Post-breeding migratory movements and stopover ecology of Swainson’s Thrushes ( Catharus ustulatus ) near Lake Ontario

Migratory birds rely on stopover sites for critical rest and refueling periods. Great Lakes sites in northeastern North America provide important stopover habitat for birds; however, little is known about how Swainson’s Thrushes (Catharus ustulatus) may use these sites, especially during post-breeding migration. We used automated radio-telemetry to estimate stopover duration of Swainson’s Thrushes captured on the south shore of Lake Ontario during southward migration in September 2020 and 2021. We evaluated physiological state of the thrushes by measuring diverse condition metrics—including plasma metabolites and leukocyte counts—and investigated relationships of stopover duration with intrinsic condition measures and day of season. Stable isotope analyses and radio-telemetry detections beyond the tagging site provided insight into natal origins and overall migration distance for this species. Thirty-six thrushes were blood-sampled and radio-tagged, and 31 birds were sampled for stable isotope analysis (18 of which were also among the former group). Plasma triglyceride was the most important predictor of stopover duration, where birds with the highest triglyceride concentrations had the shortest stopover duration. Total leukocyte counts also negatively influenced stopover duration. Uric acid, scaled mass index, and the heterophil/lymphocyte ratio were not related to stopover duration, and stopover duration did not vary temporally. Tracking data revealed that thrushes spent a mean of 11 ± 1 days at Braddock Bay prior to traveling minimum distances of c. 3,700 km to nonbreeding locations. Analysis of deuterium in feathers suggested that most individuals had plausible breeding origins in southeastern Canada and northern regions of the midwestern and eastern United States. Our study sheds light on important intrinsic factors that may influence stopover decisions of Swainson’s Thrushes and provides complementary and needed insight into the stopover ecology of this species in the Great Lakes region of North America.