The Relative Importance of Prey Availability and Intruder Pressure in Feeding Territory Size Regulation by Harriers, Circus cyaneus

The relative importance of prey availability and intruder pressure in the regulation of harrier (Circus cyaneus) territory size was investigated over two years using analytical methods chosen to permit comparison with Myers et al. (1979) study of sanderlings (Calidris alba). Relationships between territory area and two variables, prey type (mice; large, medium, and small birds) and intruder type (conspecific neighbors, conspecific floaters, and heterospecific floaters), and the consistency of these relationships between years, also were examined. Individual harrier territory areas were highly variable, ranging from 7.8 to 124.9 ha in 1984/1985, and 3.9 to 71.3 ha in 1985/1986. Of the prey variables, only mouse availability was significantly inversely correlated with territory area in both years, and slopes resulting from correlations between the logarithms of these two variables did not differ significantly from -1, the expected result if harriers were adjusting mouse availabilities. The abundance of mice in conjunction with their greater ease of capture relative to birds made them functionally more available, and hence harriers' primary prey. This may explain why mice, rather than birds, were apparently the defended resource. Of the intruder variables, neighbor variables were most strongly inversely correlated with territory area. Partial correlation analyses to determine the relative importance of intruder pressure and prey availability in regulating territory size revealed that in 1984/1985, mouse availability and intruder pressure were relatively independent and each explained some variation in territory area, whereas in 1985/1986, mouse availability, rather than intruder pressure, significantly explained all variation in territory area. Possible explanations for why territory sizes of harriers appear to be regulated more closely by food density, whereas territory sizes of sanderlings appear to be regulated more closely by intruder pressure, are based upon differences in 1) neighbor effects, 2) environmental variability, and 3) accuracy of resource assessment by intruders. The variation in intruder rates and prey availabilities observed between years suggests the need to conduct studies over several years in order to assess accurately the relative importance of these variables in territory-size regulation.