Lean mass deposition occurs at a greater rate than fat deposition during pre-breeding stopover in highly depleted songbirds in the northern Gulf of Mexico.

The Gulf of Mexico represents the largest ecological barrier between breeding and non-breeding grounds for long-distance migratory songbirds in the Nearctic-Neotropical system. Despite the prominence of the Gulf of Mexico, there are still gaps on fundamental physiological aspects of stopover of migrants in this region, including the role and relative importance of fat and lean mass depletion and deposition. We examined the arrival body condition of Nearctic-Neotropical migrants at a coastal stopover site on St. George Island, FL, in the northern Gulf of Mexico during pre-breeding migration in the spring of 2016-2018. We precisely determined lean body and fat masses on individual birds after a trans-Gulf migratory flight via quantitative magnetic resonance (QMR) technology. We hypothesized that birds with non-breeding ranges in South America would arrive with lower fat and lean masses than birds with non-breeding ranges in the Caribbean or Central America. We also hypothesized that songbirds would increase lean mass at a greater rate than fat mass, as they rebuilt muscle and organ masses. We also compared QMR lean and fat measurements to visual measures of fat and muscle scores. A total of 44 Nearctic-Neotropical migratory bird species occur on St. George Island during spring stopover. Non-breeding range did not influence the arrival fat mass or arrival lean mass in 10 focal transient species, meaning those that have no breeding or non-breeding populations on the site. Our results from recaptured individuals indicated that body mass increase during stopover derives from both lean and fat mass accumulation. Our results provide a robust quantitative assessment of songbird arrival body condition on the northern Gulf of Mexico and contribute to the understanding of the physiology of migratory songbirds after a long-distance flight, which will help inform management decisions for stopover sites located around ecological barriers.