Reduced diurnal activity and increased stopover duration by molting Swainson’s Thrushes

Abstract Migration consists of a sequence of small- to large-scale flights often separated by stopovers for refueling. Tradeoffs between minimizing migration time (more flights, shorter stopovers) and maximizing energy gain (fewer flights, longer stopovers) will affect overall migration timing. For example, some individuals make long-term stopovers in high-quality habitat that maximize energy gain (e.g., molt-migration), but movement to those habitats likely costs time. We used radio telemetry and blood plasma metabolite levels to examine physiological and behavioral tradeoffs between molt-migrant (birds molting at the molt stopover; n = 59) and post-molt (birds that presumably completed their molt elsewhere; n = 19) migrant Swainson’s Thrushes (Catharus ustulatus) near Montreal, Canada. Molt-migration was a large time investment as the average stopover duration for molt-migrants was of 47 ± 9 days (~13% of the entire annual cycle), almost twice as long as previously assumed from banding records, and far longer than stopovers of post-molting individuals (7 ± 2 days). Daily mortality rate during the molt stopover was similar to the average annual daily mortality rate. Molt-migrants’ circadian rhythms closely matched light levels, whereas post-molting birds had irregular rhythms and averaged 1 hr greater activity per day than molt-migrants. Despite being less active, molt-migrants had similar refueling rates based on metabolite profiles. As compared with migrants that completed molt earlier, molt-migrants at this stopover site had slower subsequent migration rates. Thus, birds using long-term stopovers appeared to tradeoff energy (efficient refueling) for time (slower subsequent migration). Lay Summary • Migratory birds must balance the energy and time costs of molt and migration, and the strategies they employ to overcome this challenge can vary within the same species. • We captured and tracked molt-migrant (birds molting at the molt stopover) and post-molt-migrant (birds that presumably completed their molt elsewhere before capture) Swainson’s Thrushes (Catharus ustulatus) at the same stopover site to test the expectation that molting birds remain longer and are less active but have similar refueling rates, as compared to post-molting migrants • Molting birds were less active and remained six times longer than post-molting migrants while refueling rates did not differ between the two groups. • Subsequent southward migration rates of molting birds were slo