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BBS Research Population size and density estimates The Breeding Bird Survey was introduced to monitor the population trends of a broad range of breeding bird species in the UK. However, through the recording of birds in different distance categories, it is also possible to use BBS data to assess variation in detectability and hence derive estimates of density and population size. For this, a technique known as distance sampling is used which models the decline in detectability of birds with distance from the transect line. This enables an estimate of undetected individuals to be included in its estimate of the real density and hence population size of the species of interest. Because one would expect large differences in detectability between habitats, for example, birds are likely to be more detectable in open country than they are in woodland, this method makes use of habitat data recorded by observers to model the decline in detectability with distance within different habitat classes. Published studies using BBS data include the production of habitat-specific density estimates for eight declining species in 1995 (Gregory & Baillie 1998), estimates of the percentage of British sparrow, finch and bunting populations (10 species) occurring within broad habitat classes in 1996 (Gregory 1999), and habitat association of breeding Meadow Pipits in British uplands (Vanhinsberg & Chamberlain 2001). A summary of these studies is presented below. Current research topics soon to be published includes, work examining the potential of the BBS for producing unbiased population size estimates, producing density and population size estimates for urban species, and a study examining community structure within BBS squares. Large-scale habitat use of some declining British birds Large-scale habitat use of eight species of breeding birds was considered using data from the BBS. The species were Skylark, Dunnock, Blackbird, Song Thrush, Starling, Linnet, Bullfinch and Reed Bunting. All are linked by roughly synchronous population declines over the last 25 years in southern Britain (and mostly in farmland landscapes). Discussion is limited to the conservation status of these species. Breeding densities were estimates for broad habitat types and these were used to estimate population sizes within habitat types. Confidence limits on the estimates were derived using a bootstrap procedure. For most species considered, farmland holds a high proportion of their population (in excess of 50% for four species), reflecting the predominance of this land use across Britain. This suggests that sympathetic changes in farmland practices are likely to provide the best mechanism for improving the status of these species. Substantial proportions of particular species occur outside farmland, but different species occur in different habitats. A considerable proportion of skylarks occur on upland moor, bullfinches in wooded habitats, and reed buntings in riparian habitats. Conservation of this group of species thus requires appropriate management of the wider countryside, including their main habitats. Habitats associated with human habitation hold >20% of the British populations of blackbird, song thrush and starling, and considerable number of other species. The management of parks, gardens and other ‘green space’ may have an important impact of their population and should not be neglected by conservationists. Gregory, R.D. & Baillie, S.R. 1998. Large-scale habitat use of some declining British birds. Journal of Applied Ecology 35, 785-799. Broad-scale habitat use of sparrows, finches and buntings in Britain The large-scale habitat use of a group of breeding sparrows, finches and buntings was considered using data collected across Britain. The species were House Sparrow, Tree Sparrow, Chaffinch, Greenfinch, Goldfinch, Linnet, Bullfinch, Yellowhammer, Reed Bunting, and Corn Bunting. Many, but not all of these species have declined in numbers over the last 25 years, particularly on farmland. Breeding densities were estimated for broad habitat types and these were used to estimate population sizes within habitat types. For most of the species considered, farmland holds a high proportion of their British population, in excess of 50% for six species, reflecting the predominance of this land use across Britain. Farmland specialists include Tree Sparrow, Linnet, Yellowhammer, and Corn Bunting. Substantial proportions of particular species occur outside farmland, but different species occur in different habitats. Bullfinch and Chaffinch occur predominantly on farmland and in woodland, House Sparrow, Greenfinch and Goldfinch on farmland and in build up areas, and Reed Bunting on farmland and in marshland habitat. The implications of these patterns for the conservation of the species are discussed. Gregory, R.D. 1999. Broad-scale habitat use of sparrows, finches and buntings in Britain. Vogelwelt 120, Suppl.: 163-173. Meadow Pipit densities in upland areas: indicator of heather condition Habitat associations of Meadow Pipit, breeding in British upland 1-km squares were investigated using data from the Breeding Bird Survey (1994-97), The New Atlas of Breeding Birds in Britain and Ireland 1988-91, and habitat data from Institute of Terrestrial Ecology’s (ITE) Land Cover Survey (1994). Meadow Pipit density was highest in 200m sections of BBS 1-km transects containing unenclosed grass moor, heather and bog, but there was no difference in Meadow Pipit density between these habitat types. Meadow Pipit density (number of individuals) increased with heather cover up to about 7.5 200m sections (out of ten per 1 km square) of heather, but fell thereafter. There was positive association between meadow pipit density and the extent of grass moor cover. The ITE Land Cover data gave results that were consistent with the BBS habitat data at both 1-km and 10-km square scale. There was a non-linear relationship between hill grass cover and Meadow Pipit abundance, which reached a maximum when hill grass covered between 40% and 60% of a square. Our results suggest that a mosaic of heather, bog and grassland may be the optimum habitat for Meadow Pipits. Recent changes in land management practices may have led to the disappearance or degradation of these mosaics in some areas. Meadow Pipits are widespread and easily counted and we suggest that this species might be a useful bio-indicator of changes in heather and grassland ratios that result from land management practices, such as grazing and rotational burning, though further research is required. Vanhinsbergh, D.P. & Chamberlain, D.E. (2001) Habitat associations of breeding Meadow Pipits Anthus pratensis in the British uplands. Bird Study 48, 159-172.
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