Co-migration
For my postdoc with Emily Cohen at the University of Maryland Center for Environmental Science, I am studying the community ecology of bird migration. Nocturnally migrating birds share the airspace and stopover habitats, likely forming networks of interactions that impact stopover decisions, refueling rate, and ultimately migration phenology and success. I am leveraging historical banding records to quantify the strength and persistence of species relationships across migratory stopover sites, testing whether migrations can be thought of as movements of whole communities, not just populations.
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Anthropogenic Threats during Migration
During migration, birds encounter a variety of anthropogenic threats, such as artificial light at night, free-roaming cats, and environmental toxins. These threats can directly kill a migrating bird, but they may also have indirect effects, causing declines in future reproductive success or survival. These threats can also interact to produce additive and synergistic effects on migrating birds. For example, a bird disoriented by city lights is more likely to strike a window, making it more likely to fall prey to a free-roaming cat. Claire Nemes led a lab review paper of this topic as part of the American Ornithological Society's Wesley Lanyon Award competition, in which we argue that failing to account for the indirect and interacting effects of anthropogenic threats leads to underestimates of the impacts of human activities on migrating birds.
Facultative Migration
Unlike migrants that undergo predictable, seasonal movements, facultative migrants travel with unpredictable timing and destinations. The unpredictability of their movements is often due to underlying unpredictability in the timing and location of ephemeral food resources. Facultative migrants, like pine siskins, may travel long distances, and it's unclear whether they anticipate (and therefore prepare for) movements, or rather escape areas without preparatory fattening. For my dissertation, I conducted a captive experiment and two field seasons to test how pine siskins' activity and CORT levels respond to experimentally manipulated and naturally varying levels of food availability. In response to captive food restrictions, siskins lost fat mass and showed increased activity and CORT levels, generally indicative of an "escape" response. In the wild, however, siskins showed an intermediate pattern: while they didn't prepare for departure, they didn't "escape" either. Instead, birds were more likely to depart if they had sufficient fuel stores, but among individuals with sufficient fuel, other factors (possibly weather or social information) influenced their departure decisions. Combined, these studies demonstrate how the relationships among CORT, body condition, and activity depend on food availability.
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Stopover-CORT Hypothesis
Bird migration involves a suite of physiological and behavioral changes (like excessive fattening) that must be intricately timed with the environment to ensure a successful migration. Hormones are chemical messengers that help coordinate organismal processes with environmental information. Scientists have hypothesized that the hormone corticosterone (CORT) underlies the changes in feeding behavior and activity associated with avian migration. (While CORT is often called a "stress hormone," its involvement in animals' responses to stress is only one of its roles: CORT is also associated with the increased energetic demands of predictable life history stage transitions, like hatching, fledging, and dispersal.) However, despite many field and captive studies testing the role of CORT in bird migration, the literature contains many contradictory findings. For part of my dissertation, I developed a Stopover-CORT hypothesis to help resolve these contradictions, highlighting that the relationship between CORT, feeding, and activity likely depends on stopover timing—whether the bird is a new arrival or closer to departure. I tested this Stopover-CORT hypothesis during spring migration on the Florida panhandle and during fall migration near Missoula, MT, finding that CORT predicted body composition upon arrival at the Florida stopover site and sometimes predicted departure from the Montana site. Deviations from our predictions likely depend on stopover habitat quality and whether birds are refueling in a way to minimize the time or energy expenditure of migration.
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Social information
I advised the honors thesis of rockstar University of Montana senior, Sarah Sriraman. She conducted behavioral observations of pine siskin flocks foraging at discrete sunflower fields that she regularly walked transects through to estimate food availability. Using audio recordings, Sarah matched vocalization rate and call types to food availability and siskin flock behavior. Her results offer a way to better understand how siskins use vocalizations to communicate to conspecifics about foraging patch quality and to coordinate group movements. Sarah is working on turning her thesis into her first first-author publication!
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