Size-dependent beak shape divergence between Parrot and Common Crossbills likely involves major developmental changes

Quentin Jean-Baptiste Horta-Lacueva , Mark Ravinet

We studied allometry, here defined as shape-size covariation, to explore the unresolved case of divergence between the Parrot and Common Crossbill. Using geometric morphometrics, we compared beak shape and allometry among the Parrot Crossbill, the Common Crossbill and the more distantly related Two-Barred Crossbill. The Parrot Crossbill differed from the two other species in shape changes that cannot be explained by size variations along a common allometric trajectory, and instead shows a deeper and bulkier maxilla than expected by size alone. These results indicate that Parrot Crossbill has likely evolved a distinct beak shape through modulations of developmental processes that are yet to be revealed by genomic investigations.