Building flying machines from feeding machines: the causes and consequences of body transformations in long distance migrants

Each year billions of birds migrate across our planet producing one of nature’s most incredible phenomena. People have been trying to understand this behaviour for over 2000 years and the advent of new tracking technologies is revolutionising the field, revealing some truly astonishing feats of endurance. Yet the findings often throw up more questions than answers, particularly with respect to the physiological performance. Of particular note is the remarkable ability of many of the more extreme migratory birds to transform their physiology multiple times during the annual cycle as they switch between feeding and flying machines. During migration, the sizes of some organ systems can change by more than 50% as digestive systems regress while flight muscles are grown, and stores laid down. Although individuals appear to vary in the extent to which they transform, we have little understanding of the consequences of this variation with regards to what this may mean for individual life histories and fitness or the trade-offs at play. This is largely because there are few longitudinal studies of these morphological reorganisations, as until now measuring the changes has typically required sacrificing focal animals. Here, we would combine the attributes of a unique study system with novel applications of state-of-the-art technology, including mobile x-ray radiography and ultrasonography, to address this substantial gap in our knowledge. We will develop a novel 3D imaging technique to measure the size of internal organs and carry out longitudinal analyses of migratory birds across their annual cycles. We will measure the drivers of variation in the extent and rate at which they transform including differences in habitat use, social status and wintering carry-over effects and link these to downstream consequences such as survival and reproductive success for the first time. From this we will understand how the trade-offs are managed, the consequences of individually variable strategies and the key processes that underpin them. We will tackle the following broad questions: What causes variation in the timing and extent of individual transformations between feeding and flying machines? What are the consequences of this individual variation in transformations? The proposed research will open up a suite of novel and exciting questions about the evolution and ecology of the migratory lifestyle and provide an innovative toolbox with which to investigate them. More importantly it will force us to think about the phenomenon of extreme migration in a new way, that incorporates the trade-offs associated with these flexible phenotypes. Finally, migratory species experience a range of pressures throughout their migratory cycle including the added strain of anthropogenic pressures such as habitat loss and climate change. Given the potential link between these phenotypic transformations and demography, it is crucial that we understand their underlying drivers and impacts, in order to implement effective policy and conservation measures to maintain the long-term conservation of these species.