Structured citizen science to unravel the distribution of wintering Short-eared Owls (Asio flammeus) in Portugal

Abstract

Citizen science has proven to be very useful in ornithology, especially for data collection of the distribution of discrete raptors. Short-eared Owls (Asio flammeus) are present in Portugal during winter, where their reduced vocal activity, low density and crepuscular habits hamper detection. Nevertheless, the overall decline of its European populations emphasizes the importance of determining its distribution in Portugal, estimating the size of the wintering population, and establishing a baseline for future assessments. To accomplish this, we assigned regional coordinators to 20 areas, who were responsible for recruiting amateur observers across pre-selected priority areas. In December 2021 and January 2022, the participants conducted sampling points with playbacks at sunset, followed by car transects. This sampling effort totalled 400 h and resulted in the recording of 91-112 wintering Short-eared Owls in Portugal. The 178 volunteers allowed for extensive coverage of the priority areas. The results support the previously known relevance of areas such as the Tagus estuary, but raised some concern, especially about the Sado estuary, where this species was thought to be more abundant in the ’90s. The census also yielded the first record in the Castelo Branco area. The extensive coverage of suitable habitat areas for the species would likely be impossible without the involvement of so many volunteers, and this is of major importance in elucidating the distribution of discrete raptors. This shows that well-managed citizen science programs can play an important role in elucidating the distribution and addressing knowledge gaps about raptors, regardless of their cryptic habits.