First national census of the Common Barn-owl (Tyto alba) breeding population in Portugal

Abstract

The Common Barn-owl (Tyto alba) is a cosmopolitan farmland bird that has disappeared from almost 50% of its range in mainland Portugal in the last decade. In order to improve estimates of the distribution and population abundance of the species, and contribute to understanding its long- term population trend, we carried out the First National Census of the Common Barn-owl Breed- ing Population. The census included two different methodologies: systematic point counts with playback broadcasts carried out by experienced volunteers, organised by regional coordinators, and a non-systematic census open to the general population, which consisted of passive listening point counts and an online survey. The systematic census involved 24 regional coordinators and 268 volunteers, who carried out 1 131 point counts in mainland Portugal, resulting in 340 Barn- owl records (detection frequency of 30%). The non-systematic census included 150 point counts and 414 surveys. In total, 405 Barn-owl observations were recorded, 61 at the point counts and 344 in the surveys, also covering the autonomous region of Madeira. Combining this information with the records from PortugalAves/eBird, the Barn-owl was detected in 399 10x10 km squares in mainland Portugal and 11 in the Madeira archipelago, resulting in a population estimate of 800 to 5,000 breeding pairs. Both census methods had their advantages, and we suggest they should be replicated with adaptations in future editions. This census established the baseline for future national censuses of the Barn-owl, which will enable the estimation of long-term population trends.