Barbule Density Research Articles

Description of natal down of the ostrich (Struthio camelus) and comparison with common quail (Coturnix coturnix): Developmental and evolutionary implications.

Natal down is a feather stage that differs in both form and function from the definitive feathers of adult birds. It has a simpler structure that has been speculated to be similar to the body coverings of non-avian dinosaurs. However, inference of the evolution of natal down has been limited by our understanding of its structural variation in extant birds. Most descriptive work has focused on neognathous birds, limiting our knowledge of the full diversity of feathers in extant taxa. Here, we describe the natal down of a post-hatch ostrich (Struthio camelus) and compare it to that of a post-hatch quail (Coturnix coturnix). We confirm the presence of featherless spaces (apteria) in S. camelus and the lack of barbules on the tips of natal down in both species. We also find differences between dorsal and ventral natal down structures, such as barbule density in S. camelus and the extent of the bare portion of the barb in both species. Surprisingly, we do not find that the neoptiles of either species follow the ideal morphologies for increasing insulation. Finally, we hypothesize that the different barb types present in S. camelus natal down result from a large addition of new barb ridges during development, which is not known except in feathers with a rachis. These results have implications for our understanding of how structure informs function and development in understudied feather types, such as those shared by non-avian dinosaurs.

Canalised and plastic components of melanin-based colouration: a diet-manipulation experiment in house sparrows

Whether melanin-based plumage colouration accurately reflects a bird’s quality is still controversial. To better understand potential mechanisms behind the observed variation in plumage colouration, we shifted our attention from a high-level expression of colour to low-level physiological phenomena by targeting the microstructure and pigment content of the feather. In a well-studied model system, the house sparrow (Passer domesticus), we combined an experimental manipulation of birds’ physiological condition and availability of resources that are key to the production of the studied colouration (phenylalanine and tyrosine (PT). We found that feathers from sparrows fed with the control diet had noticeably lower values of brightness, suggesting a higher quality of the ornamental “blackness” in comparison to those sampled from birds fed with a PT-reduced diet. Electron paramagnetic resonance (EPR) spectroscopy detected higher melanin concentrations in samples from the control than the PT-reduced group. Our multi-level analysis excluded mechanisms such as barbule density and melanosomes’ distribution, clearly pointing to the finest-level proxy of colour: the concentration of melanin in melanosomes themselves. Despite melanins being manufactured by birds endogenously, the efficiency of melanogenesis can be noticeably limited by diet. As a result, the birds’ plumage colouration is affected, which may entail consequences in social signalling.

No apparent trade-off between the quality of nest-grown feathers and time spent in the nest in an aerial insectivore, the Tree Swallow

Abstract Life history theory provides a framework for understanding how trade-offs generate negative trait associations. Among nestling birds, time spent in the nest, risk of predation, and lifespan covary, but some associations are only found within species while others are only observed between species. A recent comparative study suggests that allocation trade-offs may be alleviated by disinvestment in ephemeral traits, such as nest-grown feathers, that are quickly replaced. However, direct resource allocation trade-offs cannot be inferred from interspecific trait associations without complementary intraspecific studies. Here, we asked whether there is evidence for a within-species allocation trade-off between feather quality and time spent in the nest in Tree Swallows (Tachycineta bicolor). Consistent with the idea that ephemeral traits are deprioritized, nest-grown feathers had lower barb density than adult feathers. However, despite substantial variation in fledging age among nestlings, there was no evidence for a negative association between time in the nest and feather quality. Furthermore, accounting for differences in resource availability by considering provisioning rate and a nest predation treatment did not reveal a trade-off that was masked by variation in resources. Our results are most consistent with the idea that the interspecific association between development and feather quality arises from adaptive specialization, rather than from a direct allocation trade-off.

Differences in feather structure between urban and forest great tits: constraint or adaptation?

Urbanization is one of the strongest habitat transforming processes today that has resulted in changes in the ecological conditions for wild populations. In birds, the limitation of natural food sources and a warmer microclimate in cities can potentially influence the development and functioning of the plumage that may have important fitness consequences. Despite its potential significance, the plumage structure of urban birds are largely unexplored and it is unclear whether and how they respond to urban ecological processes such as different constraints and selection pressures. In this study, we compared several structural properties of contour, primary and tail feathers between two forest and two urban great tit Parus major populations. Our results show that the urban environment affects only a few structural properties of feathers and only in the plumage of first‐year birds. We found that both the plumulaceous and the pennaceous parts of their contour feathers are longer and have lower barb density in the urban than in the forest habitat. We also found that the primaries of first‐year birds have narrower rachis and higher barbule density in the cities than in the forests, but there were no differences in other wing feather traits and in any tail feather traits between habitats. We did not find differences in the feather structure of urban versus forest adult birds. The habitat differences in first‐year birds may indicate nutritional constraints or the effects of the warmer microclimate of the urban environment. These differences seem to disappear completely in adulthood that can be explained by the selective mortality of first‐year birds, or by adults being less sensitive than first‐year birds to environmental effects during their moult.

Marginal-covert moult in the House Sparrow Passer domesticus

ABSTRACT Marginal coverts are often neglected in passerine moult studies due to their small size and large number. Here, we document eight moult components of this group of wing coverts in a House Sparrow population from Barcelona in 2018–21: timing, growth rate, intensity, speed, duration, sequence, extent and barb density. We also tested the influence of various intrinsic and extrinsic factors on marginal-covert moult. Our adult population started marginal-covert moult on 29 July and finished it on 29 August, with the moult of hatch-year birds delayed 13 days in relation to the moult of adults. Growth rate of the largest marginal covert increased significantly with its size but did not differ between sexes. Intensity of marginal-covert moult replacement was not constant during the moult progress, which took 32 days on average. Moult sequence of marginal coverts started with the proximal feathers, proceeded distally replacing the central rows and was completed toward the front and rear rows. Moult extent was complete except in one case. Size of the largest marginal covert increased with calamus width, but not significantly, and barb density decreased with feather size, but not significantly, and neither measure differed between sexes. Comparison with published results indicated that growth rate is lower in marginal coverts than in primaries and rectrices, whereas barb density is greater. Linear regression of marginal-covert size on both growth rate and barb density can be used to predict growth rate and barb density of flight feathers in the House Sparrow.

Vane macrostructure of primary feathers and its adaptations to flight in birds

The selection pressures that drive flight feather morphology are poorly understood. Using a phylogenetic comparative approach and data from 178 species of birds, we investigated whether both position along the wing length and flight feather length affected vane structure. We found that barb density was lower on distal primaries than on proximal primaries of the leading feather vane. In contrast, on the trailing vane only mid-point barb density differed and, here, it showed denser barbs on the distal primaries. This difference was greater at the feather base than at the tip. Barb angle was higher along the full length of the leading edge vane on the proximal primaries than on the distal primaries. Overall, barb density decreased from base to tip on both trailing and leading vanes on both the proximal and the distal primaries. In general, barb angle was less acute at the feather base than at the tip. Barbs were in general denser in continuous flapping fliers than in soarers and the angle of barbs on both the proximal and the distal primaries was affected by flight type. However, we did not identify consistent differences in the pattern of barb angle change among flight style groups. These findings add new perspectives to our understanding of the functional morphology of the flight feather vane, although we still have limited knowledge on how aerodynamic forces, particularly during take-off and landing, affect the morphology of the feather vane. (Less)

The microstructure of white feathers predicts their visible and near-infrared reflectance properties.

Research on the optical properties of animal integuments, including fur, feather, skin and cuticle, has focussed almost exclusively on animal-visible wavelengths within the narrow range of 300–700 nm. By contrast, the near-infrared (NIR) portion of direct sunlight, spanning 700–2600 nm, has been largely ignored despite its potentially important thermal consequences. We quantified variation in visible and NIR reflectance and transmission for white body contour feathers of 50 bird species, and examined how well they are predicted by feather macro- and micro-structural morphology. Both visible and NIR reflectance of the feathers varied substantially across species. Larger, thicker, and sparser feathers that are characteristic of larger species, and feathers with rounder barbs and more closely spaced barbules, had high average reflectance, particularly within avian-visible wavelengths (300–700 nm). Feathers with rounder barbs and more closely situated barbules also had high average reflectance, particularly for NIR wavelengths. Barb roundness and barbule density were the only predictors of NIR reflectance after accounting for variation in visible reflectance and body size. Our results highlight the potential for adaptive variation in NIR reflectance mediated by feather structure, which may inform the design of functional materials to control light and heat.

Production of plumage ornaments among males and females of two closely related tropical passerine bird species

The evolution of elaborate secondary sexual traits (i.e., ornaments) is well‐studied in males but less so in females. Similarity in the appearance of ornaments between males and females supports the view that female ornaments arise as a neutral byproduct of selection on male traits due to genetic correlation between sexes, but recent research suggests an adaptive function of female ornaments in at least some contexts. Information on the degree to which production of ornaments differs between the sexes can shed light on these alternative perspectives. We therefore characterized the structural underpinnings of melanin‐based plumage production in males and females of two closely related passerine bird species (genus Malurus). Importantly, both ornamented and unornamented phenotypes in each sex are present between these two species, providing an opportunity to test the null expectation of equivalent modes of production in male and female ornamented phenotypes. In Malurus alboscapulatus, ornamented females are qualitatively similar to males, but we describe a distinctive ornamented female phenotype that differs from that of males in lacking a blue sheen and in lower feather barbule density. In M. melanocephalus, unornamented males and females are also similar in appearance, and we describe a similarity between unornamented phenotypes of males and females in both color and underlying feather barbule structure and pigment composition. Unornamented male M. melanocephalus can flexibly transition to the ornamented phenotype in weeks, and we found extreme differences in color and feather structure between these two alternative male phenotypes. These results contradict the idea that female ornaments have evolved in this system following a simple switch to male‐like plumage by demonstrating greater complexity in the production of the ornamented phenotype in males than in females.

A phylogenetic comparative analysis reveals correlations between body feather structure and habitat

SummaryBody feathers ensure both waterproofing and insulation in waterbirds, but how natural variation in the morphological properties of these appendages relates to environmental constraints remains largely unexplored. Here, we test how habitat and thermal condition affect the morphology of body feathers, using a phylogenetic comparative analysis of five structural traits [i.e., total feather length, the lengths of the pennaceous (distal) and plumulaceous (proximal) sections, barb density, and pennaceous barbule density] from a sample of 194 European bird species.Body feather total length is shorter in aquatic than in terrestrial birds, and this difference between groups is due to the shorter plumulaceous feather section in aquatic birds. Indeed, a reduced plumulaceous section in feather length probably reflects the need to limit air trapped in the plumage to adjust the buoyancy of aquatic birds. In contrast, the high pennaceous barbule density of aquatic birds compared to their terrestrial counterparts reflects water resistance of the plumage in contact with water.Our results show that birds living in environments with low ambient temperature have long plumulaceous feather lengths, low barb density, and low pennaceous barbule density. Data also suggest that plumage probably has limited function in reducing the heat absorption of species living in hot environments.Our results have broad implications for understanding the suite of selection pressures driving the evolution of body feather functional morphology. It remains to be tested, however, how other feather traits, such as the density of plumage (feathers per unit area) and the relative number of different feather types, for example downy feathers, are distributed amongst birds with different water resistance and thermoinsulative needs.Alay summaryis available for this article.

The Feather Structure of Oriental Honey Buzzards (Pernis ptilorhynchus) and Other Hawk Species in Relation to Their Foraging Behavior.

The basic structure of a bird feather may be adapted to suit a variety of functions on different parts of the body and in different species. In Oriental honey buzzards (Pernis ptilorhynchus), a species which often preys on the larvae of bees and wasps, it is thought that the bird's integument may provide protection against the stings of these insects. We investigated the structure of Oriental honey buzzard feathers from the face, head, and neck using light and scanning electron microscopy. The structure and appearance of the feathers were compared with those of two other hawk species which live in similar habitats but have different diets: the grey-faced buzzard (Butastur indicus) and the black kite (Milvus migrans). All feathers of Oriental honey buzzards that were examined were smaller than feathers from the same regions of the body of other species and had a reduced number of plumulaceous barbs; barbs were also closer together at the feather tip and had a high barbule density. The small 'scale feathers' on the face had deep barbules with a curved, armor-like appearance, which may help prevent stings from reaching the skin. A unique filamentous substance was observed on all the honey buzzard feathers, particularly those from around the eye of a male bird. It is possible that this may be related to a chemical defense mechanism to deter bees and wasps.

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.

Comparative analysis points to functionally significant variation in wing feather structure among a large and diverse sample of modern birds

Comparative analysis points to functionally significant variation in wing feather structure among a large and diverse sample of modern birds

Interspecific variation in the structural properties of flight feathers in birds indicates adaptation to flight requirements and habitat

Summary The functional significance of intra‐ and interspecific structural variations in the flight feathers of birds is poorly understood. Here, a phylogenetic comparative analysis of four structural features (rachis width, barb and barbule density and porosity) of proximal and distal primary feathers of 137 European bird species was conducted. Flight type (flapping and soaring, flapping and gliding, continuous flapping or passerine type), habitat (terrestrial, riparian or aquatic), wing characteristics (wing area, S and aspect ratio, AR) and moult strategy were all found to affect feather structure to some extent. Species characterized by low wing‐beat frequency flight (soaring and gliding) have broader feather rachises (shafts) and feather vanes with lower barb density than birds associated with more active flapping modes of flight. However, the effect of flying mode on rachis width disappeared after controlling for S and AR, suggesting that rachis width is primarily determined by wing morphology. Rachis width and feather vane density are likely related to differences in force distribution across the wingspan during different flight modes. An increase in shaft diameter, barb density and porosity from the proximal to distal wing feathers was found and was highest in species with flapping flight indicating that aerodynamic forces are more biased towards the distal feathers in flapping flyers than in soarers and gliders. Habitat affected barb and barbule density, which was greatest in aquatic species, and within this group, barb density was greater in divers than non‐divers, suggesting that the need for water repellency and resistance to water penetration may influence feather structure. However, we found little support for the importance of porosity in water repellency and water penetration, because porosity was similar in aquatic, riparian and terrestrial species and among the aquatic birds (divers and non‐divers). We also found that barb density was affected by moult pattern. Our results have broad implications for the understanding of the selection pressures driving flight feather functional morphology. Specifically, the large sample size relative to any previous studies has emphasized that the morphology of flight feathers is the result of a suite of selection pressures. As well as routine flight needs, constraints during moulting, habitat (particularly aquatic) and migratory requirements also affect flight feather morphology. Identifying the exact nature of these trade‐offs will perhaps inform the reconstruction of the flying modes of extinct birds.

Quantitative assessment of the interfacial roughness in multi-layered materials using image analysis: Application to oxidation in ceramic-based materials

A method is developed to characterize interfacial roughness of layered materials. A series of contiguous images is acquired with a Scanning Electron Microscope on polished cross-sections. Using image analysis algorithms based on mathematical morphology operations, interface profiles are extracted and mathematically processed to calculate standard surface roughness parameters. In addition, morphological criteria describing the interface tortuosity are determined, such as the “normalized rumpling index” (based on the profile developed length) and the “barbule density” that quantifies the protrusions of the layer (using the skeletonization function). The validity and accuracy of the method is evaluated by comparison with measurements performed by the conventional surface metrology techniques. This method is used to study internal oxidation of two ceramic-based materials (Si3N4–TiN composite/thermal barrier coatings) upon thermal aging. In both cases, a link is established between the roughening of the oxide layer interface and a decrease in the mechanical strength of the multi-layered material.

Chronic coccidian infestation compromises flight feather quality in house sparrowsPasser domesticus

Parasites usurp indispensable resources for birds during a moult, and this is particularly relevant for those parasites residing in host intestines. This might compromise the nutritionally demanding moult and, thus, feather functionality. Although lower feather quality has profound and multifaceted adverse effects on residual fitness, surprisingly, little is known about parasites' effect on feather traits, especially over the longer term. We conducted an aviary experiment by medicating half of a group of naturally infested house sparrows Passer domesticus against intestinal coccidians for 15 months, spanning two consecutive postnuptial moults, whereas the other half was kept infested (i.e. without medication). Coccidian infestation significantly and negatively affected the size of the uropygial gland during the second moulting period compared to the medicated group. Furthermore, wing length was significantly shorter after the second moulting in the non-medicated compared to the medicated female birds, which indicates that the negative effects of coccidians emerge only after a prolonged exposure to parasite infestation. Non-medicated birds grew poorer quality flight feathers detected in a large number of feather traits both after the first and second moults. In the case of non-medicated birds, the primaries were lighter and shorter, and had a smaller vane area, thinner rachis and decreased stiffness, although a higher barb and barbule density, which may have various consequences for fitness through reducing flight performance. Our findings demonstrate that, besides the well-known immediate consequences for host breeding success, parasites might also have serious, long-lasting effects through influencing feather quality and, ultimately, fitness of the host. © 2013 The Linnean Society of London

Haste Makes Waste but Condition Matters: Molt Rate–Feather Quality Trade-Off in a Sedentary Songbird

BackgroundThe trade-off between current and residual reproductive values is central to life history theory, although the possible mechanisms underlying this trade-off are largely unknown. The ‘molt constraint’ hypothesis suggests that molt and plumage functionality are compromised by the preceding breeding event, yet this candidate mechanism remains insufficiently explored.Methodology/Principal FindingsThe seasonal change in photoperiod was manipulated to accelerate the molt rate. This treatment simulates the case of naturally late-breeding birds. House sparrows Passer domesticus experiencing accelerated molt developed shorter flight feathers with more fault bars and body feathers with supposedly lower insulation capacity (i.e. shorter, smaller, with a higher barbule density and fewer plumulaceous barbs). However, the wing, tail and primary feather lengths were shorter in fast-molting birds if they had an inferior body condition, which has been largely overlooked in previous studies. The rachis width of flight feathers was not affected by the treatment, but it was still condition-dependent.Conclusions/SignificanceThis study shows that sedentary birds might face evolutionary costs because of the molt rate–feather quality conflict. This is the first study to experimentally demonstrate that (1) molt rate affects several aspects of body feathers as well as flight feathers and (2) the costly effects of rapid molt are condition-specific. We conclude that molt rate and its association with feather quality might be a major mediator of life history trade-offs. Our findings also suggest a novel advantage of early breeding, i.e. the facilitation of slower molt and the condition-dependent regulation of feather growth.

Case-control study of cases of bovine spongiform encephalopathy born after July 31, 1996 (BARB cases) in Great Britain

This paper reports the results of a case-control study of the bovine spongiform encephalopathy (BSE) cases born in Great Britain after the statutory reinforcement of the ban (BARB) on the...

Feather microstructure predicts size and colour intensity of a melanin-based plumage signal

Feather microstructure affects the light absorbed by plumage pigments. However, the effect of particular elements of feather microstructure on the expression of pigmentary colours or on the size of colour patches has never been investigated. Here I use a model of avian visual perception and scanning electron microscope imaging of feathers to show that part of variation in the size and colour properties of a melanin-based plumage signal of quality, the black breast stripe of great tits Parus major, is explained by three elements of feather microstructure (barbule density, barb cortex size and barb pith size). The strongest associations were between large stripes and low barbule density, between dark stripes and high barbule density, and between stripes with high relative long reflectance and high barbule density and thin barb cortex. By contrast, carotenoid-based colour was not related to microstructural elements. Thus, it is possible that not all variation in melanin-based colour is determined by melanin content, but also by feather microstructure. These findings should be considered by studies on the evolution of signals of quality.

Structural Changes in the Main Rectrices of the Superb Lyrebird Menura novaehollandiae in the Development of the Filamentary Feathers

SummaryThe development of the tail plumage of the female and male Superb Lyrebird Menura novaehollandiae proceeds through a number of moults during which the feathers formed undergo structural changes. The number of barbs in successive 50 mm lengths of the main rectrices (plain and filamentary feathers) (designated ‘barb density’) were counted, starting at the distal end. As the bird grows older, the relevant barb densities decrease. The paper identifies the changes in structure of a feather during the growing process. The case histories of several identifiable subjects are given, illustrating the independent behaviour of the individual follicles within the tail plumage of particular birds and between different birds.