title: The shapes of bird beaks are highly controlled by nondietary factors.
abstract: Bird beaks are textbook examples of ecological adaptation to diet, but their shapes are also controlled by genetic and developmental histories. To test the effects of these factors on the avian craniofacial skeleton, we conducted morphometric analyses on raptors, a polyphyletic group at the base of the landbird radiation. Despite common perception, we find that the beak is not an independently targeted module for selection. Instead, the beak and skull are highly integrated structures strongly regulated by size, with axes of shape change linked to the actions of recently identified regulatory genes. Together, size and integration account for almost 80% of the shape variation seen between different species to the exclusion of morphological dietary adaptation. Instead, birds of prey use size as a mechanism to modify their feeding ecology. The extent to which shape variation is confined to a few major axes may provide an advantage in that it facilitates rapid morphological evolution via changes in body size, but may also make raptors especially vulnerable when selection pressures act against these axes. The phylogenetic position of raptors suggests that this constraint is prevalent in all landbirds and that breaking the developmental correspondence between beak and braincase may be the key novelty in classic passerine adaptive radiations.
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol BS8 1TQ, United Kingdom; Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom; jen.bright@gmail.com.
authors.forename: Jen A
authors.initials: JA
authors.lastname: Bright
authors.affiliation: Unidad de Paleontología, Departamento de Biología, Universidad Autónoma de Madrid, Cantoblanco, Madrid 28049, Spain; Dinosaur Institute, Natural History Museum of Los Angeles County, Los Angeles, CA 90007;
authors.forename: Jesús
authors.initials: J
authors.lastname: Marugán-Lobón
authors.affiliation: Department of Archaeology, University of York, York YO10 5DD, United Kingdom; Hull York Medical School, University of York, York YO10 5DD, United Kingdom.
authors.forename: Samuel N
authors.initials: SN
authors.lastname: Cobb
authors.affiliation: School of Earth Sciences, University of Bristol, Bristol BS8 1TQ, United Kingdom;
authors.forename: Emily J
authors.identifier: 0000-0002-2618-750X
authors.initials: EJ
authors.lastname: Rayfield
mesh_terms: D000818:Animals; D001502:Beak / Q000033:anatomy & histology* / Q000502:physiology*; D005075:Biological Evolution; D049628:Body Size / Q000235:genetics*; D004032:Diet; D005247:Feeding Behavior / Q000502:physiology; D009024:Morphogenesis / Q000235:genetics*; D009929:Organ Size / Q000235:genetics; D020304:Raptors / Q000033:anatomy & histology* / Q000145:classification / Q000235:genetics*; D013045:Species Specificity
keywords: allometry; birds; geometric morphometrics; integration; modularity