The anatomy of a shrike bite: bill shape and bite performance in Loggerhead Shrikes

Phenotypic differentiation despite gene flow: Beak morphology, bite performance, and population genetics of Loggerhead Shrikes (Lanius ludovicianus)

Previous studies of Loggerhead Shrikes (Laniidae: Lanius ludovicianus) in North America have indicated considerable intraspecific genetic and phenotypic differentiation, but the congruence between genetic and phenotypic differentiation remains obscure. We examined phenotypic differences in beak shape and bite force among geographic groupings across a 950 km range, from the lower Imperial Valley to the upper Central Valley of California, USA. We integrated these analyses with a population genetic analysis of six microsatellite markers to test for correspondence between phenotypic and genetic differences among geographic groups. We found significant phenotypic differentiation despite a lack of significant genetic differentiation among groups. Pairwise beak shape and bite force distances nevertheless were correlated with genetic (F ST) distances among geographic groups. Furthermore, the phenotypic and genetic distance matrices were correlated with pairwise geographic distances. Takentogether, these results suggest that phenotypic differences might be influenced by neutral processes, inbreeding (as indicated by high heterozygosity deficiencies we observed), local adaptation, and/or phenotypic plasticity.

Biomechanics illuminates form-function relationships in bird bills

The field of comparative biomechanics examines how form, mechanical properties and environmental interactions shape the function of biological structures. Biomechanics has advanced by leaps and bounds as rapid technological progress opens up new research horizons. In this Review, I describe how our understanding of the avian bill, a morphologically diverse multifunctional appendage, has been transformed by employing a biomechanical perspective. Across functions from feeding to excavating hollows in trees and as a vocal apparatus, the study of the bill spans both solid and fluid biomechanics, rendering it useful to understand general principles across disciplines. The different shapes of the bill across bird species result in functional and mechanical trade-offs, thus representing a microcosm of many broader form-function questions. Using examples from diverse studies, I discuss how research into bird bills has been shaped over recent decades, and its influence on our understanding of avian ecology and evolution. Next, I examine how bill material properties and geometry influence performance in dietary and non-dietary contexts, simultaneously imposing trade-offs on other functions. Following an examination of the interactions of bills with fluids and their role as part of the vocal apparatus, I end with a discussion of the sensory biomechanics of the bill, focusing specifically on the bill-tip mechanosensory organ. With these case studies, I highlight how this burgeoning and consequential field represents a roadmap for our understanding of the function and evolution of biological structures.

Feather characteristics of loral zone in an insectivorous passerine: The Iberian gray shrike Lanius meridionalis, in southern France

In the French Mediterranean plain, the northern extreme of its native range, the Iberian gray shrike, Lanius meridionalis, predominantly feeds on arthropods. Its type of loral plumage plays a key role in protecting its eyes while transporting large prey. The aims are to understand the role played by feathers in protecting the animal from various types of defensive prey. I combine an inspection of large insect prey types found on larders with a review of bird specimens found in museum collections to examine the morphometric characteristics of rictal feathers and culmen. In addition, precision photographs are used to observe the posture of the plumage in natura. I could identify four categories of protective feathers: clustered bristles, semi-bristles, semi-plumes distributed in the loral area, and semi-plumes above the eyes. My results suggest that the Iberian gray shrike has a complex structure of loral feathers, specific to its foraging activity and prey types. The presence of these rictal bristles is probably a protection against the movements of larger prey items, which might damage loral zone of Lanius meridionalis.

Geometric Morphometric Analysis of the Humerus in New and Old World Vultures

The vulture guild is composed of two distinct groups, Old and New World, that provide a unique insight into how morphology varies among convergent species. All vultures are considered to be large birds of prey that utilize a style of flight called thermal soaring to search and feed primarily on carrion. Even though this flight style is exhibited among all 23 species, slight variations in their skeletal morphology may relate to their differences in ecology. We hypothesized that vulture humeral morphology varies in relation to these organisms' habitat, average body mass, courtship displays, and migratory behavior. To address this hypothesis, we used three-dimensional geometric morphometrics to measure the overall shape differences of vulture humeri. Humeral morphology was found to vary most by habitat association and migratory tendency. The humeri of vultures that inhabit forested areas exhibit features that suggest increased flapping flight compared to those in open and mountainous regions. Migratory species were found to possess more robust features near the glenohumeral joint. We found these (and other features) have some utility for predicting ecology and behavior, but we suggest that further investigation into skeletal and muscular wing elements may reveal greater understanding of the habits of extinct vulture species. This article is protected by copyright. All rights reserved.

The diet of early birds based on modern and fossil evidence and a new framework for its reconstruction

Birds are some of the most diverse organisms on Earth, with species inhabiting a wide variety of niches across every major biome. As such, birds are vital to our understanding of modern ecosystems. Unfortunately, our understanding of the evolutionary history of modern ecosystems is hampered by knowledge gaps in the origin of modern bird diversity and ecosystem ecology. A crucial part of addressing these shortcomings is improving our understanding of the earliest birds, the non-avian avialans (i.e. non-crown birds), particularly of their diet. The diet of non-avian avialans has been a matter of debate, in large part because of the ambiguous qualitative approaches that have been used to reconstruct it. Here we review methods for determining diet in modern and fossil avians (i.e. crown birds) as well as non-avian theropods, and comment on their usefulness when applied to non-avian avialans. We use this to propose a set of comparable, quantitative approaches to ascertain fossil bird diet and on this basis provide a consensus of what we currently know about fossil bird diet. While no single approach can precisely predict diet in birds, each can exclude some diets and narrow the dietary possibilities. We recommend combining (i) dental microwear, (ii) landmark-based muscular reconstruction, (iii) stable isotope geochemistry, (iv) body mass estimations, (v) traditional and/or geometric morphometric analysis, (vi) lever modelling, and (vii) finite element analysis to reconstruct fossil bird diet accurately. Our review provides specific methodologies to implement each approach and discusses complications future researchers should keep in mind. We note that current forms of assessment of dental mesowear, skull traditional morphometrics, geometric morphometrics, and certain stable isotope systems have yet to be proven effective at discerning fossil bird diet. On this basis we report the current state of knowledge of non-avian avialan diet which remains very incomplete. The ancestral dietary condition in non-avian avialans remains unclear due to scarce data and contradictory evidence in Archaeopteryx. Among early non-avian pygostylians, Confuciusornis has finite element analysis and mechanical advantage evidence pointing to herbivory, whilst Sapeornis only has mechanical advantage evidence indicating granivory, agreeing with fossilised ingested material known for this taxon. The enantiornithine ornithothoracine Shenqiornis has mechanical advantage and pedal morphometric evidence pointing to carnivory. In the hongshanornithid ornithuromorph Hongshanornis only mechanical advantage evidence indicates granivory, but this agrees with evidence of gastrolith ingestion in this taxon. Mechanical advantage and ingested fish support carnivory in the songlingornithid ornithuromorph Yanornis. Due to the sparsity of robust dietary assignments, no clear trends in non-avian avialan dietary evolution have yet emerged. Dietary diversity seems to increase through time, but this is a preservational bias associated with a predominance of data from the Early Cretaceous Jehol Lagerstätte. With this new framework and our synthesis of the current knowledge of non-avian avialan diet, we expect dietary knowledge and evolutionary trends to become much clearer in the coming years, especially as fossils from other locations and climates are found. This will allow for a deeper and more robust understanding of the role birds played in Mesozoic ecosystems and how this developed into their pivotal role in modern ecosystems.

Morphological innovation and biomechanical diversity in plunge-diving birds

Innovations in foraging behavior can drive morphological diversity by opening up new ways of interacting with the environment, or limit diversity through functional constraints associated with different foraging behaviors. Several classic examples of adaptive radiations in birds show increased variation in ecologically relevant traits. However, these cases primarily focus on geographically narrow adaptive radiations, consider only morphological evolution without a biomechanical approach, or do not investigate tradeoffs with other non-focal traits that might be affected by use of different foraging habitats. Here, we use X-ray microcomputed tomography, biomechanical modeling, and multivariate comparative methods to explore the interplay between foraging behavior and cranial morphology in kingfishers, a global radiation of birds with variable beaks and foraging behaviors, including the archetypal plunge-dive into water. Our results quantify covariation between the shape of the outer keratin covering (rhamphotheca) and the inner skeletal core of the beak, as well as highlight distinct patterns of morphospace occupation for different foraging behaviors and considerable rate variation among these skull regions. We anticipate these findings will have implications for inferring beak shapes in fossil taxa and inform biomimetic design of novel impact-reducing structures.

Come on baby, let's do the twist: the kinematics of killing in loggerhead shrikes

Shrikes use their beaks for procuring, dispatching and processing their arthropod and vertebrate prey. However, it is not clear how the raptor-like bill of this predatory songbird functions to kill vertebrate prey that may weigh more than the shrike itself. In this paper, using high-speed videography, we observed that upon seizing prey with their beaks, shrikes performed rapid (6-17 Hz; 49-71 rad s^-1) axial head-rolling movements. These movements accelerated the bodies of their prey about their own necks at g-forces of approximately 6 g, and may be sufficient to cause pathological damage to the cervical vertebrae and spinal cord. Thus, when tackling relatively large vertebrates, shrikes appear to use inertia of their prey's own body against them.