Recovery of the Peregrine Falcon Falco peregrinus in Cumbria, UK, 1966–99

Abstract

Capsule Since pesticide controls, it has expanded from a pre-pesticide baseline of 41 pairs to over 100 pairs. Aims To describe the population's pattern of recovery and identify constraints on its annual productivity. Methods Occupancy and productivity data were collected from all known and new ranges in Cumbria, UK. Statistical methods were employed to describe temporal and spatial patterns. Possible effects of weather were investigated using meteorological data. Results The population recovered from eight pairs, with birds preferentially recolonizing 41 traditional ranges. The highest occupancy levels were 100 ranges in 1997 and 1998. Many new ranges were in peripheral and atypical locations, although some resulted from the splitting of traditional ones. The central core now has the highest recorded density (> 9 pairs per 100 km2). As occupied ranges increased, the mean nearest neighbour distance declined to an asymptote (approximately 3.5 km). Simultaneously, maximum values were observed for the percentages of known and occupied ranges in which eggs were laid. The mean number fledged by occupied ranges peaked in the early 1990s. New ranges seem to be more susceptible to the effects of May rainfall but overall there was little evidence that new and traditional ranges differ in productivity. Persecution has continued, with over 10% of examined ranges experiencing intentional disturbance in the worst years. Conclusion The recovery of the Cumbrian population seems to be related to two processes: (1) greater productivity and survival forcing birds to occupy more peripheral and atypical sites; (2) a very high density in the central core possibly due to increased food supply resulting from the channelling of prey through its valley systems.