Ranging patterns are important features of animal ecology both for evolutionary ecologists who study adaptive foraging and habitat use, and for conservationists who need information on habitat requirements and minimum resource needs. Because ranging patterns are difficult and time-consuming to measure, many researchers have sought socio-ecological variables that predict range use. For example, many studies show a correlation between daily path length (day-range length) and the number of animals in a group (Chapman et al. 1995, van Schaik et al. 1983, Waser 1977, Wrangham et al. 1993). This seems reasonable because larger groups have more mouths to feed, necessitating more travel to find adequate food supplies. Group size is far easier to measure than day range, hence the attraction of this variable. However, group size must be used cautiously as a proxy for day range because their relationship is neither strong nor straightforward.

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.