In 1993, Adam Urbanek defined the Lilliput Effect as a temporary dwarfing of species or their groupings across mass extinction events. While the cause of this dwarfing was not clear, observations from ancient animals suggested that erosion of nutritive conditions necessary for growth was likely. Today, scientists across the globe agree that we are in the middle of a sixth great mass extinction, and, as predicted by the Lilliput Effect, the body sizes of vertebrates (namely birds) across continents has fallen. Yet despite evidence that biomass of many food species have diminished globally, dominate scientific narratives explaining avian size declines fail to acknowledge resource shortages as a probable driver, instead asserting that warming climates are favouring smaller, less heat-retaining morphologies. With this research, I aim to combine theory from thermal ecology with long-term observations of morphology from >200 North American bird species to determine whether size shifts represent generalised responses to warming climates or resource shortages. To achieve this, I will leverage freely-available morphometric, climatic, and land-transformation data to evaluate whether decreases in body size across species are best explained by temperature-driven patterns of selection, or changes in habitat availability. Given the large amount of data analysed for this project, I will require access to advance computing services for its completion.