Investigating landbird stopover ecology and distributions along the U.S. Coast of the Gulf of Mexico

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University of Delaware
Every spring, billions of Nearctic-Neotropical landbird migrants travel from tropical wintering grounds in Mexico, the Caribbean, and Central and South America to temperate breeding grounds in the United States and Canada. In autumn, they reverse this journey, heading south to return to their wintering grounds. During both spring and autumn migration, individuals use stopover habitat to rest and feed between flights. Habitats along the Gulf of Mexico coast are particularly critical in providing resources, as they represent the first possible landfall in the spring after the long, nonstop flight across the Gulf and the last opportunity to refuel in autumn before crossing. However, no studies have investigated the roles that synoptic weather and winds encountered during migration play on where migrants stopover along the coast in a comprehensive manner. I used archived weather surveillance radar data to better understand the stopover ecology of landbird migrants along the U.S. coast of the Gulf of Mexico. First, I quantified the influence of broad-scale weather patterns (i.e., synoptic weather) over the Gulf of Mexico on spring stopover distributions of birds along the coast. Second, I assessed the influence of low-altitude winds aloft over the Gulf of Mexico, Atlantic Ocean, and Caribbean Sea on spring stopover patterns along the entire coast and regionally. Third, I determined whether local stopover site function and stopover duration varies from autumn to spring, using 19 sites in southern Mississippi and eastern Louisiana. Overall, I found that synoptic weather and winds encountered during migration were both important in determining where birds stopped over along the U.S. coast of the Gulf of Mexico. Synoptic weather had the strongest influence in explaining mean stopover density among predictors that included longitude, latitude, distance from the coast, and the amount of hardwood forest in the surrounding landscape. Unfavorable synoptic weather types involving conditions adverse to northward spring migration altered bird stopover distributions in terms of longitude and distance from the coast. For example, strong coastal concentrations of migrants were evident on the days following headwinds over the Gulf of Mexico. Low-altitude winds encountered during migration also influenced bird stopover density across the entire coast. In particular, strong winds from the east over the Caribbean Sea and Atlantic Ocean steered migrating birds to the northern Gulf of Mexico coast. In terms of stopover function, we found that coastal stopover sites functioned primarily as rest stops in both the autumn and spring. At more inland stopover sites, site function and migrant stopover duration both depended on seasonal food availability. In the spring, food availability was generally low, so most inland sites functioned as rest stops only; at sites with food, migrants stayed longer where food availability was higher. In autumn, food resources were greater than in spring, so most inland sites functioned as refuel sites, and birds were able to feed and leave more quickly from sites with higher amounts of available food. My results offer quantitative evidence that synoptic weather and winds aloft shape the broad-scale spatial distributions of migrants during stopover and that stopover site function varies by season. My research advances scientific understanding of the role that atmospheric conditions play in bird migration and how seasonal food availability can affect stopover site function and stopover duration; ultimately, this knowledge can be used to predict the effects of future climate change on migrating landbirds and guide conservation efforts.