Welcome aboard: are birds migrating across the Mediterranean Sea using ships as stopovers during adverse weather conditions?

Abstract

Birds use stopovers during migration to interrupt endurance flight in order to minimize immediate and/or future fitness costs. Stopovers on ships is considered an exceptional and anecdotal event in the ornithological literature. This does not match the experience we had in the summer of 2021, during an oceanographic campaign in the Central Mediterranean, when we regularly observed on average 2.8 birds, of at least 13 species, stopping on board during the 25 days of the campaign. The median stopping time was 42 min, ranging from a few minutes to overnight stays on board. The probability of finding a bird stopping aboard increased with wind force and cloud cover. Birds also stopped more often in a headwind and did not stop when the wind came from different directions other than the headwind. The Central Mediterranean is one of the busiest sea routes in the world, combining the mean daily number of birds on board with the thousands of ships that pass through it during the 3 months of summer migration; we estimate that nearly 4 million birds could use ships as stopover sites. This behaviour may represent a modern-day strategy that uses ships as stopovers in the event of adverse weather conditions or could act as an ecological trap, increasing the mortality of migrants. This phenomenon deserves more research attention and further studies recording body condition and tagging of individuals on board would be informative.

Stopover Ecology of Passerine Birds on an Islet in the Bay of Biscay in Autumn Reveals a Pattern Similar to Their Use of Small Mediterranean Islands in Spring

“…The landing of birds on islands during migration is a well-known phenomenon elsewhere, such as in the Mediterranean, where these small pieces of land in the vast sea constitute migratory stopover points for numerous species (Pilastro et al , 1998; Rubolini et al , 2005; Gargallo et al , 2011; López-Iborra et al , 2022). Often, and especially on smaller islands, these stopovers last no more than a few hours and their only function is for resting and recovery from physical exhaustion (Gargallo et al , 2011), acting as a ‘rescue boat’ (Sarà et al , 2023). Studying this phenomenon, however, is useful to deepen our understanding of migratory strategies and the role of seas as ecological and evolutionary drivers of migratory bird populations.…”

“…Results from previous studies, e.g., within the Mediterranean, suggest that small islands along the Bay of Biscay offering (apparently) relatively limited feeding opportunities (Gargallo et al , 2011; Arizaga et al , 2012) are primarily used as opportunistic or emergency stopover sites ( sensu Overdijk & Navedo, 2012) by birds exhausted after the sea crossing (Schmaljohann et al , 2022; Sarà et al , 2023). We therefore predict that most individuals stopping on small islands in the Bay of Biscay make short stopovers (< 24 h) and have modest fuel deposition rates (Gargallo et al , 2011).…”