Factors contributing to local and regional variation in Black Grouse breeding success in northern Britain.
Data on Black Grouse Tetrao tetrix breeding success are presented for 1989, a good chick rearing year when 40% of chicks survived to ten weeks, and compared with 1990, a poor year when only 28% of chicks survived. This resulted in a mean of 2.7 juveniles per hen in 1989. In 1990, although mean brood size in July/August was similar to 1989, fewer hens had broods resulting in only 1.7 juveniles per hen. In 1989, estimates of breeding success from juvenile:adult ratios in shooting bags were only 4% less than estimates using dogs. In 1989, breeding success was the same on areas with and without predator control. In 1990, the proportion of hens without broods more than trebled on areas without predator control, resulting in three times fewer juveniles per hen than on areas with predator control. A cock biased sex ratio of 1.6:I was found in juveniles. Breeding success was higher than in most other British studies due to higher chick survival. Factors determining between-year differences in breeding success and their relevance to long-term declines in Britain are discussed.