Hen harriers on a Scottish grouse moor: multiple factors predict breeding density and productivity.
Predator density and productivity can be influenced by ecological and anthropogenic factors. Given the controversy over predators impacting on prey of economic and conservation importance, it is essential that these relationships are understood to provide a sound scientific basis for their conservation and management. We compared numbers and productivity of hen harriers Circus cyaneus, a protected specialist predator of conservation importance, in relation to a change in generalist predator management at Langholm, a moor managed for red grouse Lagopus lagopus scoticus shooting in southern Scotland between 1992 and 2007. During 1992-1999, the moor was managed for grouse and keepers legally controlled predators, thereafter keepering ceased. Following full protection being given to nesting harriers, their numbers increased from two breeding females in 1992 to 20 in 1997, when predation by harriers limited numbers of grouse available for shooting. After grouse management stopped in 1999, carrion crows Corvus corone and red foxes Vulpes vulpes increased and numbers of female harriers dropped to below five from 2002 onwards. Numbers of breeding harriers were negatively correlated with meadow pipit Anthus pratensis, crow, and July grouse abundance during the keepered period and positively with spring grouse abundance. Harrier clutch size was positively correlated with vole abundance. Harrier clutch survival and productivity were higher when the moor was keepered. Predation by foxes was the main cause of harrier breeding failure. Synthesis and applications. We consider this study to be the first that quantifies how control of generalist predators as part of grouse moor management can benefit harrier productivity. This adds to the importance of finding ways to ensure that grouse moors are managed for harriers, but are still economically viable. If techniques can be devised and put in place to reduce the impact of harriers on grouse, then the control of generalist predators may be viewed as a more acceptable component of conservation management for ground-nesting birds.