The BTO Heronries Census of England and Wales 1928–2000: new indices and a comparison of analytical methods

A census of Grey Heron Ardea cinerea colonies has been made annually by volunteer observers in England and Wales since a first national survey in 1928. This data set has recently been thoroughly upgraded and here we apply a new method of indexing Heron population changes to produce a new 73-year time-series, the longest time-series describing the abundance of any breeding bird in the world. The indexing methodology is based on the use of weighted ratio estimators, as developed by G.E. Thomas, but with modifications that use reasonable assumptions regarding the likely rates of colony extinction and formation. There is little reliable information about such rates of site turnover, but the results were not sensitive to the assumptions made. The new analytical method, based on counts since 1964, suggests a population estimate of just over 9000 breeding pairs (95% CI 8529–9776) in England and Wales in the year 2000, some 2000 more than suggested in earlier publications. Only minor differences in trend are apparent over the course of the time-series between the new results and those previously published. Regional analysis suggests that the population in southern England and South Wales has remained relatively constant since the 1970s, but those in the North have tended to increase in the 1990s.