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The geometry of feather barbs (barb length and barb angle) determines feather vane asymmetry and vane rigidity, which are both critical to a feather's aerodynamic performance. Here, we describe the relationship between barb geometry and aerodynamic function across the evolutionary history of asymmetrical flight feathers, from Mesozoic taxa outside of modern avian diversity (Microraptor, Archaeopteryx, Sapeornis, Confuciusornis and the enantiornithine Eopengornis) to an extensive sample of modern birds. Contrary to previous assumptions, we find that barb angle is not related to vane-width asymmetry; instead barb angle varies with vane function, whereas barb length variation determines vane asymmetry. We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes, and that cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles. This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds, revealing a fundamental aerodynamic adaptation that has persisted from the Late Jurassic. However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds. In both modern birds and enantiornithines, trailing vanes have larger barb angles than in comparatively stemward taxa like Archaeopteryx, which exhibit small trailing vane barb angles. This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology, which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor. Our findings suggest that the fully modern avian flight feather, and possibly a modern capacity for powered flight, evolved crownward of Confuciusornis, long after the origin of asymmetrical flight feathers, and much later than previously recognized.

Background Feathers with complex and fine structure are hallmark avian integument appendages, which have contributed significantly to the survival and breeding for birds. Here, we aimed to explore the differentiation, morphogenesis and development of diverse feathers in the domestic duck. Results Transcriptome profiles of skin owing feather follicle from two body parts at three physiological stages were constructed to understand the molecular network and excavate the candidate genes associated with the development of plumulaceous and flight feather structures. The venn analysis of differentially expressed genes (DEGs) between abdomen and wing skin tissues at three developmental stages showed that 38 genes owing identical differentially expression pattern. Together, our data suggest that feather morphological and structural diversity can be possibly related to the homeobox proteins. The key series-clusters, many candidate biological processes and genes were identified for the morphogenesis, growth and development of two feather types. Through comparing the results of developmental transcriptomes from plumulaceous and flight feather, we found that DEGs belonging to the family of WNT, FGF and BMP have certain differences; even the consistent DEGs of skin and feather follicle transcriptomes from abdomen and wing have the different expression patterns. Conclusions Overall, this study detected many functional genes and showed differences in the molecular mechanisms of diverse feather developments. The findings in WNT, FGF and BMP, which were consistent with biological experiments, showed more possible complex modulations. A correlative role of HOX genes was also suggested but future biological verification experiments are required. This work provided valuable information for subsequent research on the morphogenesis of feathers.

Flight feathers represent a hallmark innovation of avian evolution. Recent comparative genomic analyses identified a 284 bp avian-specific highly conserved element (ASHCE) located within the eighth intron of the SIM bHLH transcription factor 1 (Sim1) gene, postulated to act as а cis-regulatory element governing flight feather morphogenesis. To investigate its functional significance, genome-edited (GE) primordial germ cell (PGC) lines carrying targeted ASHCE deletions were generated using CRISPR/Cas9-mediated editing, with germline chimeric males subsequently mated with wild-type (WT) hens to obtain GE progeny. The resulting GE chickens harbored 257-260 bp deletions, excising approximately half of the Sim1-ASHCE sequence. Reverse transcription-quantitative real-time polymerase chain reaction (RT-gPCR) analysis showed an average 0.32-fold reduction in Sim1 expression in the forelimbs of GE embryos at day 8 (E8) compared to WT counterparts. Despite this, GE chickens developed structurally normal flight and tail feathers. /n situ hybridization localized Sim1 expression to the posterior mesenchyme surrounding flight feather buds in E8 WT embryos, but not within the buds themselves. These results suggest that partial deletion of Sim1-ASHCE, despite diminishing Sim1 expression, does not disrupt flight feather formation. The excised region appears to possess enhancer activity toward Sim1 but is dispensable for flight feather development. Complete ablation of the ASHCE will be necessary to fully resolve the regulatory role of Sim1 in avian feather morphogenesis.

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. Lay Summary

Flight feather moult is an energetically expensive stage of the annual cycle of birds. Its timing is adjusted to other important time- and energy-demanding activities, including migration. In the vast majority of migratory birds, primary moult occurs before or after migration or moulting is suspended during migration. The Black-headed Gull is an exception: it starts moulting flight feathers at the onset of the autumn migration. The course of primary moult in this species is still poorly understood due to the difficulty in obtaining sufficient data from the entire period of flight feather replacement. In this paper, we employed digital photos taken in the field to estimate the start, variation in the start date and duration of primary moult in adult and immature Black-headed Gulls, by using the Underhill–Zucchini likelihood moult model. On average, immatures started their moult on 7 June, 25 days earlier than adults, but their moult lasted 5 days longer. Consequently, there was a 20-day difference in the end of moult – that is, on average it occurred on 16 September and 6 October in immature and adult gulls, respectively. Adults showed greater variability in the mean primary moult start date than immatures, as the beginning of flight feather replacement may depend on breeding success (earlier after breeding failure) and on breeding phenology. The overlap of primary moult and migration produces a trade-off in resource allocation between those two processes, which leads to a low number of simultaneously growing primaries and a decrease in the growth rate of the two external, heaviest flight feathers. Existing reports of a later completion of primary moult in the first half of November in this species are based on the faulty assumption that the outermost primary is always the longest, which is not the case in 34% of individuals.

Molt summary tables reveal the sequence and mode of flight-feather replacement and how these feathers are divided into independent replacement series. Tables for summarizing molt are relatively new, and the rules for generating them were first formally illustrated using data from a temperate passerine. However, this first illustration failed to address (i) species with primaries divided into more than one replacement series, (ii) species with stepwise primary replacement, which almost always involves incomplete annual replacement of the primaries, and (iii) species with incomplete annual replacement within molt series characterized by single-wave replacement. Here, we review complications that arise in developing molt summary tables for such cases and we offer solutions that remove ambiguity about the direction that molt proceeds within a replacement series and about the recognition of nodal and terminal feathers that mark the beginning and end of molt series. We use these modified molt summary tables to describe the sequence of primary replacement in four groups of Gruiform birds, a group for which primary replacement has been reported to proceed from the outermost primary toward the body, unlike most other birds. Eighty molting Grey-winged Trumpeters, Psophia crepitans, and 124 molting Limpkins, Aramus guarauna , show the sequence of primary replacement is proximal in both groups; furthermore, the primaries of trumpeters are divided into two replacement series, one beginning at the outermost primary P10, and the other beginning at P3. To further evaluate the extent of this highly unusual direction of replacement in Gruiforms, we cast the data (Stresemann & Stresemann, 1966) on primary replacement in upland rails (Rallidae) and flufftails (Sarothruridae) into molt summary tables; both also replace their primaries proximally, from outermost to innermost, suggesting that this mode of primary replacement may be characteristic of Gruiformes.

To assess the exposure of avian species in Jiangsu Province, China to eight heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn), the flight feathers, eggshells and feces of total ten avian species (including four herons, four cranes, one stork and one gull) were collected during March to May in 2012. The total concentrations of As, Cd and Hg were measured by Atomic Fluorescence Spectrometer; Cr, Cu, Ni, Pb and Zn were measured by inductively coupled plasma optical emission spectrometer. The determined concentrations of Cr (3.94, 1.33–8.30 mg kg⁻¹), Cu (15.02, 7.34–35.53 mg kg⁻¹) and Zn (134.66, 77.26–242.25 mg kg⁻¹) in fresh feathers and Cd (7.93, 7.44–9.12 mg kg⁻¹), Ni (22.74, 19.38–24.71 mg kg⁻¹), Pb (85.06, 78.72–91.95 mg kg⁻¹) and Zn (63.54, 55.82–72.14 mg kg⁻¹) in eggshells were higher than the mean values of other reported data, indicating a considerable heavy metal pollution status in local area. Comparing to the heavy metal levels in early historic feathers (1992–2000), a significant elevation of concentrations has been observed in recent bird feathers. For feathers of Grus japonensis, the heavy metal concentrations increased by 19–267%. This increased tendency was consistent with local GDP (Gross Domestic Products) development. The anthropogenic economic activity especially industrial development may be a critical reason that caused the increase of heavy metal levels in local avian species.

Background Flight feathers, a type of feather that is unique to extant/extinct birds and some non-avian dinosaurs, are the most evolutionally advanced type of feather. In general, feather types are formed in the second or later generation of feathers at the first and following molting, and the first molting begins at around two weeks post hatching in chicken. However, it has been stated in some previous reports that the first molting from the natal down feathers to the flight feathers is much earlier than that for other feather types, suggesting that flight feather formation starts as an embryonic event. The aim of this study was to determine the inception of flight feather morphogenesis and to identify embryological processes specific to flight feathers in contrast to those of down feathers. Results We found that the second generation of feather that shows a flight feather-type arrangement has already started developing by chick embryonic day 18, deep in the skin of the flight feather-forming region. This was confirmed by shh gene expression that shows barb pattern, and the expression pattern revealed that the second generation of feather development in the flight feather-forming region seems to start by embryonic day 14. The first stage at which we detected a specific morphology of the feather bud in the flight feather-forming region was embryonic day 11, when internal invagination of the feather bud starts, while the external morphology of the feather bud is radial down-type. Conclusion The morphogenesis for the flight feather, the most advanced type of feather, has been drastically modified from the beginning of feather morphogenesis, suggesting that early modification of the embryonic morphogenetic process may have played a crucial role in the morphological evolution of this key innovation. Co-optation of molecular cues for axial morphogenesis in limb skeletal development may be able to modify morphogenesis of the feather bud, giving rise to flight feather-specific morphogenesis of traits.

Petrels, albatrosses, and storm-petrels are among the most beautiful yet least known of all the world's birds, living their lives at sea far from the sight of most people. Largely colored in shades of gray, black, and white, these enigmatic and fast-flying seabirds can be hard to differentiate, particularly from a moving boat. Useful worldwide, not just in North America, this photographic guide is based on unrivaled field experience and combines insightful text and hundreds of full-color images to help you identify these remarkable birds. The first book of its kind, this guide features an introduction that explains ocean habitats and the latest developments in taxonomy. Detailed species accounts describe key identification features such as flight manner, plumage variation related to age and molt, seasonal occurrence patterns, and migration routes. Species accounts are arranged into groups helpful for field identification, and an overview of unique identification challenges is provided for each group. The guide also includes distribution maps for regularly occurring species as well as a bibliography, glossary, and appendixes. The first state-of-the-art photographic guide to these enigmatic seabirdsIncludes hundreds of full-color photos throughoutFeatures detailed species accounts that describe flight, plumage, distribution, and moreProvides overviews of ocean habitats, taxonomy, and conservationOffers tips on how to observe and identify birds at sea

While researchers have made great progress investigating the molt of temperate-zone birds, few studies have examined the molt of tropical birds. We carried out this study in 2009 and 2010 in the cerrado biome, Distrito Federal, Brazil. On the basis of 334 birds captured with mist nets, we describe the timing, duration, and intensity of the flight-feather molt in eight species. Molt scores indicated the direction of replacement and points where molt series started and ended. The innermost primary usually was the first flight feather to drop, and primaries were replaced proximal to distal. Secondaries were replaced in two series, and S6 or S5 typically was the last remex to complete growth. Rectrices were replaced in a single series on each side, from R1 (central) to R6 (lateral). All species replaced their primaries according to the rules followed by most passerines. The White-eared Puffbird (Nystalus chacuru: Galbuliformes) was an exception; it replaced primaries in two molt series and possibly does not molt its secondaries completely every year. In comparison to similar temperate-zone birds, whose molts take 42–105 days, tropical birds seem to have a slower metabolism, with molt having an average duration of 122 days and intensity of 3.1 feathers growing simultaneously. Larger species required more time to molt. In four species we found overlap of molt and breeding.