We analysed the trend of the number of singing males of Cantabrian Capercaillie Tetrao urogallus cantabricus in 207 leks on the southern slope of the Cantabrian mountains, north-west Spain, in the period 1981 to 2003. The area of occupancy in 1981–1982 covered up to approximately 2,070 km2, but in 2000–2003 it was 693 km2, a 66% area reduction in 22 years. The 1981–1982 census estimated a minimum number of 274 singing male Capercaillie on the southern slope, but in 2000–2003 only an estimated 81 males were recorded, a reduction of 70%. A linear regression was used to detect and describe the changes in the size of the total population of the southern slope as well as individual subpopulations. The overall trend for the total subpopulations was a significant decrease in the number of males (y = 39.94−0.018x; R2 = 0.944, P < 0.001). The negative regression slopes indicated that the number of males had fallen in all the subpopulations considered. In the western core area the greatest annual mean rate of decline was detected in the Sierra de Picos de Ancares subpopulation (−6.22%) and in the eastern core in the Sierra de Riaño subpopulation (−3.43%). The principal local limiting factors (habitat fragmentation, human disturbance, small population size and competition) that have caused the population decline are discussed, together with the demographic, genetic and evolutionary consequences for the surviving subpopulations.