Numbers of nesting Rooks Corvus frugilegus in the United Kingdom in 1996

Abstract

In spring 1996, a nationwide census of nesting Rooks was carried out by the British Trust for Ornithology, under contract to the Department of the Environment (now the Department of the Environment, Transport and the Regions). Volunteers were asked to count Rook nests in 1997 2 × 2-km tetrads selected randomly throughout the UK. Full coverage or a valid nil return was reported for 96% of the sample tetrads, allowing the data to be treated as representative of the UK. Extrapolation from the survey's 3% sample allowed estimation, with confidence limits, of national and regional population sizes. The UK Rook population in 1996 was estimated at 1.27 million nesting pairs. This is about 40% higher than was estimated from a survey in 1975–77, but there were fewer Rooks nesting in Britain than in 1944–46. Both the direction and the scale of recent increase were supported by an analysis of change in 644 tetrads surveyed both in 1975–77 and in 1996. National data and those from local surveys suggested that population changes had not been uniform across the UK, but change estimates were partly dependent on assumptions about the thoroughness of coverage in 1975–77, and some estimates of increase may be too high. Rates of tetrad occupancy were highest in southwest England and Northern Ireland, but low in Scotland, although Scottish rookeries were more than twice as large on average as those in any other UK region. Mean rookery sizes in Britain were larger than in 1975–77, especially in England, but smaller in Northern Ireland. In all regions, however, population increase had occurred at least partly by an increase in the number of rookeries. Since 1975–77, English Rooks have shifted from elms as nesting trees to oaks and Ash, while conifer usage in Scotland, Wales and Northern Ireland appears to have decreased. The reasons for recent population increase are not known, but in some areas may stem from additional foraging opportunities provided by new outdoor piggeries and landfill sites, increased roadside carrion, and higher stocking rates on grassland.