Habitat predicts losses of red grouse to individual hen harriers.

Abstract

1. Hen harriers Circus cyaneus prey on red grouse Lagopus l. scoticus and high breeding densities of harriers can limit the number of grouse available for shooting in the autumn. Ultimately, grouse hunting contributes to the maintenance of heather moorland, an ecologically important habitat for biodiversity in general and hen harriers in particular. Predation rates vary widely among harrier individuals. Understanding which factors influence this variation would be useful to target management to mitigate the effect of harriers on grouse, such as diversionary feeding.
2. We used a simple habitat-based approach to test whether we could identify harrier nests to which most grouse were delivered. Using remote sensing habitat data, we tested whether delivery rates of dead grouse to the nest by hen harriers were higher for those pairs nesting in sites with more heather Calluna vulgaris. A relationship between heather cover and grouse delivery rates might have been expected as grouse densities were correlated with heather cover.
3. After adjusting for annual variation in grouse abundance, the rate at which grouse were delivered to harrier nests was positively associated with the proportion of heather cover within 2 km of harrier nests. This was primarily due to the positive effect of heather cover on female delivery rates.
4. This result allowed us to use habitat data to predict the harrier nests to which most grouse chicks would be delivered. Comparison of predictions of the model with observations of food delivery to nests indicated that, in terms of grouse chick delivery, the model correctly predicted the top 50% of harrier nests in five of six years.
5. We undertook an experiment where carrion was provided to certain harriers at nest sites, in order to decrease their predation on grouse. Data from this experiment showed that when harriers were given diversionary food, the relationship between grouse predation rate and habitat was removed, with grouse predation reduced to negligible levels in most cases. This demonstrated the increased benefit of feeding birds with the highest proportion of heather cover within 2 km of their nest sites, rather than feeding birds at random within the conflict population.
6. Synthesis and applications. The amount of heather cover around hen harrier nests can be used to predict which pairs will predate most grouse within a population. This information should facilitate targeted management practices, which may accrue greater benefit for grouse stocks and potentially reduce the conflict between grouse shooting and conservation of biodiversity.