Nightjar national breeding surveys in Britain
The Nightjar <i>Caprimulgus europaeus</i> is a red-listed species that is protected under Annex 1 of the EC Birds Directive. Numbers and range have declined in Britain over the greater part of the 20th century, but have begun increasing again since the start of the 21st. The species breeds mainly in southern England, but there are scattered populations as far north as central Scotland.
The 1981 national survey located 1784 calling males and estimated the maximum population to be 2100 calling males. (The dataset though only has 1642 birds, the other 142 not having sufficient location information to be mapped.) This was a lower total number than expected. The distribution also showed a contraction of the species? range in the north and west (compared to the 1968-1972 Breeding Atlas). The main cause of the decline was probably a change in climate leading to reduced breeding success, but habitat destruction and disturbance were likely also involved. Information obtained on habitats showed that conifer plantations had become the major habitat type; followed by lowland heath, open bracken areas, other woodlands and raised bogs or mosses. For more information, see Gribble (1983), <i>Bird Study</i> 30: 165-176.
The 1992 survey (organised by BTO and RSPB) located 3093 males and indicated that numbers had increased to an estimated 3400. (The dataset though only has 3082 birds, the other 11 not having sufficient location information to be mapped.) This represented an increase of more than 50% since 1981. For more information, see Morris et al. (1994), <i>Bird Study</i> 41: 181-191.
The 2004 survey (organised by BTO, RSPB, Natural England, Forestry Commission England, Wales and Scotland) located 4131 males and estimated that the total had risen again to 4606 males (+36% since 1992). (The dataset contains a total of 4197 territory records, comprising 3083 territorial males (10 without location information), six single females, 18 unsexed adults and 46 presumed migrant males ? only noted in May.) National Biodiversity Action Plan objectives for Nightjar conservation were reached in respect of population size and stability, but the target for a 5% range increase by 2003 was not met. (NOTE: In 2005, a further 192 territorial males and 3 unsexed individuals were located in locations (New Forest, Dorset Heaths and East Anglia) not covered during the 2004 survey. However, up to 7 territories may be represented in both the 2004 and 2005 datasets.)
The stated aim for each survey was to survey the entire UK population of the species. However the actual coverage in each case did not reach this ideal.
1981 Survey: 241 10-km squares in Britain were surveyed. However several were poorly covered because of lack of observers, poor weather, and because they contain large areas difficult to cover in the dark. The latter included the New Forest (Hampshire), the forests of Galloway, and the Cornish coast where the number of derelict mines pose a special hazard.
1992 Survey: A total of 2256 sites in Britain with suitable habitat were visited. The only areas not fully covered were Dumfries & Galloway and Mid Glamorgan.
2004 Survey: Potentially suitable habitat around Britain was surveyed using a hierarchical selection strategy. 3264 1-km squares within 603 10-km squares around Britain were covered.
In all three surveys squares/sites where no Nightjars were found are included with ?zero abundance? for completeness. However it is currently not possible to display these zero records on the NBN, but they can be made available upon request from the data provider.
Central points of territories were recorded, but territory size was not measured. Here we display the location at a resolution which represents the location of the territories.
The stated aim of each survey was to assess the population size and range of the Nightjar and to gather information on its use of different habitat types.
In all three surveys, trained field workers gathered the data, which is of a high quality. The data have been mapped and checked for sensitivities and typographical/geographical errors.
1981 Survey: the total number of birds recorded probably included some passage birds and unpaired individuals, but it was decided to include them since there was no consistent way to recognise and exclude them. Records for 1643 birds are included (of the survey total of 1784 birds). This discrepancy is due to several reports not having sufficient location information for them to be mapped.
1992 Survey: Dumfries & Galloway and Mid Glamorgan only had partial coverage. Some sites were visited only once or twice, leading to an underestimation of nightjars at these sites -- this was taken into consideration when estimating the total national population.
2004 Survey: Not all 1-km squares were visited for an equal number of times and not all areas of suitable habitat were covered. A correction factor for survey intensity and suitable habitat not visited has been applied to adjust the population estimate.
1981 Survey: The unit for survey coverage was the 10-km square. Observers were asked to look first at known or formerly occupied sites, and then to investigate other potentially suitable habitats within each square, particularly new forestry or felled and restock areas. For each site observers recorded a site name, six-figure grid reference, area and altitude, visit dates and numbers of calling males located, and a standardized habitat description. A minimum of two visits per site was recommended, ideally in June and July, at the optimum times for locating calling males.
1992 Survey: All sites known to have been occupied recently, all formerly occupied sites where suitable habitat might remain, and potential new sites were visited. Sites ranged from small woodland clearings (clearly defined by surrounding mature trees), to vast expanses of open heath. Remote upland conifer plantations were randomly sampled from 10-km squares known to have been recently occupied. Observers were asked to make at least two visits to each site, and where possible to make at least one of these during June, the peak time for calling. The number of males seen or heard on each occasion was recorded on a standard card, and observers were asked to mark the position of each bird located on each visit on a 1:10000 map.
2004 Survey: Survey coverage units were 1-km squares. As previously all sites known to have been occupied recently were mapped as were formerly occupied sites where suitable habitat might remain and specific sample squares were chosen on a hierarchical basis according to three levels of priority: high (sites occupied since 1992 and a random sample of 500 1-km squares containing suitable habitat within 100-km of known sites), medium (a sample of sites occupied in 1981 and area of potential future Special Protection Areas extension or designation), and low (a sample of sites with suitable habitat but no history of occupancy and additional sites which observers considered to contain suitable habitat). The strategy was to cover all high priority sites and to sample 100-km buffer zones around these (to detect local range expansion). Further a random sample of areas of potentially suitable habitat, with no recent record of occupancy, was also made. Two visits to allocated 1-km squares were made between late May and mid July. Each surveyor recorded the locations of all individual registrations (observations of individuals) from all visits and casual records. These were plotted in GIS to determine individual territories, whilst accounting for duplicated observations from adjacent recording areas or additional visits. Observers also noted the occurrence of habitat categories within 50m of each Nightjar registration.
In all cases trained volunteers carried out most of the surveys.
Please note that in the additional attributes field 'Territory', 'may only male' refers to suspected migrants, the majority of which were not occupying breeding sites.
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