Woodlark national breeding surveys in Britain
The Woodlark <i>Lullula arborea</i> is a rare breeding species and partial migrant in the UK, where it is mainly confined to southern England. It is a species of high to moderate conservation concern subject to a national Biodiversity Action Plan (BAP).
The dataset contains records from the national surveys of breeding Woodlark in 1986, 1997 and 2006. These were organised and run by the BTO (in 1986 on its own, in 1997 in conjunction with RSPB, JNCC and English Nature (now Natural England) and in 2006 in conjunction with RSPB, Natural England and the Forestry Commission in England).
In 1986 it was estimated that there were 250 territories in the UK concentrated in five regions: south-west England, New Forest/Dorset, Hampshire/Surrey border, Breckland and the Suffolk coast. The dataset records the locations of 241 pairs; the remaining 9 pairs are an estimate for those areas not covered well. Habitat occupancy differed between regions. Most birds were found in young conifer plantations and heathland, but neither of these habitats was important in the south-west where birds used a variety of habitats including marginal farmland. For more information see Sitters <i>et al</i>. (1996), <i>Bird Study</i> 43: 172-187.
In 1997 between 1426 and 1552 woodlark territories were located in Britain -- the dataset includes all 1552 territory locations but it is not certain that more than 1426 became breeding territories. The five regions of population that were apparent in 1986 all showed major increases, in particular in Breckland and the Suffolk Sandlings. Over 85% of territories occurred on heathland or within forest plantations, but the ratio varied between regions: plantations were more commonly used in the north and east, whereas more birds utilised heathland in the south. Woodlarks in Devon bred almost exclusively on farmland. More than three-quarters of the forest population was found in young plantations, particularly in stands of two and three year old trees. Most territories were located on sandy soils, with nearly 70% specifically as on acid sandy soils. For more information see Wotton & Gillings (2000), <i>Bird Study</i> 47: 212-224.
The 2006 survey gave a population estimate of 3064 territories (with 95% confidence limits 2472 - 3687), and a range expansion of 46% since 1997. The dataset records the location of the 1757 territories noted by the observers contributing to the survey. This survey demonstrated that the population had increased to meet the BAP targets set in 1996 for population size and range expansions in England, and that it is on schedule to meet targets in Wales. The species has responded to heathland conservation restoration projects and is highly dependent on forest management. For more information, see Conway <i>et al</i>. (2009), <i>Bird Study</i> 56: 310-325.
1986 survey: restricted to southern and eastern England, the known range at the time.
1997 survey: all sites where Woodlarks bred between 1986 and 1996, along with other suitable habitats in areas surrounding the known breeding range. In practice sites were only in England.
2006 survey: all sites where Woodlarks bred within the previous ten years or so, along with other suitable habitats within 10km of the known breeding range. Suitable forests were defined as plantations that contained either recently cleared compartments or those replanted in the last seven years. Other areas of open space within forests, such as heathland and rides, were also surveyed. In the southwest of England, woodlarks breed almost entirely on farmland, so it was impossible to define suitable sites for coverage. All tetrads in Devon that had ever held a breeding Woodlark were surveyed. For other counties where small numbers of woodlarks occurred or where there were historical breeding records, sites were visited on an ad hoc basis.
A total of 3619 1-km squares were surveyed, about two-thirds in core areas and one third in the sample within the buffer zones.
In all three surveys squares/sites where no woodlarks 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.
The stated aim of each survey was to provide an accurate assessment of the population and distribution of Woodlarks in Britain and how these had changed since previous surveys.
As sites to be surveyed for all three surveys were primarily chosen on the basis of being previously known sites for the time, it is likely that some occupied territories will have been overlooked. This is particularly likely in those areas, such as south-west England, where the population is thinly scattered. Furthermore, the species can be difficult to census effectively when abundance is low; and it was noted this too may have been a particular problem in the south-west. The Dorset population in 1997 may also have been underestimated, since considerably more woodlarks were reported in 1991 and 1992 than in 1997.
In all cases the surveys were widely publicized in order to attract as many 'casual' records as possible, and despite some differences in survey technique between the surveys, they are considered to be comparable.
For 2006, by using the various statistical techniques the organisers are confident that the total numbers and overall range found encompasses the whole although the dataset, as presented, clearly does not include every Woodlark territory in England in 2006.
1986 survey: Observers were asked to search all sites known to have been occupied by Woodlarks in any breeding season since 1975. Returns were requested from these sites, even if negative. Observers were also asked to investigate other potentially suitable areas. Two recording cards were used, a 'site card' for noting all breeding season (March to August) observations of Woodlarks in a 10-km square and a 'habitat card' for describing the habitat and vegetation. Observations were recorded by 1-km square within each 10km square covered.
1997 survey: Observers visited all sites where Woodlarks bred between 1986 and 1996, and other sites with suitable habitats in areas surrounding the known breeding range. Observers were asked to visit each site three times, with visits falling within the following time periods: 15/2 to 21/3, 22/3 to 25/4, 26/4 to 1/6. Almost 40% of the sites were visited more than three times, and in such circumstances observers were asked to note the actual number of visits made. It was asked that all visits should be made on clear, dry mornings with little wind. All areas of suitable habitat on each site were searched to within 100m where possible. Observers were asked to record the number of singing males and other territorial contacts located during each visit by marking them on a map (1:10000 or 1:12500) using the BTO standard territory mapping symbols. The estimated number of territories for the site (minimum and maximum) was entered on the recording form at the end of the survey. Information on land-use and habitat around each territory was also collected.
(Notes: 1) By the time of the 1997 survey much more was known about the biology of Woodlarks in Britain and so to avoid potential problems with separating territories after completion of the first brood, the survey stopped before June. 2) The unit for this survey was the site although grid references were collected as well. 3) Subsequently central territory locations were plotted in GIS, taken from original survey maps.)
2006 survey: The unit of survey coverage was the 1-km square and observers were requested to completely cover each designated square, walking within 100m of all areas of suitable habitat to maximize the detection of territorial individuals. Squares for survey were either 'core' 1-km squares, those which had been occupied in 1997 or subsequently, or were in Special Protection Areas designated for breeding Woodlarks in parts of East Anglia, the New Forest area and Dorset, or in a stratified random sample of squares containing suitable habitat or soil type, up to 10km from known breeding sites. However, the survey was widely publicized in order to attract as many 'casual' records as possible, which in 2006 did include a number of records from outside the known breeding range.
A minimum of two visits was required, one within each of the periods 15/2 - 31/3 and 1/4 - 31/5, and ideally at least 3 weeks apart. Timing of visits was recommended to take place before midday on mild, clear, dry days with little wind. Every encounter with Woodlarks was recorded on a site map and these were analysed using GIS software to determine the number of territories present. Statistical methods were used to extrapolate from sampled squares in the buffer zones, and boot-strapping resampling was used to calculate confidence limits on the figures obtained.
The survey unit was the territory, defined as containing a singing male, a pair exhibiting breeding activity, individuals present on more than one visit, or with two individuals present.
Habitat type was recorded on a hierarchical scale for all bird registrations.
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