Arriving depleted after crossing of the Mediterranean: obligatory stopover patterns underline the importance of Mediterranean islands for migrating birds

Hundreds of millions of birds reach the Mediterranean islands or Mediterranean coast of Europe every spring after having crossed the Sahara Desert and the Mediterranean Sea. Using data from three small insular stopover sites, we calculated body mass without fuel for 18 trans-Saharan passerine migrants. We subsequently used arrival fuel loads coupled with potential flight range estimates to assess the percentage of birds that are forced to perform an obligatory stopover after crossing the Mediterranean Sea due to fuel depletion. Average arrival fuel loads were among the lowest ever recorded in the Mediterranean region and minimum body mass values recorded for several species were lower than any other individual value reported. The percentage of birds that needed to replenish their energy stores before resuming their northward migration journey varied from 0% to 50% depending on the species and locality studied. Based on conservative estimates at least 180 million birds of our study species are expected to migrate through Greece, 14% of which would not be able to resume their migration without refueling. The significance of small islands and coastal sites in the Mediterranean as obligatory refuelling sites is discussed and their conservation value for migratory birds is highlighted under the perspective of climate change.