Diversionary feeding and nestling diet of Hen Harriers Circus cyaneus

Abstract

Capsule: Diversionary feeding reduced Hen Harrier Circus cyaneus nestlings' natural food intake by half. Red Grouse Lagopus lagopus scotica chicks constituted 0-4% of all nestling food items. Annually, this reduced annual grouse chick production by 0-6%.

Aim: To quantify proportions of diversionary and natural food (including grouse) delivered to Hen Harrier nestlings in relation to brood size, male status and natural prey abundance.

Methods: We recorded diversionary food provisioned to 25 Hen Harrier broods (2008-15) and studied the diet of 15 broods using observations from hides, nest cameras and regurgitated pellet analysis. Variation in nestling diet was analysed using compositional analysis.

Results: Hen Harriers took 76% of diversionary food provided. Depending on assessment method, average nestling diet was 44-53% diversionary food, 39-55% natural prey (including 24-45% passerines, 4-15% small mammals, 0-4% grouse chicks) and 0-9% unknown items. The amount of diversionary food consumed was not influenced by male status, brood size or natural prey abundance. The number of Red Grouse chicks delivered annually was 34-100% lower than expected under unfed conditions, however, the confidence intervals associated with these estimates were large.

Conclusion: Diversionary food influenced Hen Harrier nestling diet and reduced the number of Red Grouse chicks taken relative to modelled predictions. It may help reduce conflict between Hen Harrier conservation and Red Grouse shooting, but only if overall grouse productivity is thereby maintained or increased.