Effects of reduced grazing on population density and breeding success of black grouse in northern England.

Abstract

1. The maintenance or modification of grazing regimes is frequently advocated to deliver conservation targets in pastoral landscapes, but there are few quantitative studies of the effects of grazing on upland birds. This is particularly true with respect to grazing management in agri-environment schemes.
2. Numbers of black grouse Tetrao tetrix and their breeding success were therefore monitored at 20 sites in the north of England from 1996 to 2000. Ten treatment sites included areas where grazing was reduced before and during the study to <1.1 sheep ha^-I in summer and <0.5 sheep ha^-I in winter. Each was paired with a reference site that held sheep at two (summer) to three times (winter) the density on the experimental sites. The reduced grazing sites ranged from 0.4 to 3.2 km² in size and most were part of existing agreements within agri-environment schemes that had been in place for 1-5 years before 1996.
3. Numbers of black grouse males displaying increased by an average of 4.6% (SE = 2.1) year^-I at the 10 sites with reduced grazing. Displaying male trends differed significantly between treatment and normally grazed reference sites, where numbers declined annually on average by 17% (SE = 1.4).
4. Summer black grouse hen densities showed the greatest rate of increase where grazing was restricted on smaller areas of ground (0-4 km²). Declines occurred at sites where the area of restricted grazing exceeded about 1 km². The rates of change in population density, as indicated by numbers of displaying males, peaked in the early years of grazing reduction and then declined after c. 5-7 years.
5. The proportion of females that retained broods during the late chick-rearing period was 54% (SE = 0.06) at sites with reduced grazing, significantly greater than the 32% (SE = 0.06) at normally grazed reference sites. There was no difference in the size of broods between grazing treatments.
6. This study demonstrates that agri-environment schemes, which encourage extensive management of grazing land, can benefit at least some organisms of conservation importance and lead to some recovery of populations. There is a need, however, for further understanding of how such benefits can be maintained at a landscape scale and over the greater time scales involved in vegetation dynamics and bird population processes.