Variation in the mechanical properties of flight feathers of the blackcap Sylvia atricapilla in relation to migration

Migration causes temporal and energetic constraints during plumage development, which can compromise feather structure and function. In turn, given the importance of a good quality of flight feathers in migratory movements, selection may have favoured the synthesis of feathers with better mechanical properties than expected from a feather production constrained by migration necessities. However, no study has assessed whether migratory behaviour affects the relationship between the mechanical properties of feathers and their structural characteristics. We analysed bending stiffness (a feather mechanical property which is relevant to birds’ flight), rachis width and mass (two main determinants of variation in bending stiffness) of wing and tail feathers in migratory and sedentary blackcaps Sylvia atricapilla. Migratory blackcaps produced feathers with a narrower rachis in both wing and tail, but their feathers were not significantly lighter; in addition, bending stiffness was higher in migratory blackcaps than in sedentary blackcaps. Such unexpected result for bending stiffness remained when we statistically controlled for individual variation in rachis width and feather mass, which suggests the existence of specific mechanisms that help migratory blackcaps to improve the mechanical behaviour of their feathers under migration constraints.     

The effects of long‐distance migration on the evolution of moult strategies in Western‐Palearctic passerines

Although feathers are the unifying characteristic of all birds, our understanding of the causes, mechanisms, patterns and consequences of the feather moult process lags behind that of other major avian life‐history phenomena such as reproduction and long‐distance migration. Migration, which evolved in many species of the temperate and arctic zones, requires high energy expenditure to endure long‐distance journeys. About a third of Western‐Palearctic passerines perform long‐distance migrations of thousands of kilometres each year using various morphological, physiological, biomechanical, behavioural and life‐history adaptations. The need to include the largely non‐overlapping breeding, long‐distance migration and feather moult processes within the annual cycle imposes a substantial constraint on the time over which the moult process can take place. Here, we review four feather‐moult‐related adaptations which, likely due to time constraints, evolved among long‐distance Western‐Palearctic migrants: (i) increased moult speed; (ii) increased overlap between moult and breeding or migration; (iii) decreased extent of plumage moult; and (iv) moult of part or all of the plumage during the over‐wintering period in the tropics rather than in the breeding areas. We suggest that long‐distance migration shaped the evolution of moult strategies and increased the diversity of these strategies among migratory passerines. In contrast to this variation, all resident passerines in the Western Palearctic moult immediately after breeding by renewing the entire plumage of adults and in some species also juveniles, while in other species juvenile moult is partial. We identify important gaps in our current understanding of the moult process that should be addressed in the future. Notably, previous studies suggested that the ancestral moult strategy is a post‐breeding summer moult in the Western Palearctic breeding areas and that moult during the winter evolved due to the scheduling of long‐distance migration immediately after breeding. We offer an alternative hypothesis based on the notion of southern ancestry, proposing that the ancestral moult strategy was a complete moult during the ‘northern winter’ in the Afro‐tropical region in these species, for both adults and juveniles. An important aspect of the observed variation in moult strategies relates to their control mechanisms and we suggest that there is insufficient knowledge regarding the physiological mechanisms that are involved, and whether they are genetically fixed or shaped by environmental factors. Finally, research effort is needed on how global climate changes may influence avian annual routines by altering the scheduling of major processes such as long‐distance migration and feather moult.     

Moult topography and its application to the study of partial wing‐moult in two neotropical wrens

During partial moults birds replace a variable number or percentage of old feathers. This quantity, known as moult extent, has been a primary variable used in comparative studies. However, different spatial configurations of feather replacement may result from an equal number of renewed feathers. Few studies have addressed spatial aspects of moult, which may vary among species, among individuals of the same species and between episodes at the individual level. We present a novel approach to quantify the spatial configuration of a wing‐moult episode, hereafter referred to as moult topography, which comprises two elements, namely extent and vector, the latter condensing the spatial configuration of the replaced feathers on the wing plane. We apply this method to investigate preformative (post‐juvenile) wing‐feather moult pattern in the Spot‐breasted Wren Pheugopedius maculipectus and the White‐breasted Wood‐Wren Henicorhina leucosticta. We specified a null model of wing‐moult topography by which feather replacement follows a discrete anterior–posterior (vertical) axis between tracts and a discrete proximal–distal (horizontal) axis within tracts, and whereby wing feathers from a new tract are replaced only if all the feathers from the previous (anterior) tract have been replaced. Our sample of Spot‐breasted Wrens showed a strict single pattern of replacement that did not differ significantly from the null model. Our sample of White‐breasted Wood‐Wrens, however, differed significantly from the null model, showing prioritization of proximal wing feathers closer to the body. These differences might have biological relevance, for example in mate selection or in response to different environmental stressors, and might reveal the influence of these factors on the evolution of moult strategies. Overall, moult topography provides a new approach to future ecological and evolutionary studies of moult.     

Morphological properties of the last primaries,the tail feathers,and the alulae of Accipiter nisus,Columba livia,Falco peregrinus,and Falco tinnunculus

We investigated the mechanical properties (Young's modulus, bending stiffness, barb separation forces) of the tenth primary of the wings, of the alulae and of the middle tail feathers of Falco peregrinus. For comparison, we also investigated the corresponding feathers in pigeons (Columba livia), kestrels (Falco tinnunculus), and sparrowhawks (Accipiter nisus). In all four species, the Young's moduli of the feathers ranged from 5.9 to 8.4 GPa. The feather shafts of F. peregrinus had the largest cross‐sections and the highest specific bending stiffness. When normalized with respect to body mass, the specific bending stiffness of primary number 10 was highest in F. tinnunculus, while that of the alula was highest in A. nisus. In comparison, the specific bending stiffness, measured at the base of the tail feathers and in dorso‐ventral bending direction, was much higher in F. peregrinus than in the other three species. This seems to correlate with the flight styles of the birds: F. tinnunculus hovers and its primaries might therefore withstand large mechanical forces. A. nisus has often to change its flight directions during hunting and perhaps needs its alulae for this maneuvers, and in F. peregrinus, the base of the tail feathers might need a high stiffness during breaking after diving. J. Morphol. 276:33–46, 2015. © 2014 Wiley Periodicals, Inc.     

Wing shape and migration in shorebirds: a comparative study

Migration is an energetically expensive and hazardous stage of the annual cycle of non‐resident avian species, and requires certain morphological adaptations. Wing shape is one of the morphological traits that is expected to be evolutionarily shaped by migration. Aerodynamic theory predicts that long‐distance migrants should have more pointed wings with distal primaries relatively longer than proximal primaries, an arrangement that minimizes induced drag and wing inertia, but this prediction has mostly been tested in passerine species. We applied the comparative method of phylogenetically independent contrasts to assess convergent evolution between wing shape and migration within shorebirds. We confirmed the assumption that long‐distance migrants have less rounded wings than species migrating shorter distances. Furthermore, wing roundedness negatively correlates with fat load and mean distance of migratory flights, the basic components of migration strategies. After controlling for interspecific differences in body size, we found no support for a link between wing length and migration, indicating that wing shape is a more important predictor of shorebird migratory behaviour than wing length. The results suggest that total migration distance and migratory strategy may simultaneously act on the evolution of wing shape in shorebirds, and possibly in other avian species.     

Moulting strategies of a long-distance migratory seabird, the Mediterranean Cory's Shearwater Calonectris diomedea diomedea

Seabird moult is poorly understood because most species undergo moult at sea during the non-breeding season. We scored moult of wings, tail and body feathers on 102 Mediterranean Cory's Shearwaters Calonectris diomedea diomedea accidentally caught by longliners throughout the year. Primary renewal was found to be simple and descendant from the most proximal (P1) to the most distal (P10) feather. Secondaries showed a more complex moulting pattern, with three different asynchronous foci: the first starting on the innermost secondaries (S21), the second on the middle secondaries (S5) and the latest on the outermost secondaries (S1). Rectrix moult started at a later stage and was simple and descendant from the most proximal feather (R1) expanding distally. Although a few body feathers can be moulted from prelaying to hatching, moult of ventral and dorsal feathers clearly intensified during chick rearing. Different moulting sequences and uncoupled phenology between primary and secondary renewal suggest that flight efficiency is a strong constraint factor in the evolution of moulting strategies. Moreover, moult of Cory's Shearwaters was synchronous between wings and largely asynchronous between tail halves, with no more than one rectrix moulted at once. This result is probably related to the differential sensitivity of wings and the tail on flight performance, ultimately derived from different aerodynamic functions. Finally, Cory's Shearwater females renewed feathers earlier and faster than males, which may be related to the lower chick attendance of females.     

Size scaling and stiffness of avian primary feathers: implications for the flight of Mesozoic birds

The primary feathers of birds are subject to cyclical forces in flight causing their shafts (rachises) to bend. The amount the feathers deflect during flight is dependent upon the flexural stiffness of the rachises. By quantifying scaling relationships between body mass and feather linear dimensions in a large data set of living birds, we show that both feather length and feather diameter scale much closer to predictions for geometric similarity than they do to elastic similarity. Scaling allometry also indicates that the primary feathers of larger birds are relatively shorter and their rachises relatively narrower, compared to those of smaller birds. Two-point bending tests indicated that larger birds have more flexible feathers than smaller species. Discriminant functional analyses (DFA) showed that body mass, primary feather length and rachis diameter can be used to differentiate between different magnitudes of feather bending stiffness, with primary feather length explaining 63% of variance in rachis stiffness. Adding fossil measurement data to our DFA showed that Archaeopteryx and Confuciusornis do not overlap with extant birds. This strongly suggests that the bending stiffness of their primary feathers was different to extant birds and provides further evidence for distinctive flight styles and likely limited flight ability in Archaeopteryx and Confuciusornis.     

White tail spots in breeding Barn Swallows Hirundo rustica signal body condition during winter moult

The determinants and function of pigmentation of feathers and other tissues have been the focus of a large number of studies, particularly with respect to socio‐sexual communication. However, many birds exhibit depigmented white spots or bars on their feathers whose function is poorly understood. Here we assess whether white feather spots reflect phenotypic condition at the time of moult by investigating the covariation between spot size or shape and condition‐dependent feather growth rate, as gauged by width of the growth bars on the tail feathers of Barn Swallows. We found that feathers with higher growth rates had larger, less rounded white spots. In addition, variance in spot perimeter for a given spot area was larger in males than in females. This study is the first to provide evidence that features of white markings on feathers directly reflect body condition at the time of moult and can therefore reliably signal phenotypic quality in the context of socio‐sexual communication. In addition, the study highlights the potential communication function of the shape and not just the size of colour signals.     

Die Bionik der Schwungfederkaskade

Bionics of the primary cascade – solution of an optimization problem Investigating of the properties of the primary cascade in soaring birds to date resulted in some spectacular insights. It was disclosed that the primary cascade with its staggered feathers acts like a vortex diffusor and, hence, considerably reduces the wing drag. That advance enlarges the range in migration and food sourcing. Newly the reason for the circular respectively spherical positioning of the primary tips could be resolved. This spatial arrangement features an optimum with respect to enlarging the distances of the feather tips and span, respectively, at limited root bending moments at once. This synergistic solution leads to an optimized fracture behavior and favors the lightweight construction of avian wings.     

Moult in the Loggerhead Shrike Lanius ludovicianus is influenced by sex,latitude and migration

We investigated moult strategies in Loggerhead Shrikes by examining first prebasic or preformative moult patterns and by assessing the general location where individual feathers were grown using stable hydrogen isotope (δ²H) analysis. We tested the relative importance of factors known to impact moult timing and pattern, including age, sex, body size, food availability and migration. Migratory Shrikes showed evidence of suspended moult, in which feathers are moulted on both the breeding and the non‐breeding grounds with a suspension of moult during migration. Extent of moult was best explained by sex, longitude, migratory behaviour and breeding‐ground latitude. Male Hatch Year (HY) Shrikes replaced more feathers on the breeding grounds prior to migration than did HY females and moulted more extensively on the breeding grounds than did females. Non‐migratory HY Shrikes underwent a more extensive preformative moult than migratory HY Shrikes. Individuals in more southerly migratory populations moulted more extensively on the breeding grounds than did those breeding further north. Our data also indicate that individuals in the northeastern populations moulted more extensively on the breeding grounds than did those in the north and southwest. Our study underlines the complex structure and variation in moult possible within species, revealing surprising levels of differentiation between sexes and age cohorts, linked to environmental factors on the breeding grounds. Our study highlights the utility of an intrinsic marker, specifically δ²H analysis, to test hypotheses regarding the evolutionary and ecological forces driving moult. Although the methodology has not commonly been applied to this area of research, our results indicate that it can provide unprecedented insight into inter‐ and intra‐specific adaptive response to constraints, whereby individuals maximize fitness.     

Do migrant birds have more pointed wings?: A comparative study

Summary Do birds that migrate over longer distances have more pointed wings than more sedentary birds? Within several bird genera, species differ considerably in their migration distances. This makes it possible to study the extent to which different taxa show similar morphological solutions to common selection pressures. I selected 14 species, two from each of seven passerine genera, to maximize within-genus differences in migration distance. Wing lengths and the lengths of eight primary feathers around the wing tip were measured to assess wing length and shape. Primary lengths were transformed to take into account the allometric relationship between the length of each feather and wing length and then collapsed into summary measures of shape by principal component analysis. I used the method of independent contrasts to address the effects of phylogeny. Wing length showed no relationship with migration distance. There was a correlation between migration distance and wing shape. It is concluded that long-distance migration has resulted in convergent morphological evolution of long distal and short proximal primaries, resulting in wing tips close to the leading edge of the wing.     

Longer wings for faster springs – wing length relates to spring phenology in a long‐distance migrant across its range

In migratory birds, morphological adaptations for efficient migratory flight often oppose morphological adaptations for efficient behavior during resident periods. This includes adaptations in wing shape for either flying long distances or foraging in the vegetation and in climate‐driven variation of body size. In addition, the timing of migratory flights and particularly the timely arrival at local breeding sites is crucial because fitness prospects depend on site‐specific phenology. Thus, adaptations for efficient long‐distance flights might be also related to conditions at destination areas. For an obligatory long‐distance migrant, the common nightingale, we verified that wing length as the aerodynamically important trait, but not structural body size increased from the western to the eastern parts of the species range. In contrast with expectation from aerodynamic theory, however, wing length did not increase with increasing migration distances. Instead, wing length was associated with the phenology at breeding destinations, namely the speed of local spring green‐up. We argue that longer wings are beneficial for adjusting migration speed to local conditions for birds breeding in habitats with fast spring green‐up and thus short optimal arrival periods. We suggest that the speed of spring green‐up at breeding sites is a fundamental variable determining the timing of migration that fine tune phenotypes in migrants across their range.     

Phylogenetics and ecomorphology of emarginate primary feathers

Wing tip slots are a distinct morphological trait broadly expressed across the avian clade, but are generally perceived to be unique to soaring raptors. These slots are the result of emarginations on the distal leading and trailing edges of primary feathers, and allow the feathers to behave as individual airfoils. Research suggests these emarginate feathers are an adaptation to increase glide efficiency by mitigating induced drag in a manner similar to aircraft winglets. If so, we might expect birds known for gliding and soaring to exhibit emarginate feather morphology; however, that is not always the case. Here, we explore emargination across the avian clade, and examine associations between emargination and ecological and morphological variables. Pelagic birds exhibit pointed, high‐aspect ratio wings without slots, whereas soaring terrestrial birds exhibit prominent wing‐tip slots. Thus, we formed four hypotheses: (1) Emargination is segregated according to habitat (terrestrial, coastal/freshwater, pelagic). (2) Emargination is positively correlated with mass. (3) Emargination varies inversely with aspect ratio and directly with wing loading and disc loading. (4) Emargination varies according to flight style, foraging style, and diet. We found that emargination falls along a continuum that varies with habitat: Pelagic species tend to have zero emargination, coastal/freshwater birds have some emargination, and terrestrial species have a high degree of emargination. Among terrestrial and coastal/freshwater species, the degree of emargination is positively correlated with mass. We infer this may be the result of selection to mitigate induced power requirements during slow flight that otherwise scale adversely with increasing body size. Since induced power output is greatest during slow flight, we hypothesize that emargination may be an adaptation to assist vertical take‐off and landing rather than glide efficiency as previously hypothesized.     

Moult speed affects structural feather ornaments in the blue tit

Growing evidence suggests that structural feather colours honestly reflect individual quality or body condition but, contrary to pigment‐based colours, it is not clear what mechanism links condition to reflectance in structural feather colours. We experimentally accelerated the moult speed of a group of blue tits (Cyanistes caeruleus) by exposing them to a rapidly decreasing photoperiod and compared the spectral characteristics of their structural feather colours with those of control birds. Blue tits were sexually dimorphic on the UV/blue crown and on the white cheek feathers. Moult speed, however, dramatically reduced brightness and the saturation only on the UV/blue crown feathers, whereas structural white on the cheek feathers was basically unaffected by moult speed. Given that the time available for moulting is usually confined to the period between the end of the breeding season and migration or wintering, UV/blue colours, but not structural white, may convey long‐term information about an individual’s performance during the previous breeding season. The trade‐off between fast moulting and structural colour expression may represent a previously unrecognized selective advantage for early‐breeding birds.     

Population differences in the structure and coloration of great tit contour feathers

Contour feathers cover most of the avian body and play critical roles in insulation, social communication, aerodynamics, and water repellency. Feather production is costly and the development of the optimum characteristics for each function may be constrained by limited resources or time, and possibly also lead to trade‐offs among the different characteristics. Populations exposed to different environmental conditions may face different selective pressures, resulting in differences in feather structure and coloration, particularly in species with large geographical distributions. Three resident populations of great tit Parus major L. from different latitudes differed in feather structure and coloration. Individuals from the central population exhibited less dense and longer contour feathers, with a higher proportion of plumulaceous barbs than either northern or southern birds, which did not differ in their feather structure. Ultraviolet reflectance and brightness of the yellow of the contour feathers of the breast was higher for the southern than for the northern population. Birds with greener plumage (higher hue) had less dense but longer feathers, independently of the population of origin. Differences in feather structure across populations appear to be unrelated to the contour feather colour characteristics except for hue. Nutritional and time constraints during molt might explain the pattern of feather structure, whereas varying sexual selection pressure might underlie the coloration patterns observed. Our results suggest that different selective pressures or constraints shape contour feather traits in populations exposed to varying environmental conditions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114 , 82–91.     

Sexual Dimorphism and Population Differences in Structural Properties of Barn Swallow (Hirundo rustica) Wing and Tail Feathers

Sexual selection and aerodynamic forces affecting structural properties of the flight feathers of birds are poorly understood. Here, we compared the structural features of the innermost primary wing feather (P1) and the sexually dimorphic outermost (Ta6) and monomorphic second outermost (Ta5) tail feathers of barn swallows (Hirundo rustica) from a Romanian population to investigate how sexual selection and resistance to aerodynamic forces affect structural differences among these feathers. Furthermore, we compared structural properties of Ta6 of barn swallows from six European populations. Finally, we determined the relationship between feather growth bars width (GBW) and the structural properties of tail feathers. The structure of P1 indicates strong resistance against aerodynamic forces, while the narrow rachis, low vane density and low bending stiffness of tail feathers suggest reduced resistance against airflow. The highly elongated Ta6 is characterized by structural modifications such as large rachis width and increased barbule density in relation to the less elongated Ta5, which can be explained by increased length and/or high aerodynamic forces acting at the leading tail edge. However, these changes in Ta6 structure do not allow for full compensation of elongation, as reflected by the reduced bending stiffness of Ta6. Ta6 elongation in males resulted in feathers with reduced resistance, as shown by the low barb density and reduced bending stiffness compared to females. The inconsistency in sexual dimorphism and in change in quality traits of Ta6 among six European populations shows that multiple factors may contribute to shaping population differences. In general, the difference in quality traits between tail feathers cannot be explained by the GBW of feathers. Our results show that the material and structural properties of wing and tail feathers of barn swallows change as a result of aerodynamic forces and sexual selection, although the result of these changes can be contrasting.     

Migratory behaviour affects the trade-off between feather growth rate and feather quality in a passerine bird

Migratory birds have less time for moulting than sedentary birds, which may force them to produce their feathers faster at the expense of reducing feather quality. However, the effects of migration on the trade-off between moult speed and plumage quality remain to be studied in natural populations. We analysed the relationship between growth rate and quality of individual feathers, taking advantage of natural variation between migratory and sedentary populations of blackcaps Sylvia atricapilla . As predicted by life-history theory, individual blackcaps showed variable individual quality, which was revealed by positive correlations between feather growth rate and feather mass within populations. However, migrants grew up their feathers faster, producing lighter feathers than sedentary blackcaps. These results support the idea that feather growth rate and feather quality are traded against each other in blackcaps. Such a trade-off is apparently caused by different selection associated to migratory and sedentary life styles, which opens new insights into the diversification of moult patterns in birds.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 98–105.     

Linking the molecular evolution of avian beta (β) keratins to the evolution of feathers

Feathers of today's birds are constructed of beta (β)-keratins, structural proteins of the epidermis that are found solely in reptiles and birds. Discoveries of "feathered dinosaurs" continue to stimulate interest in the evolutionary origin of feathers, but few studies have attempted to link the molecular evolution of their major structural proteins (β-keratins) to the appearance of feathers in the fossil record. Using molecular dating methods, we show that before the appearance of Anchiornis (～155 Million years ago (Ma)) the basal β-keratins of birds began diverging from their archosaurian ancestor ～216?Ma. However, the subfamily of feather β-keratins, as found in living birds, did not begin diverging until ～143?Ma. Thus, the pennaceous feathers on Anchiornis, while being constructed of avian β-keratins, most likely did not contain the feather β-keratins found in the feathers of modern birds. Our results demonstrate that the evolutionary origin of feathers does not coincide with the molecular evolution of the feather β-keratins found in modern birds. More likely, during the Late Jurassic, the epidermal structures that appeared on organisms in the lineage leading to birds, including early forms of feathers, were constructed of avian β-keratins other than those found in the feathers of modern birds. Recent biophysical studies of the β-keratins in feathers support the view that the appearance of the subfamily of feather β-keratins altered the biophysical nature of the feather establishing its role in powered flight.     

The scaling of the size and stiffness of primary flight feathers

Birds encompass a large range of body sizes, yet the importance of body size on feather morphology and mechanical properties has not been characterized. In this study, I examined the scaling relationships of primary flight feathers within a phylogenetically diverse sample of avian species varying in body size by nearly three orders of magnitude. I measured the scaling relationships between body mass and feather linear dimensions as well as feather flexural stiffness. The resnlts of an independent contrasts analysis to test the effects of phylogenetic history on the characters measured had no effect on the scaling relationships observed. There was slight, but not significant, positive allometry in the scaling of shaft diameter with respect to feather length across a range of body masses. The scaling of feather length and diameter against body mass was not significantly different from isometry. Flexural stiffness, however, exhibited strong negative allometry. Therefore, larger birds have relatively more flexible feathers than smaller birds. The more flexible primary feathers of large birds may reduce stresses on the wing skeleton during take-off and landing and also make these feathers less susceptible to mechanical failure. Conversely, the greater flexibility of these feathers may also reduce their capacity to generate aerodynamic lift.     

Of 11 candidate steroids,corticosterone concentration standardized for mass is the most reliable steroid biomarker of nutritional stress across different feather types

相似文献