Dartford Warbler national surveys in the UK

Quality Control

All data were gathered by trained field workers and the data are of a high quality. The data have been mapped and checked for sensitivities and typographical/ geographical errors.
1974 Survey: Two large areas in the New Forest were not surveyed at all and another area was only partly covered. Numbers of pairs were estimated for these areas. The figure for Dorset is probably no more than 90% of the real total. It is thought unlikely that more than a few pairs nested undetected outside the known range.
1984 Survey: The totals recorded represent the minimum numbers of breeding males, taken as equivalent to pairs. We do not know to what extent this might underestimate the truth, but an error greater than 10% is thought unlikely.
1994 Survey: Some observers visited sites more than twice, but some sites were only visited once. This was shown to influence the number of territories found, and corrections have been made to estimates of national totals bearing this in mind.
2006 Survey: Not all 1-km squares were visited for an equal number of times. A survey intensity correction factor has been applied to adjust the population estimate for this.

Method steps

1. All surveys were based on visits to all possible known sites where Dartford Warblers had been recorded in the past, or other suitable habitat. Observers (volunteer ornithologists and RSPB/English Nature/BTO staff) were asked to visit each site or, in 1994 and 2006 1-km square, at least twice, once during April to mid-May, and once during mid-May to the end of June. They were asked to record the number of singing males and other contacts (for example, calling individuals and adults carrying nesting material or food) separately, to estimate the total number of territories recorded on each visit and to provide their 'best estimate' of the number of territories on that site or 1-km square during the breeding season. All numeric estimates were cross-referenced to maps showing the locations of bird registrations. In order to verify records and validate the dataset, the data have been mapped and thoroughly checked. Geographical checks have included comparing the distribution with that shown in the published paper and ensuring that records with the same area name are located close to each other.
   1974 Survey: RSPB staff searched all the heathland in Dorset as well as areas of gorsy grassland, old sand pits and forestry plantations. The New Forest was surveyed by volunteer observers allocated 1-km squares to investigate. Local volunteer ornithologists in Devon, Isle of Wight, Surrey and Sussex checked most likely sites.
   2006 Survey: Coverage was based on surveying 1-km grid squares, rather than on a site basis. The aim was to achieve complete coverage of a set of 'core' squares -- those occupied during the 1994 survey and subsequently -- and to survey a random selection of 'sample' squares from the area around those core squares. In addition, coverage of 'extra' squares was encouraged in a number of ways.

In all surveys all sites in England with suitable habitat and known sites where Dartford Warblers had been recorded in the past. Although Dartford Warblers breed in the Channel Islands, only the 2006 survey covered the islands, although these data cannot at present be shown on the NBN. The 2006 survey also included some sites in Wales as the species had spread there. The 1994 and 2006 datasets include 1-km squares that were surveyed but where no birds were found. These zero records cannot currently be shown on the NBN but are available upon request from the data provider. All presence records in all surveys represent territory centres of Dartford Warblers. The full dataset locates these to 100m resolution.
The aim in 2006 was to achieve full coverage of all the 1-km squares ('core' squares) occupied during the survey in 1994 and subsequently, together with sample coverage of a random selection of squares from within 5-km and 10-km buffers around these core squares, stratified by the presence of suitable habitat.