The Influence of Climate on Bird Morphological and Genetic Variation across Time and Space

Global climate change (GCC) is rapidly changing the adaptive landscape for plants and animals alongside other contemporary drivers of change. The consequences of GCC, such as phenological mismatches and altered inter-species interactions, have been linked to negative fitness outcomes across taxa, suggesting that adaptation to climate change is necessary for long-term persistence in many species. However, it is not always clear whether phenotypic responses occurring alongside climate change are adaptive or even whether they are, in fact, driven by shifts in climate. Changes in size and shape have been documented across the tree of life, and have been phenomenologically linked to climate change. However, morphological traits are influenced by multiple selection pressures and can be impacted by other processes, such as developmental plasticity. This has led to uncertainty regarding the extent to which observed morphological trends are driven by climate, and the contributions of plastic versus evolutionary mechanisms to these trends continue to be debated. Developing our understanding of how morphology responds to climate matters for understanding the fitness and persistence implications of these responses. In my dissertation, I investigate the links between contemporary climate change and morphological trends in birds using both a comparative analysis of shared morphological trends across species of North American migratory birds, and assessment of within-species genetic variation across spatial and temporal scales to elucidate potential mechanisms of climate-driven selection and morphological change. In Chapter 1, I provide an overview of climate-imposed pressures that may necessitate adaptive responses, mechanisms of adaptation in the context of climate change, and morphological responses associated with climate change in birds. In Chapter 2, I use hierarchical Bayesian models and a 40-year morphological dataset for five North American migratory bird species to illustrate that changes in size and shape have occurred independently of each other, and that significant relationships between climate and wing length do not persist past birds’ first year of life and do not explain long-term wing length trends. In Chapters 3 and 4, I generate a 20-year genomic time series for one species, the white-throated sparrow (Zonotrichia albicollis), to investigate the associations between genetic, morphological, and climatic changes across two decades, and compare how evolutionary responses to climate, whether linked to morphology or not, vary across space and time. In Chapter 3, I identify many candidate regions across the genome that may explain long-term morphological trends, but these regions do not overlap with those that I find to be associated with fluctuations in climate. In Chapter 4, I show that the overlap of genetic associations with climate across space and time is not significant. However, the genes in this overlap have functions that have been independently raised as involved in climate change responses in studies of other avian systems. In Chapter 5, I link my findings to each other, discuss their implications, and identify promising avenues for future research. Overall, my findings confirm the complex and varying nature of climate-morphology relationships, indicate that evolution plays a role in driving long-term morphological trends observed under contemporary climate change exist, and suggest that adaptive responses to climate change may not be easily predicted by genetic variation associated with climate across space.