Access to water affects the condition dependency of nocturnal restlessness in Garden Warblers on a Mediterranean island stopover

During migration, many songbirds encounter large ecological barriers, like deserts and seas that require substantial fuel to cross and can lead to dehydration during passage. If muscle is not catabolized to generate metabolic water, birds must seek free water on a subsequent stopover to replenish the water lost. Yet, no work has examined if birds crossing large migration barriers use access to free water in concert with energy or protein stores to make stopover decisions. We captured 61 free-living Garden Warblers (Sylvia borin) in spring at a frequently used stopover site in the Mediterranean Sea, housed them with or without drinking water, and measured nocturnal restlessness (Zugunruhe) in relation to energy stores at capture. Both groups lost equivalent flight muscle mass overnight, suggesting that water-deprived birds did not preferentially catabolize this tissue to produce metabolic water. Fat score and body mass, but not flight muscle mass, were positively correlated with nocturnal activity in both treatment groups. However, the slope of the relationship between nocturnal activity and fat score differed between groups, with water-deprived birds of high fat score showing the highest Zugunruhe activity. Our results suggest that birds with large energy stores use access to drinking water to inform their decisions about when to depart from a stopover site. Individuals with higher fat scores might be expected, regardless of flight muscle size, to depart a dry stopover site more readily than a site with freely available water. We suggest follow-up studies to further elucidate the mechanisms of this phenomenon and establish its prevalence in free-living birds.