Patterns of coevolution in the adaptive radiation of crossbills

Although evidence for coevolution and geographic variation in its apparent strength is increasing, we still have a relatively poor understanding of why coevolution varies among interactions. Here we review how variation in the occurrence of competitors, resource stability, habitat area, and time has affected the extent of trait escalation in coevolutionary arms races between crossbills (Loxia) and conifers. Competitors for conifer seeds, particularly tree squirrels, have limited the extent of crossbill-conifer coevolution; however, seed crop fluctuations reduce the extent to which tree squirrels limit crossbill-conifer coevolution. Crossbill densities increase with forest area, which results in greater escalation of seed defenses apparently as the result of stronger selection exerted by crossbills. The extent of trait escalation appears to increase toward lower latitudes where crossbill-conifer interactions have likely persisted locally for longer periods of time. However, because most crossbills occur at higher latitudes, much of the extant diversification of crossbills has occurred since the last glacial retreat, and the extent of trait escalation is limited. Nevertheless, coevolution has caused considerable trait evolution even in temperate regions. The conditions favoring coevolution between crossbills and conifers are widespread, and coevolution has played at least some role in at least three fourths of the taxa of crossbills.