Biomechanics of Feeding in Loggerhead Shrikes

A major thrust of organismal biology involves understanding how an organism’s phenotype relates to its environment, via its performance capabilities. I investigated this linkage in Loggerhead Shrikes − small North American predatory songbirds that feed on arthropods and vertebrates, whose physical and behavioral attributes differ substantially. My dissertation is composed of three components, aimed at uncovering how shrikes accommodate the disparate demands of their prey. I examined geographic variation in bill morphology of museum specimens to investigate the relationship between environmental factors and upper bill form, particularly of the hooked tip and tomial teeth, which are thought to play a key role for processing vertebrate prey. My geometric morphometric analysis revealed significant intra- and interspecific variation, mostly along an axis of increasing hook curvature and length. Whereas as dietary differences mostly explained bill shape variation among species, variance in temperature and longitude explained variation within Loggerhead Shrikes. My field component was aimed at understanding the functional and ecological consequences of morphological variation. I captured shrikes throughout California to measure bill morphology and bite performance. I collected feather and prey samples for stable isotopes analysis, to infer dietary variation. Increased bill and hook lengths and curvatures were associated with lower bite forces, whereas bill shape was unrelated to bite pressure (force/area). Thus, shrikes with different bill shapes may achieve functional equivalence by modulating bite force. Bite force, rather than bill shape, was associated with isotopic variation, and with greater consumption of vertebrates. This suggested the importance of bite performance for handling less tractable prey, and possibly for responding to spatiotemporal variation in resources. Finally, I studied how prey-processing varied with prey type and ontogeny in captive shrikes. I performed high-speed video kinematics of adult and juvenile shrikes attacking crickets and mice. My results suggested that the head-shaking behavior that shrikes use when handling vertebrates may enhance killing efficiency. Although there were few differences between age classes in procuring crickets, juveniles had greater difficulty handling mice, consistent with their reduced bill hook and tomial tooth lengths. Collectively, these results demonstrate how functional performance mediates the relationship between morphology and feeding ecology in shrikes