Avian brachial index and wing kinematics: putting movement back into bones

The relationship between wing kinematics, wing morphology and the brachial index of birds (BI=humerus length/ulna length) was examined. BI was found to differ between three groups of birds, which were classified on the basis of similar wing kinematics. In addition, a comparative analysis of a large dataset, using phylogenetically independent contrasts, suggested a significant, albeit weak, correlation between BI and four measures of wing morphology (wing loading, wing area, wing length and aspect ratio). Although wing kinematics and wing morphology are both correlated with BI in birds, the dominant selective pressure upon this ratio is probably wing kinematics. The previously identified clade specificity of BI within Neornithes is most likely because birds with similar BIs fly with kinematic similarity and closely related birds have similar flight styles. A correlation between BI and wing kinematics means that it may be possible to characterize the wing beat of fossil birds. A more robust relationship between wing morphology and BI may emerge, but only after the relationship between wing kinematics and BI is quantified. A comparative and quantitative study of wing-bone anatomy and wing kinematics is a priority for future studies of avian wing-skeleton evolution and functional morphology.     

A shortening of the manus precedes the attenuation of other wing-bone elements in the evolution of flightlessness in birds

Nudds, R. L. and Slove Davidson, J. 2010. A shortening of the manus precedes the attenuation of other wing-bone elements in the evolution of flightlessness in birds. — Acta Zoologica (Stockholm) 91 : 115–122
This is the first study to present evidence for a general pattern of wing-bone attenuation during the early stages of the evolution of flightlessness. A comparative analysis using phylogenetic independent contrasts showed that in families that contain both flighted (volant) and flightless species, the volant species have shorter wings and total-arm (humerus + ulna + manus) lengths relative to their body masses than the species within their wholly volant sister families. A shortening of the manus may typify the early stages of the evolution of flightlessness, with the humerus and ulna attenuating later, perhaps because of their role in maintaining the position of the aerodynamically important alula. A shorter wing relative to body mass was not the result of the inverse (i.e. heavier body mass relative to wing length) because mean body masses of volant members of flightless families were similar to or lower than those of their wholly volant sister families. Despite finding a common trend in the wing morphologies of volant members of flightless families, it seems unlikely that a general model of selection pressures driving the evolution of flightlessness exists. At the very least, a dichotomy between those birds that have lost the ability to fly in order to gain the ability to swim and terrestrial forms, may persist.     

Great tits (Parus major) reduce body mass in response to wing area reduction: a field experiment

Flight performance is crucial in determining whether a smallbird will survive an attack by a predator. Given the importanceof body mass in determining flight performance, it has beensuggested that birds should strategically regulate body massas a response to predation risk. However, all experiments upto now have been carried out with captive birds, comparing experimental to control birds. Here we present data from thefirst experiment in the field using a within-individuals experimentaldesign. The wing area of wild great tits, Parus major, wasreduced by reversibly taping primaries five to seven. Thisallowed for the same individual to alternatively act as controlor experimental bird. Great tits reduced body mass (but not pectoral muscle width) during episodes of wing area reduction,lending support to the view that the reduction in body massexperienced by birds during molt is a strategy rather thanthe result of energetic stress. Theoretical models establishingthe different trade-offs that determine optimal body mass should therefore take into account this important life-history episode.     

Coevolution of caudal skeleton and tail feathers in birds

Birds are capable of a wide range of aerial locomotor behaviors in part because of the derived structure and function of the avian tail. The tail apparatus consists of a several mobile (free) caudal vertebrae, a terminal skeletal element (the pygostyle), and an articulated fan of tail feathers that may be spread or folded, as well as muscular and fibroadipose structures that facilitate tail movements. Morphological variation in both the tail fan and the caudal skeleton that supports it are well documented. The structure of the tail feathers and the pygostyle each evolve in response to functional demands of differing locomotor behaviors. Here, I test whether the integument and skeleton coevolve in this important locomotor module. I quantified feather and skeletal morphology in a diverse sample of waterbirds and shorebirds using a combination of linear and geometric morphometrics. Covariation between tail fan shape and skeletal morphology was then tested using phylogenetic comparative methods. Pygostyle shape is found to be a good predictor of tail fan shape (e.g., forked, graduated), supporting the hypothesis that the tail fan and the tail skeleton have coevolved. This statistical relationship is used to reconstruct feather morphology in an exemplar fossil waterbird, Limnofregata azygosternon. Based on pygostyle morphology, this taxon is likely to have exhibited a forked tail fan similar to that of its extant sister clade Fregata, despite differing in inferred ecology and other aspects of skeletal anatomy. These methods may be useful in reconstructing rectricial morphology in other extinct birds and thus assist in characterizing the evolution of flight control surfaces in birds. J. Morphol. 275:1431–1440, 2014. © 2014 Wiley Periodicals, Inc.     

Comparison of wing morphology in three birds of prey: correlations with differences in flight behavior

Flight is the overriding characteristic of birds that has influenced most of their morphological, physiological, and behavioral features. Flight adaptations are essential for survival in the wide variety of environments that birds occupy. Therefore, locomotor structure, including skeletal and muscular characteristics, is adapted to reflect the flight style necessitated by different ecological niches. Red-tailed hawks (Buteo jamaicensis) soar to locate their prey, Cooper's hawks (Accipiter cooperii) actively chase down avian prey, and ospreys (Pandion haliaetus) soar and hover to locate fish. In this study, wing ratios, proportions of skeletal elements, and relative sizes of selected flight muscles were compared among these species. Oxidative and glycolytic enzyme activities of several muscles were also analyzed via assays for citrate synthase (CS) and for lactate dehydrogenase (LDH). It was found that structural characteristics of these three raptors differ in ways consistent with prevailing aerodynamic models. The similarity of enzymatic activities among different muscles of the three species shows low physiological differentiation and suggests that wing architecture may play a greater role in determining flight styles for these birds.     

Genome size and wing parameters in passerine birds

Despite their status as the most speciose group of terrestrial vertebrates, birds exhibit the smallest and least variable genome sizes among tetrapods. It has been suggested that this is because powered flight imposes metabolic constraints on cell size, and thus on genome size. This notion has been supported by analyses of genome size and cell size versus resting metabolic rate and other parameters across birds, but most previous studies suffer from one or more limitations that have left the question open. The present study provides new insights into this issue through an examination of newly measured genome sizes, nucleus and cell sizes, body masses and wing parameters for 74 species of birds in the order Passeriformes. A positive relationship was found between genome size and nucleus/cell size, as well as between genome size and wing loading index, which is interpreted as an indicator of adaptations for efficient flight. This represents the single largest dataset presented for birds to date, and is the first to analyse a distinctly flight-related parameter along with genome size using phylogenetic comparative analyses. The results lend additional support to the hypothesis that the small genomes of birds are indeed related in some manner to flight, though the mechanistic and historical bases for this association remain an interesting area of investigation.     

Cross-species amplification of microsatellite primers in passerine birds

Developing species specific microsatellite primers can be avoidedby using existing markers which amplify across species. However,for passerines, such cross-species markers are mostly lackingand few guidelines exist for selecting them from the wide rangeof existing markers. Here cross-species amplification tests of 40microsatellite primers in 13 passerine species show an increasein probability of amplification and polymorphism with decreasingphylogenetic distance. Primers which successfully amplified inmany species had a higher chance to be polymorphic. However,since the amplification success, across a broad range of species,of particular primersets remains difficult to predict it iscrucial to identify such markers empirically. Here we describesuch widely applicable bird (passerines) microsatellite markers.     

Differences in wing morphology between juvenile and adult European Turtle Doves Streptopelia turtur: implications for migration and predator escape

Behaviour has direct links to wing morphology in bird species. Many studies have postulated migration to be one of the most important forces of selection acting on wing morphology, particularly in relation to wing pointedness. Studies in passerines have found that adults have longer and more pointed wings than juveniles, especially in migratory species. We analysed differences in wing morphology between age groups of the European Turtle Dove, a non‐passerine migratory species that benefits from rounded wings during their daily activity, due to its ground‐feeding behaviour and acrobatic flight style. Our results show that adults of this species have longer but more rounded wings than juveniles. This suggests that in this species wing morphology in juveniles is selected to facilitate the first migration, whereas other selection forces (e.g. flight manoeuvrability) are more important after the first moult. These data also explain why juveniles are not as adept at escaping from predators or hunters as adults.     

Vertebral evolution and ontogenetic allometry: The developmental basis of extreme body shape divergence in microcephalic sea snakes

Snakes exhibit a diverse array of body shapes despite their characteristically simplified morphology. The most extreme shape changes along the precloacal axis are seen in fully aquatic sea snakes (Hydrophiinae): “microcephalic” sea snakes have tiny heads and dramatically reduced forebody girths that can be less than a third of the hindbody girth. This morphology has evolved repeatedly in sea snakes that specialize in hunting eels in burrows, but its developmental basis has not previously been examined. Here, we infer the developmental mechanisms underlying body shape changes in sea snakes by examining evolutionary patterns of changes in vertebral number and postnatal ontogenetic growth. Our results show that microcephalic species develop their characteristic shape via changes in both the embryonic and postnatal stages. Ontogenetic changes cause the hindbodies of microcephalic species to reach greater sizes relative to their forebodies in adulthood, suggesting heterochronic shifts that may be linked to homeotic effects (axial regionalization). However, microcephalic species also have greater numbers of vertebrae, especially in their forebodies, indicating that somitogenetic effects also contribute to evolutionary changes in body shape. Our findings highlight sea snakes as an excellent system for studying the development of segment number and regional identity in the snake precloacal axial skeleton.     

Altered wing phenotypes of captive-bred migratory birds lower post-release fitness

Captive breeding and release to the wild is a globally important conservation tool. However, captivity can result in phenotypic changes that incur post-release fitness costs, especially if they affect strenuous or risky behaviours. Bird wing shape is critical for migration success and suboptimal phenotypes are strongly selected against. In this study, I demonstrate surprising plasticity of bird wing phenotypes in captivity for 4/16 studied species. In a model species, captive-born juveniles with wild wing phenotypes (a 1-mm longer distal primary flight feather) survived post-release at 2.7 times the rate of those with captive phenotypes (i.e. a shorter distal feather). Subtle phenotypic changes and their fitness impacts are more common than widely realised because they are easily overlooked. To improve captive breeding for conservation, practitioners must surveil phenotypic changes and find ways to mitigate them.     

Group-foraging is not associated with longevity in North American birds

Group-foraging is common in many animal taxa and is thought to offer protection against predators and greater foraging efficiency. Such benefits may have driven evolutionary transitions from solitary to group-foraging. Greater protection against predators and greater access to resources should reduce extrinsic sources of mortality and thus select for higher longevity according to life-history theory. I assessed the association between group-foraging and longevity in a sample of 421 North American birds. Taking into account known correlates of longevity, such as age at first reproduction and body mass, foraging group size was not correlated with maximum longevity, with and without phylogenetic correction. However, longevity increased with body mass in non-passerine birds. The results suggest that the hypothesized changes in predation risk with group size may not correlate with mortality rate in foraging birds.     

High dispersal ability inhibits speciation in a continental radiation of passerine birds

Dispersal can stimulate speciation by facilitating geographical expansion across barriers or inhibit speciation by maintaining gene flow among populations. Therefore, the relationship between dispersal ability and speciation rates can be positive or negative. Furthermore, an 'intermediate dispersal' model that combines positive and negative effects predicts a unimodal relationship between dispersal and diversification. Because both dispersal ability and speciation rates are difficult to quantify, empirical evidence for the relationship between dispersal and diversification remains scarce. Using a surrogate for flight performance and a species-level DNA-based phylogeny of a large South American bird radiation (the Furnariidae), we found that lineages with higher dispersal ability experienced lower speciation rates. We propose that the degree of fragmentation or permeability of the geographical setting together with the intermediate dispersal model are crucial in reconciling previous, often contradictory findings regarding the relationship between dispersal and diversification.     

A mechanical model of wing and theoretical estimate of taper factor for three gliding birds

We tested a mechanical model of wing, which was constructed using the measurements of wingspan and wing area taken from three species of gliding birds. In this model, we estimated the taper factors of the wings for jackdaw (Corrus monedula), Harris’ hawk (Parabuteo unicinctas) and Lagger falcon (Falco jugger) as 1.8, 1.5 and 1.8, respectively. Likewise, by using the data linear regression and curve estimation method, as well as estimating the taper factors and the angle between the humerus and the body, we calculated the relationship between wingspan, wing area and the speed necessary to meet the aerodynamic requirements of sustained flight. In addition, we calculated the relationship between the speed, wing area and wingspan for a specific angle between the humerus and the body over the range of stall speed to maximum speed of gliding flight. We then compared the results for these three species of gliding birds. These comparisons suggest that the aerodynamic characteristics of Harris’ hawk wings are similar to those of the falcon but different from those of the jackdaw. This paper also presents two single equations to estimate the minimum angle between the humerus and the body as well as the minimum span ratio of a bird in gliding flight.     

The Amazon River as a dispersal barrier to passerine birds: effects of river width, habitat and taxonomy

Aim To evaluate the relative effectiveness of the lower and upper sections, respectively, of the Amazon River as a barrier to bird distribution, and to evaluate ecological and taxonomic factors affecting the efficacy of the river barrier. Location Amazon River of South America between its confluence with the Napo River in the west and its delta in the east. Methods Using published distribution maps for 448 species of passerine birds occurring along the Amazon River, we evaluated whether each was distributed along one bank only (river presumed to be a barrier) or both banks (no barrier) to test the predictions that the river was more effective as a dispersal barrier: (1) along the lower, wider portion of the river than the upper, narrower portion; (2) for species inhabiting forests than open country; (3) for species inhabiting forest understorey than forest canopy; (4) for species restricted to terra firme (never inundated upland forest) than those not restricted to terra firme and (5) for certain taxonomic groups. Results Our analyses demonstrated that the Amazon River was most effective as a dispersal barrier along its lower portion and for species restricted to forests and terra firme. However, the river was not significantly more of a barrier for species inhabiting forest understorey than forest canopy. The river was most significant as a barrier to dispersal for the antbirds (Thamnophilidae) and was less significant as a barrier to species belonging to several large families including woodcreepers (Dendrocolaptidae), ovenbirds (Furnariidae), flycatchers (Tyrannidae), cotingids (Cotingidae), tanagers (Thraupidae), seed‐eating finches (Emberizidae) and blackbirds (Icteridae). Main conclusions The robust widths of Amazonian rivers are widely considered to represent impediments to dispersal and gene flow for many taxa of birds and other animals, and may have represented agents of vicariance in the diversification of species. Our study reaffirms the effectiveness of the lower Amazon River as a current barrier to bird dispersal for forest birds and provides new insights into the effects of habitat and taxonomy on the efficacy of the river barrier. Although supportive of several predictions of the river hypothesis of biological diversification, our study is limited in addressing the historical impact of river barriers as agents of vicariance in the process of diversification.     

Loudness of birdsong is related to the body size,syntax and phonology of passerine species

Songs of passerines are generally complex, long‐range acoustic signals, and are highly diverse across species. This diversity must nevertheless be shaped by the capabilities of the avian vocal physiology. For example, within species, loudness has been shown to trade‐off with aspects of song complexity. Here, I ask if such trade‐offs with loudness influenced the evolutionary diversification of song among passerines. Comparing perceived song loudness across > 140 European and North American species showed that loudness is positively related to body size and to singing with simple trilled syntax, and negatively related to aspects of syllable complexity. Syntax and syllable phonology together explained more variation than body size did, indicating that the acoustic design of songs is an important factor determining loudness. These results show for the first time that loudness covaries with, and possibly limits, song complexity across species, suggesting that a trade‐off with loudness shaped the evolutionary diversification of passerine song.