The effect of management for red grouse shooting on the population density of breeding birds on heather-dominated moorland.
Abstract
Breeding birds, vegetation and moorland management were surveyed in 320 1-km squares on 122 estates in upland areas of eastern Scotland and northern England where red grouse (Lagopus lagopus scoticus) shooting is a widespread land use. We assessed whether population densities of 11 species of breeding birds differed between heather (Calluna vulgaris)-dominated moorland managed for red grouse shooting and other moorland with similar vegetation. We classified estates that had a full-time equivalent moorland gamekeeper as grouse moors. The mean density of red grouse shot per year was four times higher and the mean density of gamekeepers was three times higher on grouse moors than on other moors. Rotational burning of ground vegetation covered a 34% larger area on grouse moors than on other moors. Selection of heather-dominated squares resulted in similar composition of vegetation on grouse moors and other moors (about 76% heath, 12% grass, 8% bog, 2% flush and <1% bracken on both types). However, grouse moors tended to have less tall vegetation than other moors and differed significantly in some other characteristics of the vegetation, topography and soil type. Densities of breeding golden plover (Pluvialis apricaria) and lapwing (Vanellus vanellus) were five times higher and those of red grouse and curlew (Numenius arquata) twice as high on grouse moors as on other moors, while meadow pipit (Anthus pratensis), skylark (Alauda arvensis), whinchat (Saxicola rubetra) and carrion/hooded crow (Corvus corone) were 1.5, 2.3, 3.9 and 3.1 times less abundant, respectively, on grouse moors. The differences in density between moorland types remained significant (P<0.001) for golden plover and crow and approached significance (P<0.10) for lapwing and meadow pipit after allowing for variation among regions. We used Poisson regression models to relate bird density to vegetation cover, topography, climate and soil type. After adjusting for significant effects of these habitat variables, significant differences in bird density between the two moorland types remained for six species, although their magnitude was reduced. Correlations of adjusted bird density with measures of different aspects of grouse moor management provided evidence of a possible positive influence of predator control (assessed using crow density) on red grouse, golden plover and lapwing. The control of crows by gamekeepers is the most probable cause of the low densities of crows on grouse moors. There was evidence of a positive effect of heather burning on the density of red grouse and golden plover and a negative effect on meadow pipit. Multiple Poisson regression indicated that predator control and heather burning had significant separate effects on red grouse density. Significant relationships between adjusted breeding bird densities and the abundance of raptors and ravens (Cotvus corax) were few and predominantly positive. The results provide correlative evidence that moorland management benefits some breeding bird species and disbenefits others in ways that cannot readily be explained as effects of differences in vegetation type or topography. However, experimental manipulations of numbers of some predators and heather burning are required to test these findings.