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

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 Env ironment (now the Department of the Environment, Transport and the Regions) . Volunteers were asked to count Rook nests in 1997 2 x 2-km tetrads select ed randomly throughout the UK. Full coverage or a valid nil return was repo rted for 96% of the sample tetrads, allowing the data to be treated as repr esentative of the UK. Extrapolation from the survey's 3% sample allowed est imation, 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 ill 1975-77, bu t there were fewer Rooks nesting in Britain than in 1944-46. Both the direc tion and the scale of recent increase were supported by an analysis of chan ge in 644 tetrads surveyed both in 1975-77 and in 1996. National data and t hose from local surveys suggested that population changes had not been unif orm across the UK, but change estimates were partly dependent on assumption s about the thoroughness of coverage in 1975-77, and some Estimates of incr ease may be too high. Rates of tetrad occupancy were highest ill southwest England and Northern Ireland, but low in Scotland, although Scottish rooker ies were more than twice as large on average as those in ally other UK regi on. Mean rookery sizes in Britain were larger than in 1975-77, especially i ll England, but smaller in Northern Ireland. In all regions, however popula tion increase had occurred at least partly by all increase in the number of rookeries. Since 1975-77, English Rooks have shifted from elms as nesting frees to oaks and Ash, while conifer usage in Scotland, Wales and Northern Ireland appears to have decreased. The reasons for recent population increa se are not known, but in some areas may stem from additional foraging oppor tunities provided by new outdoor piggeries and landfill sites, increased ro adside carrion, and higher stocking rates on grassland.