Evolutionary versatility of the avian neck

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.