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

The impact of food competition from the red fox Vulpes vulpes on the breedi ng density of the goshawk Accipiter gentilis was studied in a 650 km(2) are a in southern Norway by investigating the density of goshawks and their gro use prey before, during, and after a period when the red fox population was low because of an epizootic of sarcoptic mange Sarcoptes scabiei. The numb er of goshawk pairs per 100 km(2) 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. Ther e was a positive correlation between the breeding density of goshawk and th e population size of grouse. Both grouse numbers and goshawk breeding densi ty increased at the same time that fox populations declined in the 1980s, w hile the re-establishment of the fox population in the 1990s was associated with decreases in grouse and goshawk numbers. During the partial absence o f red foxes, there was a continuously high grouse population. It is conclud ed that the red fox may influence the goshawk breeding density negatively b y limiting the numbers of grouse.