Bird necks display unparalleled levels of morphological diversity compared to other vertebrates, yet it is unclear what factors have structured this variation. Using 3D geometric morphometrics and multivariate statistics we show that the avian cervical column is a hierarchical morpho-functional appendage, with varying magnitudes of ecologically-driven morphological variation at different scales of organisation. Contrary to expectations given the widely-varying ecological functions of necks in different species, we find that regional modularity of the avian neck is highly conserved, with an overall structural blueprint that is significantly altered only by the most mechanically demanding ecological functions. Nevertheless, the morphologies of vertebrae within subregions of the neck show more prominent signals of adaptation to ecological pressures. We also find that both neck length allometry and the nature of neck elongation in birds are different to other vertebrates. In contrast to mammals, neck length scales isometrically with head mass and, contrary to previous work, we show that neck elongation in birds is achieved predominantly by increasing vertebral lengths rather than counts. Birds therefore possess a cervical spine that may be unique in its versatility amongst extant vertebrates, one that, since the origin of flight, has adapted to function as a surrogate forelimb in varied ecological niches.

Three-dimensional digital models were created for every cervical vertebrae (except the atlas, C1) for 54 specimens (48 distinct species, Table S2) of extant birds, from medical and microCT scans using Avizo 7.1 (Visualisation Science Group, supplementary information).