Chasing the Chunk: Tracking Fat Loads and Movement in Migratory Songbirds

By University of Montana Bird Ecology Lab November 5, 2019
Swainson's Thrush. Bob Martinka, photo.

Gray Catbird. Bob Martinka, photo.

Migration is one of the most physically taxing activities birds undertake. Songbirds migrating through Montana may fly thousands of miles in a matter of weeks to their winter ranges. For instance, University of Montana Bird Ecology Lab researchers tracked a Gray Catbird from western Montana that traveled 1,376 miles over 18 days (an average of 76 miles a day!) to its wintering grounds in NE Mexico. Birds h ave special adaptations to cope with the extremes of such taxing journeys. In addition to unique respiratory systems, birds use fat, a denser and more efficient source of energy than carbohydrates or protein, as the primary fuel source for high-endurance activity. How and where do birds replenish their fat stores, and what regulates these behaviors?

Bird’s remarkable migratory flights are fueled by food eaten at stopover sites. Stopover sites are locations along a bird’s migratory route where they stop to eat and replenish energy used on their journey. Quality stopover habitat is critical for migration. Songbirds can gain up to 10% of their body weight each day at stopover sites. Understanding the choices birds make at stopover sites has important conservation implications. Researchers at the University of Montana are studying the way physiology affects the physical demands at stopover sites.

Joely DeSimone, a PhD student in the Breuner Lab at the University of Montana, studies the way hormones regulate movement patterns in migratory birds. She is interested in the way hormones drive interactions with the environment during migration. Stopover sites provide a natural laboratory for learning about the relationship between physiology and behavior. “Birds spend more time at stopover sites than they do in long-distance flight during migration. Stopover sites provide critical habitat for recovering from flight and refueling for the next leg of their journey, and I’m interested in understanding these physiological changes,” said DeSimone.

Swainson’s Thrush. Bob Martinka, photo.

In collaboration with the University of Montana Bird Ecology Lab, DeSimone studied migrating Gray Catbirds and Swainson’s Thrushes on MPG Ranch, a conservation property in the Bitterroot Valley, this past August and September. They sought out birds along the floodplain of the Bitterroot River and woody draws in the foothills of the Sapphire Mountains. They captured birds in special nets, took a tiny blood sample, measured levels of fat and muscle on the bird’s body, and released them with a MOTUS transmitter, a promising new technology for tracking bird movement. The samples will be analyzed to learn about stress and body condition.  Combined with information from the MOTUS transmitter, DeSimone hopes to learn more about how fluctuations in corticosterone influence behavior during a bird’s stopover.

Corticosterone is an important hormone for regulating energy in birds. According to DeSimone, corticosterone plays different roles at different times during migration. Corticosterone is known to spike at arrival and departure at stopover sites. When they arrive, corticosterone helps stimulate feeding behavior. As they approach departure, corticosterone spikes again, encouraging Zugunruhe, or migratory restlessness. However, “no one has looked at the way corticosterone levels change over the duration of a stopover” reports DeSimone. She hopes her research will fill in some of the gaps.

DeSimone is combining the physiology data collected from catbirds and thrushes with information gleaned from their MOTUS tags about how long they stay in the area. Using MOTUS towers, a series of radio towers tuned to the same frequency, UM researchers are able to determine how long birds are staying in the area (visit motus.org to learn more about this technology). The tags placed on birds like small backpacks emit a uniquely coded signal, allowing researchers to recognize individuals as they move through the area. The towers can detect signals from up to 15 or more miles away. In addition, birds with MOTUS tags can be detected by the growing network of MOTUS towers throughout the continent. “Which is exciting”, said DeSimone, “to be able to track movement locally, but also nearby areas in the Bitterroot Valley and beyond”. Already this fall two Gray Catbirds with MOTUS tags have been detected by towers on the Texas gulf coast.

Desimone is still sifting through preliminary results, and plans to collect more data in subsequent years. However, some of the early analysis from the study is interesting. The tagged Gray Catbirds and Swainson’s Thrushes stayed on the ranch for an average of about 8 days. Gray Catbirds averaged a slightly longer stopover time (10.5 days) than Swainson’s Thrushes (4.5 days). Stopover length decreased as the days grew shorter, and birds were more likely to carry increased fat loads on their bodies later into the migration season. We don’t often think of birds as being super-athletes, but the demand of migration requires the astonishing ability to fuel their bodies and perform nearly unthinkable days-long flights. That is just what DeSimone is after and hopes to connect physiological attributes to changes in behavior during migration of our athletic feathered friends. To learn more, please visit the Breuner Lab’s website.