Long-term increase in the fecundity of hen harriers in Wales is explained by reduced human interference and warmer weather

The conservation status of the hen harrier Circus cyaneus in Britain, and its pivotal role in the raptor–grouse moor conflict, has rendered research on the factors influencing its population dynamics of crucial importance. Using over 20 years of data, we examine influences on the reproductive output of the small, but recently expanding, population of Welsh hen harriers. Productivity (fledglings per breeding attempt) has steadily increased with time, being the highest in recent years. Available evidence suggested that food availability could not account for these recent increases. Human interference (apparently, largely or entirely in the form of nest destruction by grouse moor gamekeepers) had a large influence on productivity, despite being restricted to a minority of the breeding population. This negative effect, and a positive effect of May temperature, was most influential on variation in breeding productivity. This study quantifies, for the first time, that cessation of persecution can result in a marked improvement in hen harrier reproductive output. Nest predation by red foxes Vulpes vulpes had no effect on variation in breeding productivity and there was no evidence that control of fox numbers by gamekeepers compensated for their depression of productivity through destroying harrier nests. Our analyses also indicated that gamekeepers probably destroyed an unknown number of nests before they were discovered. We argue that a low proportion of territorial females that apparently breed, in cases when clutch size and fledged brood size are high, may be an indication of persecution. The recent increase in the breeding productivity of Welsh harriers has probably been influential in the recent recovery of the Welsh harrier population and has apparently been due to a combination of cessation of human interference and warmer temperatures.