Arrival condition and refueling performance of long-distance migratory songbirds at a spring stopover site in the eastern Gulf of Mexico

Abstract

Barrier islands along the northern Gulf coast provide critical stopover habitats for Nearctic–Neotropical migratory songbirds using a spring trans-Gulf migration strategy. Islands bordering Apalachicola Bay, Florida, project southward into the Gulf of Mexico providing an opportunity for first landfall. In 2013 and 2014, we studied spring stopover ecology on St. George Island, Florida. We provide a detailed description of the arrival body condition and fuel deposition rates of 8 species of migratory birds. We used 3 approaches to measure fuel deposition rate: (1) estimated an hourly rate of mass change by regressing body mass or scaled body mass by time of day, (2) estimated a daily rate of mass change by examining body mass of recaptured individuals, and (3) for 4 species, measured plasma triglyceride concentrations as a proxy of individual refueling rate. The majority of migratory birds in the eastern Gulf of Mexico arrived in poor body condition with depleted fat and muscle stores. We found no significant changes in body mass or scaled body mass as the day progressed, except for Indigo Bunting (Passerina cyanea), which gained mass throughout the day. Recapture rate was low on St. George Island (<5%), but most recaptured individuals tended to lose mass. Using plasma triglyceride concentration, we found that refueling rate for Gray Catbird (Dumetella carolinensis) declined throughout the day. We found no relationship between body condition and refueling rate in Gray Catbird or Catharus thrushes. Our results suggests that long-distance migratory birds arriving on St. George Island after a nonstop migratory flight may be physiologically challenged due to poor body condition. Our results provide the first quantitative assessment of songbird refueling performance on Florida panhandle barrier islands and reinforce the importance of Gulf of Mexico barrier islands as critical stopover sites for long-distance migrants in poor body condition.