Every year, billions of birds embark on incredible journeys to migrate between breeding areas and favorable wintering grounds. Most migratory birds break up their trip and stop over along the way to rest and refuel. This alternation between an exercise-and-fasting phase (migratory flight) and a resting-and-refueling phase (stopover) requires major physiological and behavioural changes. This switch is likely triggered by hormones. We know that migratory birds mostly rely on subcutaneous fat deposits to fuel migratory flights and that the extent of fat is a key internal factor in the birds’ decision to prolong stopover or resume migration. Ghrelin is an appetite-regulating peptide released by the stomach found in most vertebrates. Studies on domesticated birds showed that ghrelin inhibits food intake and regulates fat metabolism, specifically by stimulating fat utilization and reducing the build-up of fats. Regulation of feeding behaviour and fat metabolism is crucial during migration. My PhD research revealed a key role of ghrelin in controlling the stopover behavior in a European migratory songbird, the Garden Warbler (Sylvia borin). Ghrelin reflected the extent of fat stores in birds during stopover and its peripheral administration trigged changes in food intake and in the birds’ urge to resume migration (Goymann et al 2017, https://doi.org/10.1073/pnas.1619565114).

My current project largely focuses on the functions that ghrelin exerts during the flight phase of migration. I am interested in identifying how ghrelin responds to high-intensity migratory flights and associated food deprivation, and what is the role of ghrelin on fuel metabolism during flight. In addition, a key objective of my research is to broaden our knowledge of ghrelin involvement in decision-making processes during migration. I work with two related species of American songbirds, the Blackpoll Warbler (Setophaga striata) and the Yellow-rumped Warbler (Setophaga coronata). I use an interdisciplinary approach which integrates physiology, behaviour and ecology, also combining field and laboratory experiments. The University of Veterinary Medicine, Vienna, and Western University, Canada cooperate in this project and provide a wide and complementary set of expertise in the field of avian eco-physiology and endocrinology. Below, I present my main objectives.

Ghrelin and endurance flight

During migratory flights, birds sustain high-intensity endurance exercise while fasting and exclusively relying on internal energy stores. Studies on domesticated birds showed that fasting increases ghrelin levels. However, prolonged migratory flights are associated with a reduction in size of metabolically expensive digestive organs, including the proventriculus, which is the glandular portion of the avian stomach and main site of ghrelin production. Changes in circulating ghrelin may occur in response to phenotypic flexibility of the proventriculus during migratory flights.

With this study, I aim to investigate the effects of migratory flights and associated food deprivation on circulating ghrelin, proventriculus size and expression of ghrelin in the proventriculus. To meet this objective, I make use of a climatic wind tunnel specialized for bird studies (https://birds.uwo.ca/facilities/index.html), a unique apparatus that allows us to simulate non-stop migratory flights in a controlled environment. Most Blackpoll Warblers and Yellow-rumped Warblers caught during migration can complete multi-hour flights in the wind tunnel.

I am excited to say that preliminary findings on a partial dataset revealed very interesting patterns. Exercise itself appears to affect circulating ghrelin levels, independently of the associated food deprivation. Furthermore, exercise seems to disrupt the tendency of ghrelin to positively reflect the amount of fat stores. Stay tuned to this page and my Twitter page (https://twitter.com/SaraLupi14) for more updates on the effects of exercise on ghrelin in birds!

Ghrelin and fuel metabolism

The preparation and the execution of high-intensity non-stop flights in migrating birds requires major physiological adjustments, which induce metabolic and behavioural changes. Crucial for the success of a migratory flight is the metabolic transition from fat accumulation during stopovers to fat utilization during flights. Migratory birds primarily fuel high-intense flights through the rapid mobilization of fatty acids from subcutaneous adipose tissue to the working muscles by the circulatory system. Elevated concentrations of circulating free fatty acids and glycerol, products of triglyceride breakdown, indicate the mobilization from adipose tissues. As a metabolic-regulatory peptide, ghrelin may have a key role in modulating fuel metabolism in preparation for and during flight. Ghrelin and its receptor (GHS-R1a) are widely expressed in the adipocytes and liver, main site for lipogenesis in birds. In mammals, ghrelin stimulates mass gain and reduces fat utilization. On the contrary, ghrelin administration induces fat utilization in birds. Yet, we know little about direct effects of ghrelin on metabolic correlates of migratory flights.

With this study, I aim to investigate the role of ghrelin in the regulation of fuel metabolism during flight, with particular attention to fat metabolism. To meet this objective, I measure how specific fuel metabolites respond to flight and experimentally elevated ghrelin in captive birds flown in the wind tunnel.

Ghrelin and migratory behaviour

What messengers tell the brain of a migratory bird that its fuel reserves are sufficient to resume migration? A number of hormones are involved in the regulation of metabolism, energy mobilization, food intake and fat storage in birds, e.g. glucocorticoids, ghrelin, leptin, adiponectin, orexin, and obestatin. In the past years, we started to gather important information on the involvement of the two isoforms of ghrelin, acylated and unacylated ghrelin, on stopover decisions in migratory birds. We found that injections of unacylated ghrelin reduce food intake and enhance the urge to migrate in free-living migrating birds temporarily caged at a stopover site during spring migration. On the other side, injections of acylated ghrelin reduced the urge to migrate (Goymann et al 2017).

With this study, I aim to investigate the role of acylated and unacylated ghrelin on migratory behavior using a combination of field and laboratory experiments. In a field experiment, I use automated radio telemetry methods (https://motus.org) to ascertain if changes in stopover decisions caused by increased ghrelin concentrations can modify departure decisions and migratory strategy of free-living migratory birds. In a laboratory experiment, I video record individually caged birds to study the behavioural response to experimentally elevated ghrelin while controlling for potentially masking factors, such as amount of food available, temperature and humidity.

Funding

Funded by Marie Skłodowska-Curie Individual Global Fellowship

H2020-MSCA-IF-2017 Project Number: 798739, GHRELMIGRA