The impacts of buzzards on red grouse

The impact of buzzards on red grouse

We looked at the year-round diet of buzzards at Langholm Moor. © Laurie Campbell

BACKGROUND

The Langholm Moor Demonstration Project (2008-2017) aimed to restore economically sustainable driven grouse shooting while maintaining a viable population of hen harriers and to extend and improve the heather habitat.

ACKNOWLEDGEMENTS

This study was funded by Buccleuch Estates, Game & Wildlife Conservation Trust, Scottish Natural Heritage and Natural England through contributions to the Langholm Moor Demonstration Project. Human-wildlife conflicts often centre on economic loss caused by wildlife. Despite this being a major issue for some land managers, estimating total prey losses to predation, including that by legally protected predators, can be difficult. Estimating impacts of protected wildlife on economically important prey can also help management decisions to be evidence-led. The recent recovery in numbers and range of common buzzards in Britain has brought them into conflict with some gamebird managers. The magnitude of any impact is poorly understood, having seldom been quantified by empirical field data. A three-year PhD studentship funded by the Langholm Moor Demonstration Project, was conducted with Newcastle University to consider the year-round diet of buzzards at Langholm Moor. Taking into account buzzard abundance and foraging range, the study examined whether buzzards may have impacted the project aim of restoring red grouse numbers sufficiently to re-establish driven grouse shooting.

Experimental culling of buzzards to measure their potential impact on grouse was deemed unacceptable for both welfare and logistic reasons. In their absence, bioenergetics models were used that combined measures of buzzard abundance from field surveys with studies of their diet assessed by using cameras and prey remains at nests and pellet analysis over the winter. The resultant measures of seasonal grouse consumption by buzzards were used in conjunction with sample counts of grouse abundance to estimate potential impact on red grouse on Langholm Moor, a 115km² moor in south-west Scotland, managed to restore red grouse shooting.

Grouse consumption by an individual pair of breeding buzzards and their chicks varied between pairs and years, averaging 0-5 adult grouse and 0-6 grouse chicks per annum depending on assessment method. This rate was lower than previous estimates for two other raptor species, hen harrier and peregrine, present on the study site. Total consumption by buzzards could, however, have been greater given that an estimated 55-73 buzzards were present on the study site year-round during the study period, making buzzards three-times more abundant than both hen harrier and peregrine combined. Averaging across diet assessment methods, consumption models estimated that during each of the three breeding seasons (April-July 2011-2013), the

buzzards foraging on our study site consumed 73-141 adult grouse and 77-185 chicks (depending on year). This represented 5-11% of adult grouse present in April (22-67% of estimated adult mortality) and 2-5% of chicks that hatched (3-9% of estimated chick mortality). During two non-breeding seasons (August-March), consumption models using pellet analysis estimated that buzzards ate a total of 242-400 grouse, equivalent to 7-11% of those present at the start of August and 14-33% of those estimated to have died during the non-breeding season.

We concluded that consumption of red grouse by buzzards had the potential to lead to non-trivial economic loss to grouse managers. This was one of several factors associated with the incomplete restoration of grouse numbers and the inability of the project to restore sustained driven grouse shooting within the 10-year project duration. This conclusion assumed that buzzards killed the grouse that they ate, and that such grouse mortality was additive to other causes. Buzzards are widely recognised as a scavenger, and the proportion of grouse that they consumed through scavenging birds that were already dead, as opposed to the proportion that they killed, is unknown. Similarly, it is unlikely that all grouse killed and consumed by buzzards would have survived long enough to breed. Instead, a proportion would have succumbed to another source of mortality. Furthermore buzzards eat predators of grouse such as crows and small mustelids, so they may have helped offset the impact of these species. Thus, the impact estimates presented here, while supporting the suggestion that buzzards could have been a factor limiting grouse numbers at Langholm, must be treated with caution. Further experimental evidence, from either a buzzard removal study or one associated with supplying buzzards with diversionary food, would be required before more definitive conclusions on impacts could be drawn. KEY FINDINGS

Red grouse formed a minor part of buzzard diet at

Langholm Moor.

The high numbers of buzzards present meant that collectively their year-round consumption of red grouse could have contributed to the incomplete grouse recovery.

Stronger experimental evidence involving buzzard removal or diversionary feeding would be required to provide more accurate estimates of buzzard impact on grouse and test the assumptions described here.

Richard Francksen David Baines Sonja Ludwig