PHENOLOGY AND WINTERING POPULATION ESTIMATES OF AUKS IN THE MEDITERRANEAN SEA BASED ON COUNTS FROM A STRATEGIC COASTAL LOCATION IN THE STRAIT OF GIBRALTAR

Some migratory seabird populations have declined sharply in recent decades. Monitoring their migratory movements from strategic locations provides a useful tool to understand their phenology and estimate their population sizes. During the autumn 2020 and spring 2021 migration periods, we conducted a total of 183 hours of counts of two auk species, Razorbill Alca torda and Atlantic Puffin Fratercula arctica, from a strategic coastal point (Tarifa Island in the Strait of Gibraltar). We used Generalized Additive Models (GzAM) to estimate the number of birds that could have entered or left the Mediterranean Sea on the days that no counting took place. Auks entered the Mediterranean from early October to early winter, peaking in the second half of November. In spring, Razorbill movements leaving the Mediterranean were concentrated from mid-January to mid-April, while the peak Atlantic Puffin movement occurred from mid-February to May. We estimate that 4,000-5,000 Razorbills and 15,000 Atlantic Puffins made up the populations migrating through the Strait of Gibraltar in the year of the study. These figures are notably lower than those previously reported for auks passing through this migratory bottleneck. Hence, the results of this study, despite coming from a single year of monitoring, may contribute significantly to assessing the numbers of auks that winter in the Mediterranean and could eventually draw attention to possible declines in their numbers.