Bird Flight Energetics - from tissues to free-flight

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Birds are amongst the most diverse, successful and ecologically important groups on earth and flight is key to their success. However, flight is one of the most energetically expensive modes of locomotion and there are few aspects of a bird's ecology, behaviour and physiology that are not affected by its energetic demands. In all modes of locomotion, energetics and locomotor performance are linked via an energy transduction cascade in which muscles convert chemical energy (derived from food) into mechanical work that is transferred to the environment to produce movement. Free-flight energy expenditure is difficult to measure directly, but proxies (components of the energy transduction chain such as heart rate and body acceleration) have been shown to be generally related to metabolic rate. However, underlying assumptions and simplifications inherent in these indirect approaches have not been rigorously assessed and validated for animals during flight. Ideally, the transfer of energy between all levels of organization should be determined: a full understanding of the system is required to improve the predictive power of using proxies as indicators of metabolic rate. This is achievable in bird flight by combining research expertise in muscle physiology and flight energetics at Leeds with expertise in studying free-flight using physiological and biomechanical sensors and modelling flight energetics at Bangor.The overall aim of this project is to use a multidisciplinary approach to determine the relationship between the mechanical performance and energy utilisation of birds during flight across a range of speeds, its partitioning at the level of individual muscles and non-muscular systems, and the functional linkage to the indirect measures of heart rate and dynamic body acceleration. To achieve this goal, we will track the transduction of energy by quantifying the following. First, we will determine the whole organism metabolic rate of species with a U-shaped power curve, by measuring the rates of oxygen consumption and carbon dioxide production during flight in a wind tunnel, while simultaneously recording heart rate, dynamic body acceleration and kinematics. This has not yet been undertaken for any flying animal. Second we will use regional blood flow as a measure of tissue-level energy expenditure, enabling us to separate out the factors contributing to overall flight energy expenditure by quantifying the energy used by all of the muscles and by other, non-muscular physiological systems (e.g. respiratory, circulatory and homeostatic) in relation to flight speed. Third, the mechanical performance of the major flight muscles will be determined by measuring their length change and activity patterns during flight and simulating these conditions in vitro to measure force and power generation. By recording the 3D kinematics of the wings and body we will be able to characterize the instantaneous accelerations and, by extension, the instantaneous aerodynamic forces. By integrating energetics and mechanical measurements we will obtain the most comprehensive understanding of the energy transduction chain for any flying animal. Ultimately, we will establish the detailed relationship between whole organismal and tissue-level metabolic energy expenditure with that of proxies of energy turnover that can be measured in the field and that lie at opposite ends of the energy transduction chain: heart rate and 3-axis accelerometry and 3-axis gyroscope. These integrated measurements will allow us to refine and improve the predictive power of using such proxies as indicators of metabolic rate. The wind tunnel based studies will provide a firm footing understanding animal flight behaviour in the field. These results may also inform decisions in conservation, land use planning and public health issues; to mitigate the combined effects of habitat fragmentation and climate change, or when birds are implicated in the spread of disease.