Depleted lean body mass after crossing an ecological barrier differentially affects stopover duration and refuelling rate among species of long-distance migratory birds

1. During the long-distance migratory flights of birds, lean mass breakdown occurs in concert with fat catabolism and is expected to have repercussions on total stopover duration because birds require time to rebuild lean tissue before accumulating fat reserves. Despite this, little is known about the role of in-flight lean mass breakdown on stopover duration because direct measurements are restricted by the destructive nature of traditional body composition analysis and the technological limitations of tracking small birds over large expanses. 2. We used non-lethal, non-invasive Quantitative Magnetic Resonance technology and plasma metabolite profiling to measure the body composition and physiological state of free-living birds captured at a migratory stopover site after flight across the Gulf of Mexico, and an automated radiotelemetry array covering similar to 5000km(2) to track stopover duration and regional movements. We tested whether stopover duration is prolonged in individuals arriving with lower lean mass and investigated how lean mass affects regional movements. 3. Stopover duration decreased by 22% for each additional gram of lean mass in Northern Waterthrush (Parkesia noveboracensis), but this relationship was not apparent in Swainson's Thrush (Catharus ustulatus), Grey-cheeked Thrush (Catharus minimus) or Yellow-billed Cuckoo (Coccyzus americanus), even though these species also arrived with depleted lean mass. Stopover duration increased for Swainson's Thrush with higher plasma uric acid, a marker of protein catabolism. Northern Waterthrush with higher plasma triglycerides had longer stopover duration. 4. Our findings suggest that migratory birds may compensate for substantial lean mass losses by increasing refuelling rate or moving to a different stopover site, and highlights species-level differences in lean mass breakdown and the associated impacts on physiological function. Our results highlight the strategies used by different species to recover from a trans-Gulf of Mexico flight and resume migration, which improves our understanding of the annual cycle of migratory birds.