Gamebirds on chalk grassland

Key findings

• Released pheasants and red-legged partridges spend a lot of time feeding on chalk grassland.
• A wide range of invertebrates are eaten by both species of gamebird.
• We could find no effect of this feeding on invertebrate populations, including butterflies.

In 2002, as part of a study looking at the effect of released gamebirds across the countryside, we started a project to investigate a concern of English Nature about released gamebirds on chalk grassland in the South Wessex Downs. The area has steep valleys and coombes, which are ideal for driving high birds. These steep valleys form part of a network of shrinking areas of chalk grassland.

Chalk grassland is estimated to cover 25-30,000 hectares across Britain, with major concentrations in Wiltshire and Dorset. The area has declined by almost 50% in the last 50 years as a result of agricultural intensification. The South Wessex Downs is internationally important for chalk grassland and for the diverse range of species associated with it. A number of the species found on chalk grassland in central southern Britain are at the northern limits of their range and as a consequence the habitat tends to support a high proportion of rare or nationally-scarce species, such as the Adonis blue butterfly, a key species in this study.

We looked at six chalk grassland sites in 2003 and 2004: three with large-scale releasing programmes (more than 15,000 gamebirds; pheasants and red-legged partridges) and three with little or no releasing. In winter 2002, we watched released gamebirds and collected faecal samples on chalk grassland to assess what the birds had been eating. We found that released birds spent a lot of time feeding on the grassland (66% for pheasants and 40% for red-legged partridges) and that they consumed a variety of invertebrate groups (see Figure 1). This suggested that direct consumption (of invertebrates and potentially vegetation) was likely to be the mechanism for any gamebird effect on the chalk grassland ecosystem.

Figure 1. Invertebrate composition of faecal samples from released gamebirds, collected in winter 2002 from chalk grassland

Spiders		Flies
Grasshoppers		Bees / Wasps / Ants
Bugs		Beetles
Lacewings		Thrips
Butterflies / moths		Snails

With this in mind, we designed an exclosure experiment to establish whether invertebrate numbers were affected by released gamebirds on areas to which they had access compared with areas from which they were excluded. In 2003 and 2004 at each of the six sites, we set up six plots before the gamebirds were released, each consisting of a square metre exclosure and a similarly sized open control pen. These remained in place during the shooting season. In the following spring, we placed emergence boxes over both exclosed and open areas and set pitfall traps in each for invertebrates. We also counted emerging Adonis blue butterflies.

In the first year, we looked at emerging invertebrates and found no difference between the release sites and control sites in the number or diversity of invertebrates emerging from the open and exclosed pens. The numbers of Adonis blue butterflies caught in the emergence boxes were very low (see Figure 2, right hand axis) and we were unable to establish any differences. In the second year, we focused on two of the invertebrate groups shown to be consumed in large numbers (see Figure 1): beetles and spiders. Again we found no differences between release and control sites in beetle and spider numbers (see Figure 2, left hand axis).

We will look for differences in the species assemblages found at the release and control sites, as there could be changes caused by gamebirds eating specific groups. So far, our work indicates no dramatic impact of released birds on the Adonis blue butterfly, but we have insufficient data to test for subtle effects.

Figure 1.

	Beetles
	Spiders
	Adonis blue butterflies

We could find no effect of released gamebirds on chalk grassland invertebrates, but plan a further project in spring 2006 to investigate more subtle effects.