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The avian order Anseriformes (waterfowl) is an ecologically diverse and globally distributed clade of approximately 150 species. Owing to their ecological and economic importance there is an extensive literature documenting the feeding behaviors and diet of waterfowl, making this an ideal clade in which to study the morphological and functional evolution of feeding systems. The central objective of this dissertation is to integrate dietary and morphological data using shape analysis, evolutionary modeling, and biomechanical simulation to identify processes underlying the evolution of functional systems. The dietary dataset, compiled from the literature for 99 waterfowl species, represents the most comprehensive compilation of avian diet data to date applied to phylogenetic comparative analyses. These data are used to test the hypothesis that the evolution of herbivory is associated with an increase in body mass due to a potential advantage in the digestion of low-quality diets (chapter 2). The results reveal at least five independent transitions toward increased herbivory in waterfowl and that increased herbivory is not associated with increased body mass when phylogeny is taken into account. The morphometric dataset, collected from over 130 museum specimens and representing 50 waterfowl species, is one of the first surveys of bird skull shape to incorporate both three-dimensional beak curvature and the geometry of the cranial linkage mechanism, the bones underlying cranial kinesis. To evaluate the force transmission properties of the cranial linkage a new biomechanical simulation platform is developed, with the capacity to simulate cranial kinesis in birds and fishes (chapter 3). Diet and beak shape data are combined to test the correlation between beak shape and diet in waterfowl and examine how the major axes of beak shape variation relate to a performance trade-off in waterfowl between filter-feeding and grazing (chapter 4). Beak shape and diet in waterfowl are strongly correlated, with the first major axis of variation in beak shape corresponding to different positions along a performance trade-off gradient between filter-feeding and grazing. The morphometric dataset of waterfowl skull shapes is then used to test the hypothesis that the avian cranial linkage geometry evolves in association with beak feeding behaviors (chapter 5). The beak and linkage of waterfowl skulls exhibit strong morphological and functional integration, with significant differences in linkage shape and function among waterfowl with differing feeding behaviors. In addition to exploring the processes underlying the evolutionary trajectories of functional systems, this dissertation also encompasses the broader themes of collection-based biomechanical simulation and major transitions in the evolution of waterfowl feeding ecology (chapter 6).


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