Citizen science results of GTAN-SPEA: censuses of the short-eared owl (Asio flammeus) and barn owl (Tyto alba) and Noctua-Portugal Program

Abstract

The Working Group on Nocturnal Birds of SPEA (GTAN-SPEA) promotes since 2010 the monitoring program for nocturnal birds called “Noctua-Portugal”. This citizen-science initiative has allowed population trend estimates to be calculated for the 10 target species. After 13 sampling seasons (2009/10 to 2021/22), the data collected by more than 140 volunteer collaborators in 91 sampling squares points to a strong decline in the Scops Owl (Otus scops) and the Little Owl (Athene noctua), and a moderate decline in Barn Owl (Tyto alba), Long-eared owl (Asio otus) and Eurasian Stone Curlew (Burhinus oedicnemus). On the other hand, the Eagle Owl (Bubo bubo), the Tawny Owl (Strix aluco), and the two nightjars (Caprimulgus europaeus, C. ruficollis) populations seem to be stable. It has not been possible to estimate the population trend for the Short-eared Owl (Asio flammeus) due to the scarcity of data. To overcome this limitation, in the winter of 2021/22, GTAN-SPEA promoted the 1st national census of the wintering population of the Short-eared Owl. Counts were carried out by 178 volunteers in 20 priority areas for the species in mainland Portugal, with the support of regional coordinators. Counts based on 340 listening points and 210 road surveys were carried out over two weekends. During the census, between 91 and 112 Short-eared Owls were counted. Most individuals were detected on the coast, mainly in the Tagus Estuary and Ria de Aveiro. In 2023, GTAN-SPEA promoted the 1st national census of the Barn Owl, thus seeking to better understand the causes behind the declining trend of this nocturnal bird associated with agricultural and urban areas, which often nests in artificial structures. This initiative was open to the entire population, relying on two methodological approaches: a systematic census and a non-systematic census (by completing an online survey). 24 regional coordinators and 268 volunteers participated in the systematic census, who carried out 1129 listening points in mainland Portugal. 340 barn owls were detected, corresponding to a detection frequency of 30%. The detection frequency per district varied between 13% (Castelo Branco) and 61% (Coimbra). In the non-systematic census, 256 surveys and 150 listening points were submitted. In total, 257 barn owls were recorded, 201 in the surveys and 56 in the listening points.