Estimating the population and distribution trends of owls in Portugal using citizen science data from Program NOCTUAPortugal and other sources

Abstract

During the last eight years (2010-2017), the Working Group on Nocturnal Birds of SPEA (GTAN-SPEA) has carried out a volunteer monitoring program (NOCTUA-Portugal) to determine the population trends of seven owl species at a national scale. We used generalized estimating equations models to assess trends. In addition, we used the distribution data from breeding bird atlases, observations sent by collaborators, and data from the public online database PortugalAves/eBird to detect changes in distribution in a four-decade period (1978-2017). We defined four periods to study potential changes in distribution of owls in Portugal: 1978-1984 (1st breeding bird atlas); 1999-2005 (2nd breeding bird atlas); 2006-2014 (interval between 2nd and 3rd breeding bird atlases); and 2015-2017 (ongoing 3rd breeding bird atlas). Three species had negative population and distribution trends: Common Barn-owl (Tyto alba), Eurasian Scopsowl (Otus scops), and Little Owl (Athene noctua). The Eurasian Eagle-owl (Bubo bubo) had a slightly positive population and distribution trends, while the Tawny Owl (Strix aluco) was stable or slightly declined. The population trend of the Northern Long-eared Owl (Asio otus) was unknown because of large fluctuations, and due to the low number of records it was not possible to determine a population trend for the Short-eared Owl (Asio flammeus).