Numbers and breeding success of Capercaillies Tetrao urogallus and Black Grouse T. tetrix at Abernethy Forest, Scotland

Abstract

Capsule In most years, changes in numbers are associated with variations in breeding success. Aims To describe the annual variation in numbers and breeding success of Capercaillies Tetrao urogallus and Black Grouse Tetrao tetrix at Abernethy Forest, and their inter‐relationships. Methods Numbers and breeding success of Capercaillies and Black Grouse were recorded annually at Abernethy Forest (a Scots Pine Pinus sylvestris forest comprising ancient native, or semi‐natural, pinewood and plantations) during 1989–2009. Indices of abundance and densities of Capercaillies were obtained along transects, while counts of males at leks were obtained for both species. Breeding success (number of chicks per female) was obtained using dogs to locate females and chicks. Results Capercaillie: the index of abundance increased to a peak in winter 1996/97 (2.7 times greater than in 1992/93) and then declined. There was no long‐term trend. In winters 2003/04 and 2004/05, the mean density was 4.2 per km2 (95% CLs 3.1–5.7). The total number was 140 (95% CLs 100–220) for 2003/04 and 170 (95% CLs 110–280) for 2004/05, comprising about 8% of the Scottish population. The number of males at leks peaked in 1995 (to 46 males) and again in 2004 (41 males), and there was no long‐term trend. The annual breeding success varied from 0 to 2.93 chicks per female (mean = 0.64). The mean was within the 95% CLs of an independent estimate of the productivity required to maintain numbers. In a free‐running model, annual breeding success and survival rates (which were assumed to improve when mortality owing to fence collisions was removed) largely accounted for the annual variation in the index of abundance, as measured from winter counts along transects during 1990/91 to 2002/03. If mortality associated with collisions with fences had continued, the index would have declined, assuming no immigration. Black Grouse: The number of male birds at leks increased to a peak in 1997 (to 165 males), before falling back to a smaller number (about 50 males) in the early 2000s. There was a smaller peak in 2007. The annual breeding success varied from 0 to 4.71 chicks per female (mean = 1.76). Conclusion Numbers of Capercaillies and Black Grouse varied over a 19‐year period at Abernethy Forest, but did not show either upward or downward trends, while the national population of Capercaillies dipped to a low level in 1998/99, and the Black Grouse population continued to decline. In most years, changes in numbers of both species were associated with variation in breeding production. Mortality caused by collisions with fences would have led to a decline in Capercaillie numbers if fences had not been removed.