Climate change is a major threat to Australia's biodiversity. One recently identified yet unexplained response to climate change involves reductions in animal body size. We will provide critical information on the generality of this phenomenon and the processes involved. We will assemble a continent-wide inventory of body sizes in Australian birds over the last 50-150 years to calculate a community-level index of size change. We will combine this with a continent-wide time-series of ecosystem productivity and novel indices of individual condition to determine the likelihood of species declining in size, and the relative importance of changing temperature and primary productivity.