The population size of Capercaille Tetrao urogallus in Scotland was estimated from annual counts of males at 151 known (79-100) spring lek sites and 45 counts of males, females, and their broods in August during the period 2010-2020, combined with published survival estimates. Population size declined from an estimated 580 birds (95% CL: 462-698) in 2011 to 304 (239-369) in 2020 and the extinction probability was 23% after 25 years, 95% after 50 years and 100% after 100 years. Removing mortality associated with collisions against fences lowered extinction probabilities to 0%, 3% and 40%, respectively, emphasizing fence removal as a management priority. Greatest declines occurred in Deeside & Donside, Morayshire & Ross-shire and Perthshire & Trossachs. In the Badenoch & Strathspey stronghold, where an estimated 83% of males remain, numbers of occupied leks and lekking males have decreased in the last five years and are associated with reduced breeding success over the last 10 years. Our estimates of population size are only half that derived from the last official national survey in winter 2015/16. We discuss possible reasons for this difference including underestimates from lek-based surveys linked to male detection probabilities. Finally, we recommend improvements to the current annual monitoring scheme, including repeat counts at leks and use of novel remote acoustic devices and restoration of annual brood counts in remaining key forests.

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