Hen Harrier Circus cyaneus hunting behaviour was examined in relation to the abundance and distribution of prey on two areas of heather moorland in Scotland. During 578 hours of observation, harriers were seen hunting for 18 hours, in which they took 86 prey. The proportion of large prey (Red Grouse Lagopus lagopus scoticus chicks and lagomorph young) increased in the diet as their abundance increased, whereas the proportion of small prey (passerines and small mammals) decreased. Where large prey were scarce, harriers hunted according to passerine distribution. Where grouse were common, harriers hunted in accordance with grouse distribution. In agreement with predictions from patch models, increasing grouse density resulted in harriers spending less time attacking each brood. Interactions between harriers and grouse were observed 41 times, of which 20 resulted in capture. Adult grouse vigorously defended their young against harriers, and males responded more often than females. Observations of harriers hunting and at the nest indicated that once they had caught a grouse chick they rarely returned to the same brood to catch another.