Dartford Warbler national surveys in the UK
In the United Kingdom, Dartford Warblers (<i>Sylvia undata</i>) are mainly confined to the remaining fragments of lowland heath and, as a consequence of the interest in the flora and fauna of this much-threatened habitat, their populations and ecology have been well studied. Historically, the Dartford Warbler had a much wider distribution, and was presumably more abundant, than in recent years. Since the nineteenth century, the population went into steep decline, at least in part due to habitat loss and fragmentation. The most recent survey in 2006, however, recorded a major expansion in range and population size. The UK population has been well monitored, with full surveys in 1974, 1984, 1994 and 2006. The first two surveys were funded by RSPB; the 1994 survey was funded by RSPB and English Nature (now Natural England); the 2006 survey was funded by RSPB, BTO, Natural England and the Forestry Commission (England) under the Statutory Conservation Agencies and RSPB Breeding Bird Scheme (SCARABBS).
Though the population in the UK is small compared with that of southern and western France and Spain, the species is considered to be of conservation concern in Europe because of losses in some of the most valuable habitats in Spain, including the Mediterranean maquis.
1974 Survey: This dataset includes every recorded territory. However, in the New Forest, although 203 pairs were actually found, two large areas thought to support about 40 pairs were not recorded at all and another area was only partially covered. Therefore, the total population count includes an element of estimation. The total population was estimated to be 557 pairs, which was close to the 460 pairs estimated at the last peak in 1960-61. However, the centre of the distribution had moved further west. The species is very susceptible to cold winters, but the population was high in 1974 after a long run of mild winter weather. The preferred habitat was mature heather with a generous mixture of gorse of medium height. Small fragments of heath were found to be less densely occupied than larger ones. For more information see Bibby & Tubbs (1975), <i>British Birds</i> 68: p177-195.
1984 Survey: The number of territories recorded in 1984 was 423. The totals recorded in this dataset represent the minimum number of breeding males, taken as equivalent to pairs, which were counted on two or more visits between April and June. Increases in territory numbers were especially noted in Surrey, which had a strong population after recovery from extinction in 1961, and Cornwall was occupied after a 40-year absence. In the centre of the range, about 10-15% of the population decline is attributed to the fact that colder winters preceded the 1984 survey compared with the mild winters preceding the 1974 survey. A loss of 75 territories was due to growth of forestry plantations, temporarily suitable in 1974 but by 1984 too old (and permanently unsuitable), and unfortunately not replaced by other new plantings. The amount of suitable habitat remained about the same in the New Forest, but declined by about 10% in Dorset. Further losses in Dorset were due to degradation of sites, and the effects of fragmentation and isolation. For more information, see Robbins & Bibby (1985), <i>British Birds</i> 78: 269-280.
1994 Survey: A total of 1,600-1,670 territories was recorded, though it is likely that the actual population was slightly higher (1,800-1,890 territories), representing a near four-fold increase in population since the 1984 survey. Observers were asked to visit each site (or 1km grid square in the New Forest) at least twice, once during April to mid-May and once during mid-May to the end of June. This dataset includes every territory recorded by the observers. The observers recorded the number of singing males and information such as whether the bird was calling or carrying nest material etc. They used this to information to provide their best estimate
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.
The stated aim of all four surveys was to assess the population size and range of the Dartford Warbler in Britain.
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.
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.
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