Contents of report
Breeding Bird Survey
Common Birds Census
Constant Effort Sites
Nest Record Scheme
Waterways Bird Survey
What the categories mean
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The value of the monitoring work undertaken by the BTO is recognised in the Government's Biodiversity Steering Group report (Anon. 1995). The BTO's results, particularly on declining farmland species, are highlighted as showing how broadly based surveillance can identify important new trends. More generally, the report states that monitoring is essential if the broad aims, specific objectives and precise targets of the Government's Biodiversity Action Plans are to be achieved. It notes that:

  • baselines must be established;
  • regular and systematic recording must be made to detect change;
  • the reasons for change should be studied to inform action.

The BTO's monitoring schemes fulfil a considerable portion of these needs for a wide range of bird species in the UK.

1.1 The BTO's monitoring of breeding birds in the UK

The Integrated Population Monitoring Programme has been developed by the BTO under the BTO/JNCC contract to monitor the numbers, breeding performance and survival rates of a wide range of bird species. It has the following specific aims (Baillie 1990, 1991):

  (a) To establish thresholds that will be used to notify conservation bodies of requirements for further research or conservation action.
  (b) To identify the stage of the life cycle at which changes are taking place.
  (c) To provide data that will assist in identifying the causes of change.
  (d) To distinguish changes in populations induced by human activities from those that are natural population fluctuations.

The programme brings together data from several long-running BTO schemes.

  • Changes in numbers of breeding birds are measured by:
    • The Common Birds Census (CBC) - which ran from 1962-2000, this scheme maps the territories of common birds on 2-300 farmland and woodland plots of about 60 and 20 ha area, on average, respectively.
    • The Waterways Bird Survey (WBS) - which began in 1974 and maps the territories of birds on rivers, streams and canals on 1-300 plots, covering an average length of 4.5 km each.
    • The Constant Effort Sites Scheme (CES) - which began in 1983 and is based on bird ringing at over 100 sites where the catching effort is kept constant each year, so that changes in numbers of birds caught reflect population changes.
    • The BTO/JNCC/RSPB Breeding Bird Survey (BBS) - which began in 1994, has replaced the CBC as the major monitoring scheme for landbirds. It is based on 2300 1-km squares, in which bird-watchers count and record birds along a 2 km transect walked in a standardised manner within each square. All habitats and regions are well covered by the survey because the squares are chosen randomly by computer.
  • Changes in breeding performance are measured by:
    • The Nest Record Scheme - which began in 1939 and collates standardised information on up to 35,000 individual nesting attempts per year. This allows the measurement of
      • Laying dates
      • Clutch sizes
      • Brood sizes
      • Nesting success over egg and chick stages
    • The CES provides information on overall productivity for a range of species by measuring the ratio of the numbers of juveniles to numbers of adults caught each year.
  • Changes in survival are measured by:
    • The National Ringing Scheme - which provides information on the finding circumstances and longevity of ringed birds found dead by members of the public.
    • The CES can provide information on survival based on the recapture of ringed birds at CES sites.

An overview of how the schemes fit together is shown in the diagram below, which also shows how the BTO aims to combine all this information to understand what makes the populations change (in so-called "population models").

1.2 The value of combining results from different monitoring schemes
  Increasingly it is being realised that monitoring changes in the numbers of animals is not enough for conservationists (Goss-Custard 1993). The monitoring of breeding performance and survival rates are essential to allow efficient interpretation of changes in population size (Temple & Wiens 1989) and, in the case of long-lived species, to provide early warning of impending changes in population size (Pienkowski 1991).
  Without access to good long-term datasets of breeding performance and survival, remedial conservation action has to be taken without a sound basis or has to wait until some detailed investigative research has been undertaken. In addition, for long-lived species, declines in population size may only occur after long periods of low survival or reproduction.
  The classic example is that of the Peregrine, which in the UK suffered from poor breeding performance during the 1940s and 1950s, due to DDT contamination. This decreased the buffering capacity of the non-breeding population to withstand the severe mortality of breeding adults that occurred due to cyclodiene poisoning from the middle 1950s onwards (Ratcliffe 1993). Monitoring of breeding numbers did not reveal the problem as efficiently as an "early warning" based on the monitoring of breeding performance (Pienkowski 1991).
Another recent example where declines in breeding performance have preceded declines in population size is provided by the catastrophic breeding failures of seabirds, and particularly Arctic Terns, in Shetland (Monaghan et al. 1989, Walsh et al. 1995).
  Farmland birds

The BTO identified that rapid declines of farmland birds was a key conservation problem in the mid-1980s (O'Connor & Shrubb 1986, Fuller et al. 1995), but the causes of the declines were not readily apparent. The BTO has been able to investigate the causes of these declines because of its long-term historical databases (Siriwardena et al. 1998a, 2000). The alternative approach of funding intensive studies of the 10-20 species separately would have been very costly, taken several years to complete and would not necessarily have been representative of the UK.

The study was undertaken jointly with Oxford University, was funded by the UK Government, and looked at changes in population size, breeding performance and survival in relation to changes in farming practice. The study showed that each species has tended to respond to different aspects of the agricultural environment but that these tended to be symptomatic of the trend towards intensification and regional specialisation. Overall, declines in survival rates were found to be the main factor driving population declines. However, for Linnet the main factor appears to have been a decline in nesting success at the egg stage. As a result, the study was able to identify areas for future research, thereby helping conservation bodies to target their scarce resources in the most efficient manner.


Other examples where the combined (or integrated) analysis of BTO datasets have helped to pinpoint the causes of population declines include:

  • Declines in breeding performance appear to have driven the population decline of Lapwing (Peach et al. 1994).
  • Declines in survival rates during the first year of life are sufficient to have driven the population decline of Song Thrush (Baillie 1990, Thomson et al. 1997).
  • Declining over-wintering survival, associated with below average rainfall in the Sahel wintering quarters, was the most important factor determining population change of Sedge Warblers (Peach et al. 1991).
  Biodiversity Action Plans
  The ability to distinguish quickly, the stage of the life-cycle most affected during population declines is particularly important for the conservation agencies considering the plight of species listed on the Conservation Importance Lists (JNCC 1996; Anon. 1995, 1998). (These lists were drawn up using data from the BTO's Common Birds Census (and other sources of information) to prioritise species of birds of conservation concern). Indeed, analysis of BTO datasets is included as a key point in several of the UK Government's Biodiversity Steering Group Action Plans for rapidly declining species.
  Of course, this is not the only function of the BTO's Integrated Population Monitoring programme, because, once conservation actions have been initiated, their successes will be monitored and be assessed against the background information provided by the BTO's long-term schemes. This is the only way that conservation bodies can measure the effectiveness of their actions at a national scale in a cost-effective manner.
1.3 The aims of this report
  The aims of this report are as follows:
  1) To provide a species-by-species overview of the trends in breeding population size and reproductive success of birds covered by BTO monitoring schemes over the past 30 years.
  2) To cover the majority of breeding species, excluding colonial seabirds, which are well covered by the JNCC's Seabird Monitoring Programme (Upton et al. 2000), and excluding the majority of species already covered by the Rare Breeding Birds Panel (Ogilvie 1998). Most wintering populations of waterfowl are well covered by the Wetland Bird Survey annual reports (e.g. Pollitt et al. 2000).
  3) To cover the UK as a whole, and to provide habitat and regional analyses where practical.
  4) To provide early warning alerts to JNCC and Country Agencies about worrying declines in population size or reproductive success, with special reference to species on the Conservation Importance Lists.
  The report will be updated regularly and it is meant to be a working document to be used primarily by conservation practitioners as a ready reference guide to the current changes in status of breeding birds in the UK. (Breeding distributions are not included as these are already fully documented in the New Breeding Atlas (Gibbons et al. 1993) and breeding population sizes are not included because these are to be reported regularly by the Avian Population Estimates Panel (Stone et al. 1997)). However, by producing this as a web-report, we hope that it will be regularly used by a wider audience, especially BTO members and the general bird-watching public. We also hope that it will be used more widely and will become a useful resource for schools, colleges and universities, the media, ecological consultants, decision makers, local government and the more general world of industry and commerce.
  The report is the third in a series produced as part of the BTO's work carried out under its Partnership with the Joint Nature Conservation Committee (on the behalf of Natural England, Scottish Natural Heritage, the Countryside Council for Wales, and the Environment and Heritage Service in Northern Ireland), as part of its programme of research into nature conservation. It is the result of the sustained long-term fieldwork efforts of many thousands of the BTO's volunteer supports. Without their enthusiasm for collecting these hard-won facts, the cause of conservation in the UK would be very much the poorer.

The report should be cited as: 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. and Wernham, C.V. (2001) Breeding Birds in the Wider Countryside: their conservation status 2000. BTO Research Report No. 252. BTO, Thetford. (

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