Advances in genomics and bioinformatics will enable us to reconstruct demographic history, and past and ongoing gene flow for the different call types. This will provide a valuable complement to previous studies of crossbill adaptation and reproductive isolation for understanding the evolution of extant crossbill diversity. Additional studies of reproductive isolation in the wild, as well as the importance of song to mate choice, which can be conducted in aviaries, are also needed. Studies so far have focused on the learning and stability of contact calls across years. Future studies should examine the development of other vocalizations that we and presumably crossbills use to discriminate between call types such as toop calls and song to determine the extent to which they are learned and whether learning is open-ended as for contact calls.
eBird has advanced our understanding of the distributions and movements of the call types, and, with expanded coverage and reports (with recordings to confirm call types), eBird should continue to increase our knowledge. Advances in animal tracking should allow the spatial monitoring of nomadic crossbills in real time and provide insights on the scale and timing of individual movements and the extent to which flocks and pairs remain together over time. If enough individuals are tracked over multiple years, we should learn whether specific regions are critical to certain call types. Individuals within a call type are known to preferentially remain in habitats depending on the match of their phenotype (bill depth) to the food resource (128). Future studies should examine the extent to which such preferences (i.e., matching habitat choice) contribute to local adaptation and divergence within call types.
Certain aspects of crossbill biology are especially difficult to study (e.g., those requiring following individuals over multiple years), however, the relative ease at which feeding intake rates can be measured directly and be related to crossbill biology is almost unique to crossbills. Although such data have been used to elucidate many aspects of crossbill biology (e.g., 131, 17, 4, 11, 188, 130), we encourage others to gather such data when addressing questions that are likely tied to resource use, which is most of crossbill behavior, ecology, and evolution. In situations where marked individuals can be followed, it would be useful to determine the stability of vocalizations, and whether for example mated pairs are more likely to call match if they remain paired for multiple breeding attempts.
Climate change likely poses the greatest threat to crossbill diversity and challenge for their conservation. Such change is likely to shift and reduce the distribution of many conifers that crossbills rely upon, and warming temperatures are likely to accelerate seed release from cones and reduce the distributions of crossbills, as shown for crossbills in Europe (119). Similar studies in North America would be valuable.