understand population changes, and (3) that there are clear objectives (Baillie 1990, Furness & Greenwood 1993)

3 COMMON BIRD CENSUS The Common Bird Census (CBC) has been the main monitoring tool for common birds in the UK over the last 30 years. The CBC is based on territory mapping and is one of the longest running schemes of its kind in Europe, having been established in the early 1960s (Marchant et al. 1990). It has proved highly valuable in advancing our knowledge and understanding of population fluctuations among British birds (e.g. O'Connor & Shrubb 1986, Marchant et al. 1990, Baillie 1990, Greenwood & Baillie 1991, Peach et al. 1991, Fuller et al. 1995, Greenwood et al. 1995). There is general consensus that territory mapping provides a reasonable estimate of most territorial populations (Dawson 1981, Verner 1985, Verner & Ritter 1988, Bibby et al. 1992) and the aims and methods of the CBC have been reviewed on a periodic basis (O'Connor & Fuller 1984, Marchant et al. 1990, Baillie & Marchant 1992). Despite the considerable achievement of the CBC, the scheme has a number of limitations and the BTO has been exploring alternative methods for population monitoring for some time (Baillie & Marchant 1992, Gregory et al. 1994). The limitations of the CBC are as follows: 1. Survey sites are not chosen at random. In consequence, there is the potential for species, geographical and habitat bias. In fact, the CBC has been restricted to farmland and woodland, and other habitats have been excluded. The distribution of survey plots mirrors the distribution of the human population and is baised towards the south and east of the UK. 2. Fieldwork, data compilation and mapping analysis are extremely time consuming (Gregory et al. 1994). Fieldwork involves nine or ten visits to a census plot each breeding season and many hours in data collation. Note that in Britain territory analysis is carried out by BTO staff rather than by volunteers, as is the case in other countries (van Dijk 1992). The time required for territory mapping fieldwork and analysis is roughly seven times that of an equivalent technique, such as point counts (Gregory et al. 1994). Taken together, these factors have limited the expansion of CBC coverage in the UK and to some extent limited the use of the data, although much can be done at the analytical stage to alleviate problems (Greenwood et al. 1995). Over a number of years the BTO has tested a number of new approaches to population monitoring in a UK setting (Gregory & Baillie 2004). The aim is to develop a monitoring scheme based on a formal sampling strategy for volunteer birdwatchers which uses simple and efficient methods and thus allows large numbers of people to participate over a wide geographical area. ALTERNATIVES TO TERRITORY MAPPING Line and point-count transects The obvious alternatives to territory mapping are line or point count transects, both of which are relatively efficient and have been used widely in population monitoring in Europe and North America (Bibby et al. 1992). A number of field trials have tested the suitability of these methods (Gregory et al. 1994, Gregory & Baillie 2004). They showed the comparability of monitoring results derived from different counting methods, the efficiency of the methods, and the practicability of using randomly chosen survey sites in the UK. A line transect approach was recommended because it was preferred by volunteers and was slightly more precise. Furthermore, line transects can be applied effetively in a wider range of habitats than point counts (Bibby et al. 1992)

4 The final, and arguably most important, aspect of the new scheme was the formal sampling strategy. Our trials, and similar experience with national surveys of single-species (Shrubb & Lack 1991, Donald & Evans 1995), have demonstrated the feasibility of a random sampling design in the UK. The advantages of this are considerable as it provides the only mechanism for producing what is demonstably unbiased and thus representative information (Bibby et al. 1992). A desk-based study assessed a number of different strategies for the selection of survey squares (Gregory & Baillie 1994, 2004). These simulations showed that simple random sampling compared very well with any of the more complicated strategies involving landscape type. It was concluded that a random selection, stratified by potential observer density, should be used for the new survey. BREEDING BIRD SURVEY Against this background, an annual survey of terrestrial breeding birds was introduced in the UK in 1994 ( The strategic aims of the BBS are: 1) to be based on a formal randomised sampling design, 2) to improve the geographical representation of monitoring, 3) to improve the habitat representation of monitoring, and 4) to increase species coverage of monitoring (as a consequence of the other points). The BBS thus aims to provide precise information on year-to-year and longer term changes in population levels for a broad spectrum of common species across a range of regions and habitats. The parallel recording of land use and habitat change will facilitate a better understanding of the factors responsible for population changes, and this will be particularly important for populations in decline. In a wider context, the BBS aims to promote a greater understanding of the population biology of British birds through a partnership between large numbers of voluteer and a small number of professional ornithologists. Selection of survey squares Survey squares were selected as a random sample, stratified by potential observer density, from within regions across the UK (Gregory & Baillie 1994). The 128 BTO regions were reduced to 83 regions by the amalgamation of the smallest ones. Sampling from fewer, larger regions avoids the problem of extremely small administrative regions. The number of squares to be surveyed in each region was calculated by multiplying observer desity by a constant and setting a minimum level of coverage. We anticipate that professional will be necessary to ensure minimum coverage in remote areas. At a local level squares are allocated to volunteers through a network of voluntary regional organisers. Each organiser receives a list of target squares for their region with the instruction that squares should be allocated in strict order from the top downwards. This is essential in maintaining the random design of the survey. The same squares are surveyed year after year and new volunteers are found if the original volunteer drops out. The way in which observer changes are handled in the analysis will be addressed once sufficient data have been gathered. Regional organisers play a vital role in coordinating fieldwork and overseeing the return of data. In limited circumstances, they can report squares as being uncoverable, for example, because a landowner refuses access permission or the square is complelely inaccessible. In such cases, the square can effectively be ignored, although organisers submit information on land use and habitat type to check for any bias introduced by their exclusion

5 Survey methods BBS fieldwork involves three visits to a survey square each year. The first visit is to record details of the habitat and to mark the survey route (Fig. 2). Bird counts are carried out on the second and third visits. The survey route comprises two parallel lines, each 1 km in length (although for practical reasons there is often substantial deviation about the ideal). Each of these lines is divided into five sections, making a total of ten 200 m sections, and birds and habitats are recorded within these units. Habitat type and land use are recorded annually on a habitat form. This form describes both the habitat surveyed along the actual and the 'ideal' transect if there is a deviation. By recording the 'ideal' transect route we are able to access whether observers avoid or prefer particular habitats within their squares. Figure 2. An idealised BBS route through a 1 1 km square. The lower drawings show proposed survey routes through 1 1 km squares

6 All the survey forms were designed so that the data could be readily computerised. For example, coded information on species names, county codes, weather conditions, and habitat type allow detailed information to be summarised and input efficiently. Habitat information is recorded on the first reconnaissance visit to the square. The appropriate habitat codes are chosen from an established hierarchical system (Crick 1992), which is common to a range of BTO schemes. Observers record the primary and secondary habitat for each transect section with up to four levels of detail (Fig. 3). The primary habitat is the dominant habitat type within a particular transect section. In the example given, transect 1 comprises tilled land with a hedgerow without trees, an active farmyard, and the dominant crop being an autumn cereal. There is no secondary habitat within the section and so this space is left blank. Note that the ideal transect lies within 20 m of the actual route and the habitats do not differ (Fig. 2). Transect 2 is a similar area of farmland which contains a small woodland. The first habitat codes are the same as those above. The second codes are for a young coniferous plantation with low human disturbance, a moderate shrub layer, and sparse field layer. Note that the ideal route is 50 m from the actual survey route and the ideal goes through the young plantation. Figure 3. An example of a BBS habitat recording form (see text for details). Count visits are timed so that the first is in the early part of the breeding season (April to mid- May) and the second in the late part (mid-may to the end of June). It is recommended that visits should be at least four weeks apart. The average visit time is around 90 minutes. Volunteer counters are asked to begin their counts between 6 am and 7 am so that they coincide with maximum bird activity, but avoid concentrated song activity at dawn (Bibby et al. 1992). Volunteers record all the birds they see or hear as they walk methodically along their transect routes. They are encouraged to

7 pause, listen and scan for birds as they walk along their transects. Birds are noted in three distance categories (within 25m, between m, or over 100 m to either side of the line) measured at right angles to the transect line, or as in flight. Recording birds in distance bands is important because it gives a measure of bird detectability in different habitats and allows relative population density to be estimated using DISTANCE sampling (Bibby et al. 1992, Buckland et al. 1993). Note that the estimates of absolute densities derived using this method are based on a number of key assumptions. Birds are noted in the field on specially designed forms using two-letter species-codes, which were developed within the CBC (Fig. 4). Observers are encouraged to summarise this information on count summary forms as soon after the fieldwork as is convenient. Header information on the forms includes the observers' name, address, telephone number, the square reference, county code, date, whether the bird count was early or late in the season, weather conditions, and the starting and finishing times of the two halves of the transect (Fig. 4). Counties are recorded using 4-letter codes which are used across BTO schemes. Weather codes describe cloud cover, rain, wind, and visibility, on arbitrary three-point scale. We discourage bird counts in heavy rain, poor visibility, or strong winds. Figure 4. An example of a BBS field recording form and summary sheet (see text for details)

8 The majority of forms are returned to BTO Headquarters through our network of regional organisers. Thus organisers are in a position to chase up outstanding forms, answer queries and note interesting observations. BTO acknowledges the receipt of each form directly with each observer and we provide them with information on census work and related projects on a regular basis. On receipt, count summary and habitat forms are double-checked by staff for clarity and obvious errors, before being sent to be input by an outside agency. Field forms are kept for reference and to allow any further checking. A series of computer progams are used to check the computerised data set. These programs look for inconsistency or errors in the different codes (a number of similar species codes are commonly confused) and check for unlikely bird counts or occurences. Results from the first year Coordinates for a total of km squares were distributed to regional organisers in (64 %) of these squares were surveyed (Fig. 5), 734 (30 %) were not surveyed and 147 (6 %) were reported as uncoverable. A small number of squares 68 (4 %) were surveyed by professional teams in north and west Scotland funded by RSPB. This is an area with very few volunteer counters. The map shows an encouraging spread of survey squares although there are a number of gaps which we intend to fill in the coming years. The priority in 1995 has been to improve the proportion of squares that are surveyed and only those regions achieving a high level of coverage in 1994 received any new squares for The eventual sample size of BBS is to be km² squares. Figure 5. Distribution of Breeding Bird Survey squares in The sampling strategy is designed so that there are larger numbers of squares where there are more potential observers and the map shows a concentration of points around large cities

9 The range of species recorded in 1994 was extremely broad, the total species count was 190, with 76 species recorded from over 100 squares, and 21 species from between squares (Table 1). This suggests that we would be able to monitor these species with at least a moderate degree of precision. The most widespread species were Chaffinch Fringilla coelebs, Woodpigeon Columba palumbus, Blackbird Turdus merula, and Wren Troglodytes troglodytes. A wide range of habitats was covered (Table 2). Farmland predominates within the sample, as it dominates land use in the UK (Barr et al. 1993), the next most common land uses were human sites (including urban, suburban and rural sites) and woodland, followed a range of rarer land types (Table 2). We expect the coverage of species and habitats to increase modestly as the scheme grows. DISCUSSION The launch of the BBS as an annual bird monitoring scheme marks an important and significant advance in British ornithology. In the medium- to long-term the BBS will take over the monitoring role of the CBC by producing population indices for a range of common breeding birds. Calibration of the trends from the two schemes will be essential to maintain the long-term time series initiated by the CBC in the 1960s. We plan to have a considerable period of overlap between the two surveys. It will be some time before there are sufficient BBS data to produce meaningful time trends and in the meantime data from the CBC will be invaluable source of information. Further work is required in the development of the analysis for year-to-year monitoring data from stratified BBS samples, and longer-term population changes. The national statistics will be derived from a region-by-region analysis. The fact that different regions have different sampling intensities can be incorporated in the analysis and will not bias estimates of change. Population changes are likely to be weighted by the size of regions, sampling intensity within regions, and the number of bird registrations within occupied squares. We will explore the use of bird counts in different distance categories and the use of early and late counts on a species-by-species basis. For example, it might be sensible to treat residents, early-arriving migrants and late-arriving migrants in different ways. In this way, it should be possible to maximise the precision of between-year changes for groups of species with particular lifestyles. Recent advances in the analysis of monitoring data, particularly the implementation of methods based on the Loglinear Poisson regression (ter Braak et al. 1994, van Strein et al. 2004) provides great potential for use with BBS data. It is the nature of such time-series that the data contain missing values, are spatially auto-correlated, and that the bird counts will be non-normally distributed. All these factors, however, can be assessed and incorporated in the regression models developed by van Strein et al. (2004) to create robust indicies. BBS data will form an integral part of the BTO's Integrated Population Monitoring (IPM) programme by providing information on populatin changes among common birds. The IPM aims to develop a better understanding of bird populations by integrating information on bird numbers with that on survival and breeding success (Baillie 1990, Greenwood et al. 1993). It is then possible to build population models to explore the dynamical behaviour of a particular species and build in environmental variables such as weather, in order to distinguish natural population fluctuations from those brought about by the actions of man

10 Table 1. Species recorded from the Breeding Bird Survey Species are listed in descending order of occurence for A) species recorded from over 100 survey squares and B) species recorded from between 50 and 100 squares. Species Proportion of survey squares occupied (%) Number of survey squares occupied A. Over 100 squares occupied Chaffinch Woodpigeon Blackbird Wren Carrion Crow Robin Blue Tit Starling Skylark Great Tit Dunnock Swallow Song Thrush Magpie Willow Warbler House Sparrow Greenfinch Pheasant Jackdaw Yellowhammer Linnet Goldfinch Rook Collared Dove Swift Blackcap Mistle Thrush Whitethroat Pied Wagtail Mallard Cuckoo Chiffchaff House Martin Meadow Pipit Lapwing Long-tailed Tit Kestrel Stock Dove Jay Coal Tit Black-headed Gull Moorhen Bullfinch Great Spotted Woodpecker Green Woodpecker Feral Pigeon Curlew Goldcrest Grey Heron Species Proportion of survey squares occupied (%) Number of survey squares occupied contd. Herring Gull Lesser Black-backed Gull Buzzard Reed Bunting Garden Warbler Red-legged Partridge Sparrowhawk Treecreeper Nuthatch Lesser Whitethroat Wheatear Grey Partridge Canada Goose Oystercatcher Spotted Flycatcher Sedge Warbler Corn Bunting Turtle Dove Yellow Wagtail Coot Raven Tree Sparrow Mute Swan Grey Wagtail Tree Pipit Common Gull Snipe B. Between 50 and 100 squares occupied Siskin 6 96 Redstart 6 95 Tufted Duck 6 94 Lesser Redpoll 6 90 Marsh Tit 6 88 Shelduck 5 85 Red Grouse 5 81 Golden Plover 5 81 Cormorant 5 80 Hooded Crow 5 72 Little Owl 4 66 Whinchat 4 65 Willow Tit 4 63 Wood Warbler 4 63 Great Black-backed Gull 4 63 Redshank 4 57 Tawny Owl 4 56 Reed Warbler 4 56 Sand Martin 3 54 Common Sandpiper 3 54 Stonechat

11 Table 2. Habitat composition of 200 metre transect sections within the Breeding Bird Survey These figures do not reflect the habitat composition of the UK as a whole as they are not corrected for regional variation in sampling effort. Major Habitat Percentage of transect sections Number of transect sections Farmland Human Sites Woodland Heathland & Bogs Grassland Scrubland Water Bodies Inland Rock Coastal Miscellaneous TOTAL ACKNOWLEDGEMENTS We thank the many BTO volunteers, both fieldworkers and regional organisers who have contributed to the monitoring programmes discussed in this paper. The CBC and PCP were supported under a contract from the JNCC (on behalf of English Nature, Countryside Council for Wales, Scottish Natural Heritage, and under a contract from the Department of the Environment (Northern Ireland). The project to evaluate sampling strategies was funded by RSPB. The BBS is jointly funded by the BTO, the JNCC and the RSPB. We are grateful to the members of the BTO's Integrated Population Monitoring Working Group, Prof. Steve Buckland, Drs. Rhys Green, Nicholas Aebischer, John Goss-Custard and Dorian Moss for helpful advice. We also thank Drs. David Stroud, Ken Smith, David Gibbons, Jeremy Greenwood, Will Peach, and Humphrey Crick for helpful discussion. John Marchant, Dawn Balmer, Andy Wilson, and the late Dr Steve Carter provided assistance. REFERENCES Baillie, S.R. (1990): Integrated population monitoring of breeding birds in Britain & Ireland. Ibis 132: Baillie, S.R. & J.H. Marchant (1992): The use of breeding bird censuses to monitor common birds in Britain and Ireland - current practice and future prospects. Vogelwelt 113: Barr, C.J., R.G.H. Bunce. R.T. Clarke, R.M. Fuller, M.T. Furse, M.K. Gillespie, G.B. Groom, C.J. Hallam, M. Hornung, D.C. Howard & M.J. Ness (1993): Countryside Survey Main Report. London: Department of the Environment. Bibby, C.J., N.D. Burgess & D.A. Hill (1992): Bird Census Techniques. London: Academic Press. ter Braak, C.F.F., A.J. van Strien, R. Meijer & T.J. Verstrael (1994): Analysis of monitoring data with many missing values: which method? In Hagemeijer, E.J.M. & T.J. Verstrael (eds.): Bird Numbers 1992: Distribution, monitoring and ecological aspects. Proc. 12th Int. Conf. IBCC and EOAC, Noordwijkerhout, The Netherlands: Beek-Ubbergen: SOVON. Buckland, S.T., D.R. Anderson, K.P. Burnham & J.L. Laake (1993): Distance sampling: estimating abundance of biological populations. London: Chapman & Hall. Crick, H.Q.P. (1992): A bird-habitat coding system for use in Britain and Ireland incorporating aspects of land-management and human activity. Bird Study 39:1-12. Dawson, D.G. (1981): The usefulness of absolute ("census") and relative ("sampling" or "index") measures of abundance. Studies in Avian Biology 6:

12 van Dijk, A.J. (1992): The Breeding bird monitoring programme of SOVON in the Netherlands. Vogelwelt 113: Donald, P.F. & A.D. Evans (1995): Poplation size and habitat selection of Corn Bunting Miliaria calandra breeding in Britain in Bird Study 42: Fuller, R.J., R.D. Gregory, D.W. Gibbons, J.H. Marchant, J.D. Wilson, S.R. Baillie & N. Carter (1995): Population declines and range contractions among lowland farmland birds in Britain. Conservation Biology 9, Furness, R.W. & J.J.D. Greenwood (eds., 1993): Birds as monitors of environmental change. London: Chapman & Hall. Gibbons, D.W., J.B. Reid & R.A. Chapman (1993): The New Atlas of Breeding Birds in Britain and Ireland: London: Poyser. Greenwood, J.J.D. & S.R. Baillie (1991): Effects of density- dependence and weather on population changes of English passerines using a non-experimental paradigm. Ibis 133 (suppl.): Greenwood, J.J.D., S.R. Baillie, R.D. Gregory & W.J. Peach (1995): Some new approaches to conservation monitorng of British birds. Ibis 137 (suppl.): Greenwood, J.J.D., S.R. Baillie, H.Q.P. Crick, J.H. Marchant & W.J. Peach (1993): Integrated population monitoring: detecting the effects of diverse changes. In Furness, R.W. & J.J.D. Greenwood (eds.): Birds as Monitors of Environmental Change: London: Chapman & Hall. Gregory, R.D. & S.R. Baillie (1994): Evaluation of sampling strategies for 1-km squares for inclusion in the Breeding Bird Survey. Thetford: BTO Research Report 139. Gregory, R.D. & S.R. Baillie (2004): Survey design and sampling strategies for monitoring breeding birds. Bird Census News, Vol. 13, 2000 (2004): Gregory, R.D., J.H. Marchant, S.R. Baillie & J.J.D. Greenwood (1994): A comparison of population changes among British breeding birds using territory mapping and point-count data. In Hagemeijer, E.J.M. & T.J. Verstrael (eds.): Bird Numbers 1992: Distribution, monitoring and ecological aspects. Proc. 12th Int. Conf. IBCC and EOAC, Noordwijkerhout, The Netherlands: Beek-Ubbergen: SOVON. Marchant, J.H., R. Hudson, S.P. Carter & P. Whittington (1990): Population trends in British breeding birds. Tring: British Trust for Ornithology. O'Connor R.J. & R.J. Fuller (1984): A re-evaluation of the aims and methods of the Common Bird Census. Tring: BTO Research Report 15. O'Connor R.J. & M. Shrubb (1986): Farming and birds. Cambridge: Cambridge University Press. Peach, W.J., S.R. Baillie, L.G. Underhill (1991): Survival of British Sedge Warblers Acrocephalus schoenobaenus in relation to west African rainfall. Ibis 133 (suppl.): Shrubb, M. & P.C. Lack (1991): The numbers and distribution of Lapwings Vanellus vanellus nesting in England and Wales in Bird Study 38: van Strien, A.J., E.J.M. Hagemeijer & T.J. Verstrael: Assessing trends in birds monitoring data. Bird Census News, Vol. 13, 2000 (2004) Verner, J. (1985): Assessment of counting techniques. In Johnston, R.F. (ed.): Current Ornithology 2: New York: Plenum Press. Verner, J. & L.V. Ritter (1988): A comparison of transects and spot mapping in Oak-Pine woodlands of California. Condor 90: