Sleep Posture Influences Metabolic Rate and Vigilance in the Common Whitethroat (Curruca Communis)

Migration is an important life-history strategy that is adopted by a significant proportion of bird species from temperate areas. Birds initiate migration after accumulating considerable energy reserves, primarily in the form of fat and muscle. Sustained exercise, such as during the crossing of ecological barriers, leads to the depletion of energy reservesand increased physiological stress. Stopover sites, where birds rest and restore energy, play a fundamental role in mitigating these challenges. The duration of resting at stopover sites is influenced by environmental and physiological conditions upon arrival, and the amount of body fat reserves plays an important role. While sleep is recognized as essential for all organisms, its importance is accentuated during migration, where energy management becomes a survival constraint. Previous research indicated that individuals with larger fat reserves tend to sleep less and favor an untucked sleep posture, influencing energy recovery and anti-predatory vigilance. We explored the relationship between sleep behavior and posture, metabolic state, and energy conservation strategies during migration in the common whitethroat (Curruca communis). We were able to confirm that sleeping in a tucked position results in metabolic energy savings, at the cost of reduced vigilance. However, whitethroats did not show alterations of their sleep patterns as a response to the amount of stored reserves. This suggests that they may not be taking full advantage of the metabolic gains of sleeping in a tucked posture, at least at this stage of their migratory journey. We suggest that, to achieve optimal fuel accumulation and maximize stopover efficiency, whitethroats prioritize increased foraging over modulating their sleep patterns.