Variation in jaw size has been crucial to the evolution and adaptation of vertebrates. On p. 674, Jennifer Fish, Richard Schneider and colleagues explore the mechanisms by which duck and quail achieve distinct jaw sizes, testing the hypothesis that differences in neural crest (NC) biology contribute to species-specific differences in jaw size. The researchers show that the total sizes of the pre-migratory NC progenitor populations in duck and quail are similar. However, the midbrain region, which generates jaw NC precursors, is wider and shorter in duck owing to an anterior shift in brain regionalisation. Furthermore, they report, more pre-migratory NC precursors are allocated to the midbrain in duck, which gives rise to an increased number of post-migratory NC cells within the duck mandibular arch. Finally, differences in proliferation lead to an increase in the size of the duck mandibular arch relative to that of the quail. Thus, the authors propose, the larger jaw size of duck is the result of at least three distinct developmental events.