Physiological conditions of trans-Saharan migrants after crossing the
Mediterranean are not associated with the weather at sea
Abstract
Migration is an energy-intensive phase of birds’ life cycle, often
including the crossing of large ecological barriers on non-stop flights.
The release of corticosterone (CORT), an adrenocortical hormone (also
known as the stress hormone), generally rises at the onset of migration
to facilitate and sustain high-energy metabolism. However, during
prolonged flights, unfavourable meteorological events may affect the
ability of migrants to cope with other stressors. In this study, we
evaluated the effects of weather along the Mediterranean Sea on the
physiological conditions of birds that reached a stopover island. We
assessed fuel stores and CORT variations during pre-breeding migration
in two trans-Saharan species, the Common Whitethroat (Curruca communis)
and the Garden Warbler (Sylvia borin), upon arrival after the
Mediterranean Sea crossing. By examining tailwinds and air temperature
at different altitudes in the period preceding the arrival, we found
that even with unfavourable weather conditions birds arrived at the
stopover site with residual energy reserves. Also, adverse weather
conditions did not influence baseline CORT levels. Interestingly, while
both species exhibited a normal stress response to additional stressors,
Garden Warblers showed peak CORT levels correlated with higher
temperatures. These results suggest that migrants are generally
well-equipped to handle variable weather conditions and may avoid
migrating during extreme weather. As only birds surviving the sea
crossing were studied, we suspect that most birds in poor condition did
not reach the stopover site. Species-specific adaptations and diverse
pre-migration ecological contexts may lead to different migration
strategies to face this long-distance journey. Future research should
explore to what extent physiological adjustments may aid migrants in
dealing with the current increase in extreme meteorological events.