Variable evidence for convergence in morphology and function across avian nectarivores

Abstract

Nectar-feeding birds provide an excellent system in which to examine form-function relationships over evolutionary time. There are many independent origins of nectarivory in birds, and nectar feeding is a lifestyle with many inherent biophysical constraints. We review the morphology and function of the feeding apparatus, the locomotor apparatus, and the digestive and renal systems across avian nectarivores with the goals of synthesizing available information and identifying the extent to which different aspects of anatomy have morphologically and functionally converged. In doing so, we have systematically tabulated the occurrence of putative adaptations to nectarivory across birds and created what is, to our knowledge, the first comprehensive summary of adaptations to nectarivory across body systems and taxa. We also provide the first phylogenetically informed estimate of the number of times nectarivory has evolved within Aves. Based on this synthesis of existing knowledge, we identify current knowledge gaps and provide suggestions for future research questions and methods of data collection that will increase our understanding of the distribution of adaptations across bodily systems and taxa, and the relationship between those adaptations and ecological and evolutionary factors. We hope that this synthesis will serve as a landmark for the current state of the field, prompting investigators to begin collecting new data and addressing questions that have heretofore been impossible to answer about the ecology, evolution, and functional morphology of avian nectarivory.

Nectar feeding beyond the tongue: hummingbirds drink using phase-shifted bill opening, flexible tongue flaps and wringing at the tips

“…The first hypotheses on how nectar is moved through the bill to be swallowed date back to the 19th century, with renewed attention in the 1980s and within the last 20 years (reviewed in Rico-Guevara, 2014 ; Cuban et al, 2022 ). There seem to be a variety of potential solutions to the fluid dynamics challenge of efficiently and delicately extracting minute amounts of nectar, which often requires thin and elongated beaks and protrusible tongues to reach inside long and tubular flowers ( Hewes et al, 2022 ). A narrow bill as a special tool to navigate corollas presents the additional challenge of moving the liquid to the, in most species, distant throat.…”

“…With this first experimental exploration of intraoral transport of nectar, the foundation is set for comparative biomechanical studies to evaluate the diverse nectar feeding hypotheses ( Cuban et al, 2022 ) and associated adaptations of unrelated clades of nectarivorous taxa (e.g. honeyeaters, sunbirds and many others; Hewes et al, 2022 ), to better elucidate the extent of convergent and alternative solutions to this unique feeding challenge. Our findings highlight the importance of studying the morphofunctional adaptations of not only the tip but also the base of the tongue of nectarivorous taxa.…”

Morphological specialization to nectarivory in Phyllostomus discolor (Wagner, 1843) (Chiroptera: Phyllostomidae)

“…How is morphology shaped by pollinator–plant relationships? Many angiosperms have evolved adaptations for animal pollination, including supplying nectar as reward, conspicuously colored petals and bracts, aromas, and other traits that attract pollinators; likewise, many animals have evolved adaptations for the exploitation of floral nectar (Altringham, 2011; Hewes et al, 2022; Pellmyr, 2002). This has led to impressive cases of coevolution, such as that seen between Darwin's orchid and the sphinx moth that pollinates it while feeding on its nectar (Wasserthal, 1997).…”

Convergent evolution of noncoding elements associated with short tarsus length in birds

“…Birds have emerged as an excellent system to study convergent evolution 10,23 . They have comparatively small, less-complex genomes that are easy to assemble 24,25 and a wealth of phenotypic data that can be mapped on dense phylogenies 14,26,27 .…”