Does food competition from red fox (Vulpes vulpes) influence the breeding density of goshawk (Accipiter gentilis)? Evidence from a natural experiment

Abstract

The impact of food competition from the red fox Vulpes vulpes on the breeding density of the goshawk Accipiter gentilis was studied in a 650 km² area in southern Norway by investigating the density of goshawks and their grouse prey before, during, and after a period when the red fox population was low because of an epizootic of sarcoptic mange Sarcoptes scabiei. The number of goshawk pairs per 100 km² was approximately nine/year in the 1950s, three/year in 1972–75 and in 1980–85, and four/year in 1988–91. In each of these periods, nesting territories of goshawks were regularly spaced. There was a positive correlation between the breeding density of goshawk and the population size of grouse. Both grouse numbers and goshawk breeding density increased at the same time that fox populations declined in the 1980s, while the re-establishment of the fox population in the 1990s was associated with decreases in grouse and goshawk numbers. During the partial absence of red foxes, there was a continuously high grouse population. It is concluded that the red fox may influence the goshawk breeding density negatively by limiting the numbers of grouse.