The relative effects of raptor predation and shooting on overwinter mortality of grey partridges in the United Kingdom.
Abstract
Factual information is key to resolving conflicts between raptor conservation and gamebird management, especially when the conservation status of one of the species involved is threatened. The grey partridge Perdix perdix is a UK Biodiversity Action Plan species because of a marked decline in abundance caused by agricultural intensification. Recently, the number of raptors present on farmland and the commercial shooting of red-legged partridges Alectoris rufa have both increased. To inform conservation action, the relative impacts of these two factors on grey partridge populations urgently require quantification. On our study site areas of low density of grey partridges coincided with areas of high raptor density. However, these areas were managed intensively for shooting and for two areas that suffered local partridge extinction, the 3-year average percentage of partridges shot exceeded 50%. Grey partridge mortality to raptors between autumn and spring lay between 9.5% of autumn density (assumes losses to raptors occurred before shooting) and 15% of post-shooting density (if all losses to raptors were post-shooting). A deterministic model suggested that this rate of loss would reduce the equilibrium density of spring pairs by 11-26% relative to a situation without raptors. In comparison, shooting losses across the study area amounted to 35-39% of autumn density, more than double the losses to raptor predation, with a predicted reduction of 68-85% in equilibrium density of spring pairs. Synthesis and applications. Shooting based on large-scale releases of red-legged partridges acts in a density-independent manner and can lead to local grey partridge extinction. Removing the grey partridge from the UK quarry list would be counterproductive, as most action to boost wild grey partridge densities is carried out by enthusiasts with shooting as the incentive. However, it is imperative that managers of intensive shoots based on gamebird release adopt measures to reduce shooting pressure on wild grey partridges at low density. These include training shooters to distinguish between grey and red-legged partridges and implementing a warning system (whistle) to alert the gun line when birds of the non-target species are approaching. Such voluntary measures are effective in addressing overshooting, which has greater implications for grey partridge conservation than raptor predation.