Hen Harriers were studied on a grouse moor in northeastern Scotland from 1970-74. Numbers in spring were fairly stable, and the 24 nesting territories occupied in 1974 were regularly dispersed with a mean nearest neighbour distance of 1.52 ± s. e. 0.09 km. Many pairs apparently failed to breed and left the area. The mean clutch size of those remaining was 4.70 ± s. e. 0.24 (n= 27) and the mean number of young fledged per successful nest was 3.11 ± s. e. 0.26 (n= 19). Young males were more likely to disperse far from the study area than young females.
The variety of prey species observed from hides at five nests was very limited, and comprised mainly pipits, grouse chicks and lagomorphs. Estimates by weight suggest that grouse and lagomorphs accounted for 89% of all prey. Males brought more small items and fewer large items to nests than females, and on average provided 72% of all prey items seen during watches from the hides.
On the basis of studies of prey at three nests in 1974, and counts of grouse in spring and late summer, harrier predation was estimated to have reduced the number of grouse which might otherwise have survived to late July that year by a maximum of 7.4%.