Long-term monitoring of endangered birds is essential to estimate population trends and to identify potential causes of population decline. This is particularly important for alpine birds inhabiting mountain areas at the boundaries of their range. here we analyse the population trend of Capercaillie in the Spanish Central Pyrenees based on annual surveys carried out between 2000 and 2017. We found a significant population decline (around 58%) in the number of birds counted in leks. Most capercaillies inhabit coniferous forests of Black Pine with abundant Bilberry and rhododendron understorey. The number of males declined at lower altitudes and in more exposed orientations, in a scenario consistent with the differential rate of loss of habitat quality due to climate change. We hypothesised that one of the main causes of the Capercaillie decline could be low breeding success (average annual productivity 0.67 chicks per female). In light of the decline rates observed, the Pyrenean population should be relisted as endangered in the Spanish National Catalogue of Endangered Species. Affording a higher degree of protection should guarantee the adoption of management measures to reverse or slow down the general trend of decline of the species in the south of its range.