The relationship between Red-Billed Chough Pyrrhocorax pyrrhocorax (L) breeding populations and grazing pressure on the Calf of Man

Abstract

A link between the numbers of Loghtan Sheep Ovis aries var. and breeding pairs of Red-Billed Choughs Pyrrhocorax pyrrhocorax has been described for the Calf of Man, Isle of Man, UK, through the analysis of data from 1969 to 1982. Recent research has included further data on populations of Choughs and Sheep grazing, as well as data on brood sizes, Rabbit Oryctolagus cuniculus populations and summer rainfall. Data from 1969 to 1994 show a significant relationship between Sheep and Chough numbers, although data for the period 1983 to 1994 in isolation show no significant association. The mean (± se) brood size ringed in the nest in the period 1959–94 was 2.91 ± 0.011. Between 1972 and 1994, as Sheep numbers peaked, mean brood size decreased, only to rise again as Sheep numbers declined. High Rabbit populations correspond statistically with large Chough broods and increased fledging success. Declines in the Chough breeding population appear to be most closely linked to declines in Rabbit numbers due to outbreaks of myxomatosis, even during periods when Sheep numbers were maintained. The recovery of the Chough population after 1970 corresponds with a change in the effects of myxomatosis outbreaks. Rabbit grazing appears to be equally as important as Sheep grazing in the maintenance of the sward characteristics favoured by breeding Choughs. Combined data on Sheep and Rabbit grazing describe the fluctuations observed in Chough breeding success more fully than data for either grazer alone.