The Peculiar Necks of Herons and Anhingas: A Study of Cervical Morphology in Pelecanimorph Birds.

The remarkable diversity in avian neck morphology likely influences neck movement in behaviors such as preening and prey capture. Ardeids (herons, egrets, and bitterns) and anhingas are thought to have evolved specialized mid-neck vertebrae to facilitate their ambush predation motion, which involves a rapid straightening of the neck to propel the head toward prey. Although prior studies have suggested that both groups possess distinctive cervical vertebral morphology, a broader comparative framework incorporating extensive taxonomic sampling, information-rich shape data, and phylogenetic context is needed to evaluate the uniqueness of their neck organization relative to close relatives. Here, we examined cervical vertebral morphology in 24 species across the Pelecanimorphae, with a focus on Ardeidae and Anhingidae. Using three-dimensional geometric morphometrics and phylogenetic comparative methods, we quantified vertebral shape variation and cervical regionalization across the five morphologically distinct regions of the avian neck. We assessed patterns of cervical regionalization across pelecanimorph lineages and tested for vertebral shape differentiation in ardeids, while including anhingas descriptively in morphospace comparisons due to limited sampling. We hypothesized that ardeids are morphologically distinct in one or more cervical regions and that both ardeids and anhingas exhibit unique patterns of cervical regionalization relative to other pelecanimorphs. Our results indicate that (1) ardeids and several lineages within Suliformes, including anhingas, exhibit distinctive patterns of cervical regionalization, and (2) ardeids evolved uniquely shaped vertebrae across cervical regions 1-4 relative to other pelecanimorphs. Anhingas exhibit cervical regionalization patterns similar to those of ardeids, consistent with convergent evolution of neck organization. Additionally, substantial morphological diversity within ardeids may reflect ecological differences in neck use and foraging strategies, highlighting the need for future studies linking cervical form and function. Overall, this study underscores the evolutionary lability of the avian neck and its potential role in facilitating ecological diversification across avian lineages.