Red grouse populations and shooting

• At low shooting bags, proportionally fewer young grouse were shot than expected from the July counts.
• More old males were shot than expected from July counts when the bag was high.
• More old males were shot at the start of the shooting season.

This study on red grouse compared age and sex ratios in the bag with those in the population before shooting. We collected data on shooting selectivity in 2005 and 2006 between mid-August and the end of September on nine moors in northern England. All data were from driven grouse shooting days where beaters drive grouse in the direction of a line of guns. A day of grouse shooting usually consists of four to five separate shooting locations (drives). We attended 45 drives and determined the total number of grouse shot, broken down by age and sex, for each drive. We calculated the length and the area of each drive and noted the number of beaters involved.

To compare the age and sex ratios of shot birds with the age and sex ratio of the population before shooting, we counted grouse in July 2005 and 2006 in the same areas where we collected shooting data. For all 45 drives we visited during the shooting season we knew the age structure, but we only had available the sex-ratio of the old birds for 33 drives. The counts were part of our long-term data collection to determine the ratio of old birds to young of the year and the sex ratio of the old birds.

Figure 1. The young-to-old ratio of the bag divided by the young-to-old ratio of the July count (log-transformed) for different bag sizes A log (bag/count) ratio above 0 for young/old means that a higher proportion of young birds were shot than occurred in the population before shooting.

We considered the relative ‘young-to-old ratio’ and relative ‘sex-ratio’ between the counts and the bag in relation to variations in the number of beaters per square kilometre (100 hectares), the length of the drive, the total number of grouse shot and whether the data came from the first, second or the third/fourth time the same area was driven in a season.

Although hunters cannot consciously select for a specific sex or age class during the shooting process, fewer young than old grouse were shot at low bag sizes, on average, than would be expected from the population composition before shooting (see Figure 1). The susceptibility of old males to shooting increased with bag size (see Figure 2). It was high early in the season, but decreased with the number of times an area was shot (see Figure 3).

The susceptibility of old birds to shooting might be driven by territoriality. Aggressive behaviour, which plays an important role for establishment of territories, increases with density and therefore old birds, especially males, might be more inclined to return to their territory and are therefore less likely to fly over the line of hunters in driven grouse shooting. However, single males that fly over the line of butts might be easier targets than females with family groups.

Our results have important implications for understanding red grouse population dynamics since recent research has found that parasites alone do not explain cycles in red grouse. Shooting and parasites might interact at high density such that old highly parasitized grouse remain in the population after shooting.

The study stresses that the assumption made in many studies that harvest records reflect the age and sex ratio of the population, and therefore reflect productivity, can be misleading.