a b s t r a c tThe endemic Azores Bullfinch or Priolo (Pyrrhula murina) is one of the most threatened European passerines, historically restricted to a single population in the Pico da Vara/Ribeira do Guilherme Special Protection Area (São Miguel Island, Azores, Portugal). Annual census of this species started in 1991. It usually took one observer 1-2 months to complete the census and even then, it was not logistically reasonable to include some areas with very steep slope in the core of the protected area. In order to bypass these constraints, an alternative and ambitious monitoring scheme that would allow estimating the whole species throughout its entire range was proposed in 2007. The new scheme, which complemented and improved previous estimates, consisted in performing a thorough instantaneous census using a distance sampling approach, thus allowing a detailed snapshot of effective occupation, density, habitat preferences and distribution range of the Azores Bullfinch. The Priolo Atlas turned into a volunteered geographic information and citizen science-based ornithological initiative which every 4 years congregates more than 50 well-trained volunteers in the Azores. With the first two editions (2008 and 2012), this initiative has already established itself as a critical tool to monitor the population of this endemic species.

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