Migrating birds accomplish the extraordinary feat of trans-oceanic and even trans-hemispheric treks, a true marvel of the animal kingdom. Prior to these excursions, many bird species display dramatic changes in morphology, withering less crucial organs and increasing essential flight components like heart and flight muscle. Although bulking up muscle in humans necessitates an increase in their use or exercise, recent evidence in birds suggests an additional endogenous capacity for muscle gain, independent of behaviour or the environment. Barnacle geese (Branta leucopsis) are known to increase production of key metabolic proteins prior to migration. Wild, flighted birds of this species have significantly higher levels of these proteins than do flightless captive or juvenile geese. This suggests that for this species, flight training may be crucial to migratory readiness. With this knowledge, Steve Portugal of the University of Birmingham (now at the UK's Royal Veterinary College) and his international team set out to further investigate these trends, predicting that long-distance migrants like the barnacle goose might increase flight activity prior to migration to achieve a heftier physique and stimulate essential protein production.
The team caught wild barnacle geese near Norway's Ny-Ålesund research station, implanted custom-made heart rate loggers into the birds' abdomens, and recaptured the birds the following year. The data loggers monitored the heart rate of the birds continuously throughout the year, including both the spring and autumn migrations. As it is known that heart rate increases dramatically during flight, these records allowed the researchers to construct flight activity patterns. Using this strategy, the team calculated the time in flight for each day for each goose in addition to comparing heart rate patterns during distinct annual phases (pre-migration, migration, breeding and wing-moult). Portugal's crew discovered that, contrary to their original hypothesis, there was no difference in the time spent flying per day in the pre-migratory phase compared with any other period. As a buildup of flight muscle prior to migration has previously been documented in this species, this implies that increased flight activity is not required to achieve their burly physique.
Other bird species show strong relationships between pre-migratory mass gain and increased flight muscle. It is possible, then, that the corresponding increases in wing loading due to the heftier build documented in pre-migration barnacle geese triggers an increase in flight muscle. With this loading effect, the usual amount of time spent flying (just 22 min day−1 for these geese) may be adequate to achieve buildup of flight muscle prior to migration, and potentially induce essential protein production. It is also possible that any endogenous capacity for muscle building in waterfowl may support this pre-migratory couch potato approach. Nonetheless, this study reveals that a flight-training regime is not required to prime the barnacle goose for its impressive long-distance migration. As flight is the most costly form of vertebrate locomotion, 10–20 times more energetically expensive than the resting state, this may also reflect a strategy to conserve vital energy stores for the remarkable journey that lies ahead.