Divergent migratory strategies lead to variable refueling performance amongst Gray Catbirds (Dumetella carolinensis) during spring stopover in the Gulf of Mexico

Each spring, migratory Gray Catbirds (Dumetella carolinensis, henceforth referred to as Catbirds) that have wintered in Central America and the Caribbean, fly across the Gulf of Mexico during their northern migration. These long-distance flights are primarily fueled by lipid reserves, but protein is also catabolized leading to significant depletions in organ and muscle tissue mass upon arrival at stopover sites. Here, Catbirds must not only recover from their previous intensive flight across the Gulf but must also provision for the next leg of migration as they continue to their breeding grounds in North America. There is still much to be known about how reduced lean mass at stopover may constrain refueling performance. Catbirds present a unique opportunity to study refueling performance relating to differential migratory strategies as these songbirds have significant geographic spread in their North American breeding grounds. We caught Catbirds during spring migration on St. George Island, FL over three consecutive years. We measured total fat and lean mass using non-invasive quantitative magnetic resonance, basal metabolic rate using standard flow-through respirometry techniques, and plasma triglycerides to determine refueling rate. We then used feather deuterium (δD) to estimate breeding location and migratory distance. We determined, with high probability, three migratory strategies used by this group of Catbirds at stopover, yet found no differences in total, fat, or lean body mass amongst birds attributed to be short, medium, or long-distance migrants. However, migratory distance explains the relationship between total mass and refueling rates, with long-distance migrants having a larger increase in plasma triglyceride concentration per unit increase of body mass compared to short-distance migrants. Furthermore, we document a physiological strategy whereby long-distance migrants can deposit greater amounts of lean and fat mass with a smaller associated increase of metabolic rate than predicted for their size.