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

Authors: Ana Morales, Barbara Frei, Greg W. Mitchell, Camille Bégin-Marchand, and Kyle H. Elliott

Year: 2022

Publication: Ornithology

Publication Link:

Keywords: Catharus ustulatus, circadian rhythm, metabolites, migration rate, molt-migration, radio telemetry, stopover


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, bird using long-term
stopovers appeared to tradeoff energy (efficient refueling) for time (slower subsequent migration).

• 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 slower than the post-molting birds.
• molt-migration at these important stopovers provides insights
into why some individuals undergo
molt-migration and others do not.

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