This thesis presents a detailed examination of the fueling strategies and sources of energy used by Neartic-Neotropic migrants to complete their journey. In chapter one, I provide a literature review on the fueling strategies and sources of energy used by migrants to fulfill the tremendous demands of migration. I focused on migrant songbirds of the New World, but I also drew on literature from Old World migrants, as well as shorebirds. I explored the use of fuel storage compounds by examining the advantages and deficits of each and I reviewed the usage of each compound during flight, in relation to the needs of the migrant and the availability of stopover sites. Furthermore, I reviewed the energetics of migration using field and lab based studies, and I discussed the importance of refueling at stopover habitats, which led to a discussion on the costs associated with visiting stopover sites. I examined the use of an energy conservation strategy, in which digestive organs are shut down and reduced in size, by reviewing the correlation among migrants using this strategy and the distance they must travel, as well as the availability of stopover sites en route. I then compared birds using this strategy to short distance migrants that forage frequently en route to demonstrate the existence of a tradeoff birds must make due to the physiological condition of their digestive organs, which I used to explain why short-distance migrants must refrain from utilizing this strategy. In addition, I examined the use of protein stores en route by evaluating the relation among birds that must resort to “burning” their digestive organs, and the type of terrain they must cross. I also looked at the influence of migrants’ diet composition before and during migration, and its relation to the amount of protein migrants use during flight, as well as the dietary influence on the building of depleted protein and fat stores during periods of refueling. In chapter two, I present original research from Dr. Caleb Gordon’s lab obtained from six years of mist netting at a stopover site in Illinois. I build on research presented in Benjamin Larsen’s thesis (2007). Ben provided two predictions for classifying the fueling strategies of eastern migratory songbirds, he proposed the existence of an associated taxonomic classification among these strategies, and demonstrated that two methods used commonly in research for measuring mass change are useful for indicating two types of tissue assimilation because they rely on different time scales. I added more years of mist net data, as well as more songbird species to the data set. I also examined evidence for mass-gain thresholds among within season recaptured birds, and looked for evidence of divergent fueling strategies among sexes within the same species. In addition, I refined the two divergent fueling predictions previously set forth, as well as the tests and analyses Ben previously used to examine the data. In this chapter, I examined two possible approaches used by eastern songbirds to fueling migration in an effort to determine the proportional reliance on nutrient stores acquired prior to beginning flight versus nutrients obtained en route at stopover habitats because migrants that fuel frequently during flight are important for conservation reasons. I also looked for evidence of mass-gain thresholds among recaptured birds, which would provide support for the proposed tradeoff migrants must make when “choosing” to utilize one fueling strategy over the other. I examined fueling differences among sexes within the same species in an effort to determine, if sexes fuel differently, which would aid in understanding, and explaining complex fueling behaviors that prohibit the classification of species into one of the two categories. I presented this chapter in the form of a manuscript intended for publication in the ornithological journal, The Auk.
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