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).
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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.
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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.
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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