Abstract
The role of increased predator numbers in the general decline of bird populations in the late 20th century remains controversial, particularly in the case of the Eurasian Sparrowhawk, for which there are contradictory results concerning its effect on the abundance of potential prey species. Previous studies of breeding season census data for Sparrowhawks and prey species in Britain have measured predator abundance either as raw presence-absence data or as an estimate derived from spatially explicit modelling, and have found little evidence of association between predator and prey populations. Here, a predator index derived from site-level binary logistic modelling was used in a regression analysis of breeding census data on 42 prey species, with significant effects emerging in 27 species (16 positive, 11 negative). The frequency of significant positive associations may indicate the tracking of prey abundance by Sparrowhawks, which would tend to cancel out any negative predation effect on prey populations, rendering it difficult to detect using census data. If so, the negative effects that have emerged for some species may underestimate the impact of increased Sparrowhawk numbers on the prey populations concerned. Nevertheless, estimates of the effect on national populations of prey species, obtained by combining effect sizes with a measure of the increase in site occupancy by the predator, suggest the possibility of a substantial impact. It cannot, therefore, be ruled out that the increase in abundance and range expansion of Sparrowhawks made a significant contribution to contemporaneous declines in the populations of many of its prey species.