AMHERST, Mass. â€“ Ornithology researchers Alexander Gerson and his postdoctoral research associate Derrick Groom at the University of Massachusetts Amherst, with others, compared the short-term effects of resting vs. long duration flight on the rate of lean body mass loss in birds. They found that the total quantity and rate of protein loss was not different in the two conditions, but both were affected by ambient humidity.
â€œThis work advances our understanding of the role of the unique and dramatic fluctuations we observe in muscle and organ masses in migratory birds,â€� the authors point out. Details appear in a recent issue of in the Proceedings of the Royal Society B.
Eco-physiologist Gerson and Groom conducted experiments with Swainsonâ€™s thrushes in this study supported by the National Science Foundation; work they believe is the first to compare the acute effects of metabolic state, rest versus exercise, on lean mass dynamics in birds in a controlled experimental setting.
They collaborated with colleagues at Long Point Bird Observatory in Ontario, Canada, and at the Advanced Facility for Avian Research at Western University in nearby London. It is home to the worldâ€™s only hypobaric climate-controlled wind tunnel specifically designed to study bird flight. The Swansonâ€™s thrush migrates thousands of miles each year from the boreal forest in Canada to Venezuela and Central America.
Groom says, â€œItâ€™s been known for a long time that fat is the major fuel for supporting migration, but people donâ€™t generally know that birds also burn protein, which shrinks organs such as the stomach, the intestine and the liver. They can reduce their digestive tract by as much as 50 percent after long flights, and birds can lose a quarter of their muscle mass.â€�
Further, â€œThey donâ€™t just burn through fat and then turn to the muscle, it all happens simultaneously,â€� he adds. He and Gerson investigated how and why birds burn protein, and in particular the role of dehydration in the process.
â€œWe wanted to test a couple of hypotheses to find out more,â€� Groom notes. The â€œprotein-for-waterâ€� hypothesis says that breaking down protein offsets some of the water loss birds naturally experience when flying. Another idea they tested is the â€œtissue-specific turnover rateâ€� hypothesis, which says that protein breakdown is due to the fact that the animal is not eating while migrating and proteins naturally break down with fasting.
In addition to the wind tunnel designed to study long duration flight in birds Ââ€“ one of just three in world â€“ Groom, Gerson and colleagues used a field-portable quantitative magnetic resonance imaging (QMRI) machine for body composition analysis, to assess the rate of body fat and muscle mass loss in groups of flying birds compared to birds at rest in the wind tunnel in low- and higher humidity conditions.
Birds flew up to 12 hours in the wind tunnel, but flights were stopped if an individual stopped flying three times within five minutes. Eleven birds that were willing to fly in the wind tunnel were flown in low- and high humidity environmental conditions and evaluated as well as after rest in the tunnel. All birds were allowed at least three days to recover between experimental flights or resting bouts.
The researchers found that â€œinstead of metabolic rate, ambient humidity was a key regulator of lean mass catabolism,â€� and dryer environments elicited greater protein breakdown than more humid environments. Groom adds, â€œWe found that both resting birds and flight birds lost protein regardless of humidity level, but birds in the dry environment tended to lose more protein than birds in humid environment. There was no difference between birds that flew versus those that rested in terms of lean mass loss.â€�
Gerson says, â€œBasically it means that both hypotheses do work together. Protein loss is occurring regardless of whether the animal is active or not, and the role of the ambient environment can enhance whatever else is going on.â€� He adds, â€œWe were surprised,â€� he adds. â€œWe thought for sure that the flying birds would burn more protein than the resting birds, but they didnâ€™t.â€�
They write, â€œWe found that both hypotheses are supported, that birds constitutively burn protein when undergoing both fast and flight, but it can be exacerbated by environmental stress.â€� Further, â€œSince climate change may cause a rise in the environmental stressors (changes in temperature, humidity, and wind speed and direction), migratory birds may experience augmented rates of protein breakdown in flight which could negatively impact migration success.â€�
Groom points out, â€œThe broader picture is that this result makes us really think about whether a bird can burn enough fat to get to where they want to go, but it also makes us think about whether they have enough protein. We also learned more about how ambient environment affect the birdâ€™s ability to complete a migration.â€�
Because many birds are omnivorous and get much of their protein from insects, he and Gerson point out, identifying and preserving habitat that can support their protein needs along the migration route is critically important.