A fundamental challenge of biology is to understand the extent to which different processes generate biological diversity. One of the most prominent hypotheses to explain patterns of diversification is that of an arms race between predators and their prey. The proposed study will determine the extent to which arms races between crossbills (Loxia) and conifers have contributed to the adaptive radiation of crossbills. These results will greatly advance our understanding of how community context and resource characteristics affect the geographic mosaic of coevolution, and thus provide the impetus for further theoretical advances. Previously used techniques, including measures of crossbill feeding rates and cone measurements, will be used to test predictions concerning the types of changes in cone traits that should arise if crossbills and squirrels have influenced cone structure, and to test whether crossbills have adapted in a reciprocal manner to the changed cones. The results of these tests will determine (1) how resource characteristics and competitor type affect the form of the geographic mosaic for crossbills and conifers and (2) to what extent arms races have contributed to the adaptive radiation of crossbills
