Barb geometry of asymmetrical feathers reveals a transitional morphology in the evolution of avian flight

The observed looseness or reduced rigidity in the vanes of flightless taxa is primarily caused by degradation in the microstructure of their interlocking feather barbules.

“Similarities in primary feather barb geometry between flying and flightless birds support the conclusions of Livezey [31,34], that the observed 'looseness 0 or reduced rigidity in the vanes of flightless taxa is primarily caused by degradation in the microstructure of their interlocking feather barbules.”

• cites [31] 31. Livezey BC, Humphrey PS. 1986 Flightlessness in steamer-ducks (Anatidae: Tachyeres): its morphological bases and probable evolution. Evolution 40, 540. (doi:10.2307/2408576) sourceCould not verify

  The Adaptive Significance of Flightlessness section (page unavailable) describes the remiges of flightless steamer-ducks as shorter and stiffer. The passages found do not address vane looseness or barbule microstructure, so they cannot verify the claimed cause; searches for looseness, vanes, and barbules found no passage on that mechanism. (LLM, conf=0.72)

  Experimental — accuracy not yet measured
• cites [34] 34. Vazquez RJ. 1992 Functional osteology of the avian wrist and the evolution of flapping flight. J. Morphol. 211, 259-268. (doi:10.1002/jmor.1052110303) sourceNot checked — no full text of the source

Most secondarily flightless species in the survey still retain functionally asymmetric primaries.

“Congruent with the findings of several previous studies [30,31,34], most secondarily flightless species in our survey still retain functionally asymmetric primaries.”

• cites [30] 30. Livezey BC. 2003 Evolution of flightlessness in rails (Gruiformes, Rallidae). Ornithol. Monogr. 53, 1-654. sourceNot checked — no full text of the source
• cites [31] 31. Livezey BC, Humphrey PS. 1986 Flightlessness in steamer-ducks (Anatidae: Tachyeres): its morphological bases and probable evolution. Evolution 40, 540. (doi:10.2307/2408576) sourceCould not verify

  The ‘Morphology of Flightlessness in Tachyeres’ section (page unavailable) reports that flightless steamer-ducks retain the usual number of primary remiges, but it does not describe whether those feathers are asymmetric. Searches for primary-feather asymmetry, vane asymmetry, and symmetrical or asymmetrical flight feathers found no passage addressing that feature or the proportion of flightless species retaining it. (LLM, conf=0.73)

  Experimental — accuracy not yet measured
• cites [34] 34. Vazquez RJ. 1992 Functional osteology of the avian wrist and the evolution of flapping flight. J. Morphol. 211, 259-268. (doi:10.1002/jmor.1052110303) sourceNot checked — no full text of the source

The origin of the modern flight feather is coincident with the inferred origin of multiple flight-related features, such as the alula and an ossified sternal keel.

“The origin of the modern flight feather-characterized by small barb angles in cutting-edge vanes and larger barb angles in trailing vanes-is coincident with the inferred origin of multiple flight-related features, such as the alula and an ossified sternal keel [40,41].”

• cites [40] 40. Zheng X, Wang X, O'connor J, Zhou Z. 2012 Insight into the early evolution of the avian sternum from juvenile enantiornithines. Nat. Commun. 3, 1116. (doi:10.1038/ncomms2104) rspb.royalsocietypublishing.org Proc. R. Soc. B 282: 20142864 sourcePartly supported

  The introductory passage (page unavailable) places the appearance of a projecting sternal keel in Ornithothoraces, supporting the cited paper’s role as evidence for that flight-related feature. It does not address the origin of modern flight feathers or the alula, or establish that their origins coincided with the keel’s. (LLM, conf=0.83)

  Experimental — accuracy not yet measured
• cites [41] sourcePartly supported

  The introduction states that a projecting sternal keel appears in Ornithothoraces, supporting the claim’s keel component. The paper does not compare the keel’s inferred origin with the origin of modern flight feathers or the alula, so it does not establish the claimed coincidence. The supplied passages have section labels but no page numbers. (LLM, conf=0.82)

  Experimental — accuracy not yet measured

Barb geometry significantly differs among functionally distinct portions of flight feather vanes.

“We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes”

• AssumptionThe authors state this as an assumption: Flight feather vanes can be partitioned into four functional categories (cutting-edge leading, supported leading, free-edge trailing, supported trailing) based on hypothesized aerodynamic functions.

Cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds.

“This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds”

Cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles.

“cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles”

• Also stated in: § (a) Modern flying birds · plain

  “cutting-edge leading vanes were restricted to a distinct region of morphospace characterized by small barb angles (less than 248).”

• AssumptionThe authors state this as an assumption: Flight feather vanes can be partitioned into four functional categories (cutting-edge leading, supported leading, free-edge trailing, supported trailing) based on hypothesized aerodynamic functions.

Barb angle is not related to vane-width asymmetry.

“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.”

• Also stated in: § 4. Discussion (a) Modern flying birds · plain

  “Contrary to previous assumptions [2,17,19,20], variation in barb angle is not related to variation in vane width or vane-width asymmetry”

Barb length variation determines vane asymmetry.

“whereas barb length variation determines vane asymmetry”

• Also stated in: § 4. Discussion (a) Modern flying birds · boosted

  “Our survey of modern birds demonstrates that flight feather vane-width asymmetry is primarily determined by barb length.”

Barb angle varies with vane function.

“instead barb angle varies with vane function”

• Also stated in: § 4. Discussion (a) Modern flying birds · plain

  “instead varies with respect to the specific aerodynamic function of that portion of the feather vane”

In Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds.

“However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds.”

The primaries of most modern secondarily flightless birds are similar to those of modern flying birds in vane asymmetry and barb geometry.

“Our survey of modern, secondarily flightless birds found that the primaries of most flightless birds are similar to modern flying birds in terms of their vane asymmetry and barb geometry.”

Barb geometry throughout the remaining portions of the early Mesozoic flight feather and wing is quantitatively distinct from that of modern birds.

“Although the barb geometry of cutting-edge leading vanes in Mesozoic stem birds is essentially modern, barb geometry throughout the remaining portions of the early Mesozoic flight feather and wing are quantitatively distinct from those of modern birds.”

Barb angles were relatively small in the trailing vanes of early Mesozoic stem birds, unlike in modern birds.

“Unlike in modern birds, barb angles were relatively small in the trailing vanes of early Mesozoic stem birds (figure 4b).”

• Also stated in: § (c) Mesozoic stem birds · plain

  “Trailing vane barb angles of all other Mesozoic taxa (black circles) were relatively small compared with modern birds ( purple circles).”

Variation in vane width, and vane-width asymmetry, were primarily driven by barb length.

“Variation in vane width, and vane-width asymmetry, were instead primarily driven by barb length.”

Most flightless species retained the small leading vane barb angles and large trailing vane barb angles of their volant ancestors.

“Most flightless species retained the small leading vane barb angles and large trailing vane barb angles of their volant ancestors”

The outer hindwing feathers of the four-winged dromaeosaurid Microraptor were similar in vane structure to the primary feathers of two-winged Mesozoic taxa.

“our observations document that the outer hindwing feathers of the four-winged dromaeosaurid Microraptor were similar in vane structure to the primary feathers of two-winged Mesozoic taxa, corroborating previous interpretations of their aerodynamic function [33].”

• cites [33] 33. Livezey BC. 1992 Morphological corollaries and ecological implications of flightlessness in the kakapo (Psittaciformes: Strigops habroptilus). J. Morphol. 213, 105-145. (doi:10.1002/jmor.1052130108) sourceNot checked — no full text of the source

The elongated wing feathers of Mesozoic Paraves exhibit small barb angles in cutting-edge leading vanes that are comparable with those of modern flying birds.

“The elongated wing feathers of Mesozoic Paraves exhibit small barb angles in cutting-edge leading vanes that are comparable with those of modern flying birds (figure 4b).”

When classified by vane partition, free-edge trailing vanes, supported trailing vanes and supported leading vanes overlapped extensively in morphospace due to their similar ranges of large barb angles.

“When classified by vane partition, free-edge trailing vanes, supported trailing vanes and supported leading vanes overlapped extensively with each other in morphospace due to their similar ranges of large barb angles”

• AssumptionThe authors state this as an assumption: Flight feather vanes can be partitioned into four functional categories (cutting-edge leading, supported leading, free-edge trailing, supported trailing) based on hypothesized aerodynamic functions.

Within the flight feathers of modern birds, there is a similar range of barb geometries across different orders, different flight styles, different feather positions within the wing, and different positions along the length of feathers.

“Within the flight feathers of modern birds, we found a similar range of barb geometries across different orders, different flight styles, different feather positions within the wing, and different positions along the length of feathers”

Vane-width asymmetry was sufficient in all species to position the rachis within the 35% functional threshold for automatic pitch control, except for the flightless rail Megacrex inepta.

“Vane-width asymmetry was sufficient in all species to position the rachis within the 35% functional threshold for automatic pitch control, except for the flightless rail Megacrex inepta”

Supported leading vanes and trailing vanes have relatively large barb angles regardless of their vane width, or the degree of vane-width asymmetry.

“Supported leading vanes and trailing vanes have relatively large barb angles regardless of their vane width, or the degree of vane-width asymmetry.”

• AssumptionThe authors state this as an assumption: Flight feather vanes can be partitioned into four functional categories (cutting-edge leading, supported leading, free-edge trailing, supported trailing) based on hypothesized aerodynamic functions.

Barb angles are only small in leading vanes that function as the cutting edge of a feather or of an entire wing during flight, regardless of leading vane width.

“Barb angles are only small in leading vanes that function as the cutting edge of a feather or of an entire wing during flight, regardless of leading vane width.”

• AssumptionThe authors state this as an assumption: Flight feather vanes can be partitioned into four functional categories (cutting-edge leading, supported leading, free-edge trailing, supported trailing) based on hypothesized aerodynamic functions.

Variation in barb angle was largely independent of vane width or vane-width asymmetry.

“In contrast to previous assumptions, variation in barb angle was largely independent of vane width or vane-width asymmetry”

Leading and trailing vanes only partially overlapped in morphospace owing to relatively larger angles in the trailing vane, and relatively smaller angles in the leading vane.

“Conversely, leading and trailing vanes only partially overlapped in morphospace owing to relatively larger angles in the trailing vane, and relatively smaller angles in the leading vane”

There is no significant relationship between barb length and barb angle within the leading and trailing vanes of feathers among the entire sample of modern birds.

“Using phylogenetic regression we found no significant relationship between barb length and barb angle within the leading and trailing vanes of feathers among the entire sample of modern birds”

Free-edge and supported trailing vanes with similar vane widths did not significantly differ in their barb geometry.

“Conversely, freeedge and supported trailing vanes with similar vane widths did not significantly differ in their barb geometry”

• AssumptionThe authors state this as an assumption: Flight feather vanes can be partitioned into four functional categories (cutting-edge leading, supported leading, free-edge trailing, supported trailing) based on hypothesized aerodynamic functions.

Overall, the primaries of most flightless birds were indistinguishable from those of flying birds in terms of their vane asymmetry and barb geometry.

“Overall, the primaries of most flightless birds were indistinguishable from those of flying birds in terms of their vane asymmetry and barb geometry.”

The pattern of small barb angles in cutting-edge vanes holds across a phylogenetic and functionally diverse sample of modern flying birds.

“This pattern holds across a phylogenetic and functionally diverse sample of modern flying birds”

There is no significant correlation between barb angle asymmetry and vane-width asymmetry across most feathers.

“(b) There is no significant correlation between barb angle asymmetry and vane-width asymmetry across most feathers.”

Cutting-edge leading vanes are restricted to a distinct region of morphospace characterized by small barb angles.

“Cutting-edge leading vanes (red circles) are restricted to a distinct region of morphospace characterized by small barb angles.”

There is no significant relationship between barb angle and vane width across most feather vanes.

“(a) There is no significant relationship between barb angle and vane width across most feather vanes.”

Leading vane barb angles of flightless crown birds and Mesozoic stem birds fell within the range of flying cutting-edge vanes, except for the flightless M. inepta, Po. sandwichensis, X. lyalli and G. australis.

“(b) Leading vane barb angles of flightless crown birds (orange circles) and Mesozoic stem birds (white and grey shapes) fell within the range of flying cutting-edge vanes (red shading) except for the flightless M. inepta, Po. sandwichensis, X. lyalli and G. australis (orange circles outside of red shading).”

Barb length asymmetry significantly increases with increasing vane-width asymmetry across all feathers.

“(d ) Barb length asymmetry significantly increases with increasing vane-width asymmetry across all feathers.”

Barb length significantly increases with increasing vane width across all feather vanes.

“(c) Barb length significantly increases with increasing vane width across all feather vanes.”

Vane-width asymmetry was sufficient in all flight feathers of both stem and crown birds to position the rachis within the critical 35% limit for pitch control, except for the extant flightless rail M. inepta.

“(a) Vane-width asymmetry was sufficient in all flight feathers of both stem and crown birds to position the rachis within the critical 35% limit for pitch control (grey box), except for the extant flightless rail M. inepta.”

The asymmetrical primaries of Mesozoic taxa such as Microraptor and Archaeopteryx provide prima facie evidence for the evolution of some form of aerial theropod locomotion by the Jurassic.

“The asymmetrical primaries of Mesozoic taxa such as Microraptor and Archaeopteryx (collectively referred to here as stem birds) provide prima facie evidence for the evolution of some form of aerial theropod locomotion by the Jurassic [6].”

• cites [6] 6. Feduccia A, Tordoff H. 1979 Feathers of Archaeopteryx: asymmetric vanes indicate aerodynamic function. Science 203, 1021. (doi:10.1126/science.203.4384.1021) sourceNot checked — no full text of the source

In many birds, primary vane emarginations result in separated feather tips when the wing is spread, which reduces induced drag on the wing.

“In many birds, primary vane emarginations (i.e.sharp changes in the width of a vane along its length) result in separated feather tips when the wing is spread (figure 2a), which reduces induced drag on the wing [15,16].”

• cites [15] 15. Graham RR. 1932 Safety devices in wings of birds. J. R. Aeronaut. Soc. 36, 24 -58. sourceNot checked — no full text of the source
• cites [16] 16. Tucker V. 1995 Drag reduction by wing tip slots in a gliding Harris' hawk, Parabuteo unicinctus. J. Exp. Biol. 198, 775-781. sourceNot checked — no full text of the source

The cutting-edge vane morphology reveals a fundamental aerodynamic adaptation that has persisted from the Late Jurassic.

“revealing a fundamental aerodynamic adaptation that has persisted from the Late Jurassic.”

Positioning the rachis within 35% of the chord length behind the leading edge allows a flight feather to dynamically maintain feather stability, or pitch in airflow.

“Positioning the rachis within 35% of the chord length behind the leading edge allows a flight feather to dynamically maintain feather stability, or pitch in airflow [7,9].”

• cites [7] 7. Norberg RA ˚. 1985 Function of vane asymmetry and shaft curvature in bird flight feathers: inferences on flight ability of Archaeopteryx. In The beginnings of birds (eds MK Hecht, JH Ostrom, G Viohl, P Wellnhofer), pp. 303-318. Eichstatt, Germany: Freunde des Jura-Museums.Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites [9] 9. Norberg RA ˚. 1995 Feather asymmetry in Archaeopteryx. Nature 374, 221-222. sourceNot checked — no full text of the source

The evolutionary transition in flight feather morphology has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor.

“which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor.”

The discovery reveals a previously unrecognized evolutionary transition in flight feather morphology.

“This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology”

The separated tips of emarginated feathers act as independent aerofoils during flight.

“The separated tips of emarginated feathers act as independent aerofoils during flight [15,16].”

• cites [15] 15. Graham RR. 1932 Safety devices in wings of birds. J. R. Aeronaut. Soc. 36, 24 -58. sourceNot checked — no full text of the source
• cites [16] 16. Tucker V. 1995 Drag reduction by wing tip slots in a gliding Harris' hawk, Parabuteo unicinctus. J. Exp. Biol. 198, 775-781. sourceNot checked — no full text of the source

In some species, such as the steamer ducks (Tachyeres sp.) and the great auk (Pinguinus impennis), these barb geometries may reflect natural selection for a hydrodynamic function during wing-propelled swimming.

“In some species, such as the steamer ducks (Tachyeres sp.) and the great auk (Pinguinus impennis), these barb geometries may reflect natural selection for a hydrodynamic function during wing-propelled swimming [32].”

• cites [32] 32. Xu X, Zhou Z, Wang X, Kuang X, Zhang F, Du X. 2003 Four-winged dinosaurs from China. Nature 421, 335 -340. (doi:10.1038/ nature01342) sourceNot checked — no full text of the source

The developmental capacity for fine, differential control of barb angle expansion within a feather is integral to the acquisition of an aerodynamically modern flight feather.

“Nevertheless, the developmental capacity for fine, differential control of barb angle expansion within a feather is integral to the acquisition of an aerodynamically modern flight feather.”

The absence of large trailing vane barb angles in early Mesozoic stem birds suggests that they lacked the capacity to differentially control barb angle.

“The absence of large trailing vane barb angles in early Mesozoic stem birds suggests that they lacked the capacity to differentially control barb angle.”

Aerial locomotion in Archaeopteryx, Confuciusornis and Sapeornis may also have consisted primarily of passive gliding.

“The observation that this plesiomorphic morphology is shared with Archaeopteryx, Confuciusornis and Sapeornis, raises the prospect that aerial locomotion in these taxa also consisted primarily of passive gliding.”

The presence of vane asymmetry and cutting-edge barb geometry in Mesozoic wing feathers suggests that stem birds were capable of some form of aerial locomotion from at least the Late Jurassic.

“Since vane asymmetry and cutting-edge barb geometry would be advantageous features for any feathers that function as an aerofoil within airflow, the presence of these two characters in Mesozoic wing feathers suggest that stem birds were capable of some form of aerial locomotion from at least the Late Jurassic.”

Larger barb angles in the trailing vanes evolved subsequent to the origin of asymmetrical flight feathers and the origin of aerial locomotion.

“Larger barb angles in the trailing vanes evolved subsequent to the origin of asymmetrical flight feathers and the origin of aerial locomotion.”

None of the Mesozoic specimens exhibit any signs of taphonomic distortion that might influence barb measurements, such as evidence of unnatural overlapping or barb separation.

“However, none of the specimens exhibit any signs of taphonomic distortion that might influence barb measurements, such as evidence of unnatural overlapping or barb separation.”

The modern capacity to independently differentiate barb angles between the leading and trailing vanes of a feather evolved substantially after the origin of asymmetrical feathers with an aerodynamic function.

“Our comparative analysis of flight feather barb geometry in Mesozoic stem birds indicates that the modern capacity to independently differentiate barb angles between the leading and trailing vanes of a feather evolved substantially after the origin of asymmetrical feathers with an aerodynamic function.”

Vane asymmetry and cutting-edge barb geometry in themselves do not provide specific evidence of either passive gliding or active, powered flight in early stem birds.

“However, these two characteristics in themselves do not provide specific evidence of either passive gliding or active, powered flight in early stem birds.”

Enantiornithines shared derived aerodynamic similarities in their flight stroke with modern birds that earlier stem birds lacked.

“This finding implies that enantiornithines shared derived aerodynamic similarities in their flight stroke with modern birds that earlier stem birds lacked.”

Small barb angles in cutting-edge vanes, and the aerodynamic stability they confer, is a fundamental aerodynamic adaptation of avian flight.

“suggesting that small barb angles in cutting-edge vanes, and the aerodynamic stability they confer, is a fundamental aerodynamic adaptation of avian flight.”

The asymmetrical flight feathers of the early Mesozoic stem birds investigated represent a new transitional morphology exhibiting some, but not all, of the features of modern avian flight feathers.

“The asymmetrical flight feathers of the early Mesozoic stem birds investigated here represent a new transitional morphology exhibiting some, but not all, of the features of modern avian flight feathers.”

Fully modern asymmetrical flight feather morphology, combining large supported vane barb angles with small cutting-edge leading vane barb angles, evolved well after the origin of vane asymmetry in Paraves, at some point along the internode between the lineage leading to Confuciusornis and the most recent common ancestor of Enantiornithes and Ornithurae.

“Fully modern asymmetrical flight feather morphology, combining large supported vane barb angles with small cutting-edge leading vane barb angles, evolved well after the origin of vane asymmetry in Paraves, at some point along the internode between the lineage leading to Confuciusornis, and the most recent common ancestor of Enantiornithes and Ornithurae (figure 1).”

• Also stated in: § (c) Mesozoic stem birds · boosted

  “Our data demonstrate that the entirely modern flight feather morphology (with small cutting-edge and large trailing vane barb angles) evolved along the internode separating Confuciusornis and the Enantiornithes þ Ornithurae clade (figure 1).”

Smaller barb angles increase a vane's rigidity and response to the aerodynamic forces experienced in flight.

“However, smaller barb angles also increase a vane's rigidity and response to the aerodynamic forces experienced in flight [2,19].”

• cites [2] 2. Lucas AM, Stettenheim PR. 1972 Avian anatomy: integument. Agricultural handbook 362. Washington, DC: US Dept. Agriculture.Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites [19] 19. Ennos A, Hickson J, Roberts A. 1995 Functional morphology of the vanes of the flight feathers of the pigeon Columba livia. J. Exp. Biol. 198, 1219. sourceNot checked — no full text of the source

Larger barb angles help increase the flexibility of a vane.

“Larger barb angles help increase the flexibility of a vane [19]”

• cites [19] 19. Ennos A, Hickson J, Roberts A. 1995 Functional morphology of the vanes of the flight feathers of the pigeon Columba livia. J. Exp. Biol. 198, 1219. sourceNot checked — no full text of the source

Mesozoic paravians stemward of Enantiornithes may have been incapable of fully modern high-powered flight.

“our data are congruent with the idea that Mesozoic paravians stemward of Enantiornithes may have been incapable of fully modern high-powered flight [12,[35][36][37][38].”

• cites [12,[35][36][37][38]Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Friction barbules on the obverse surface of trailing vanes mechanically catch the reverse surface of overlapping leading vanes to prevent separation of the feathers during the flight stroke.

“Friction barbules on the obverse surface of trailing vanes mechanically 'catch 0 the reverse surface of overlapping leading vanes to prevent separation of the feathers during the flight stroke [15].”

• cites [15] 15. Graham RR. 1932 Safety devices in wings of birds. J. R. Aeronaut. Soc. 36, 24 -58. sourceNot checked — no full text of the source

The developmental capacity to independently vary barb angle and barb length allows for a theoretical vane-width morphospace that is highly redundant.

“The developmental capacity to independently vary barb angle and barb length allows for a theoretical vane-width morphospace that is highly redundant [17,18]”

• cites [17] 17. Feo TJ, Prum RO. 2014 Theoretical morphology and development of flight feather vane asymmetry with experimental tests in parrots. J. Exp. Zool. (Mol. Dev. Evol.) 322, 240 -255. (doi:10.1002/jez.b.22573) sourceNot checked — no full text of the source
• cites [18] 18. Prum RO, Williamson S. 2001 Theory of the growth and evolution of feather shape. J. Exp. Zool. (Mol. Dev. Evol.) 291, 30 -57. (doi:10.1002/jez.4) sourceNot checked — no full text of the source

Some form of aerial locomotion was plesiomorphic for the most exclusive clade including Microraptor and modern birds.

“The presence of small cutting-edge barb angles, in conjunction with sufficient vane asymmetry for feather pitch stability, support the conclusion that some form of aerial locomotion was plesiomorphic for the most exclusive clade including Microraptor and modern birds (figure 1) [33].”

• cites [33] 33. Livezey BC. 1992 Morphological corollaries and ecological implications of flightlessness in the kakapo (Psittaciformes: Strigops habroptilus). J. Morphol. 213, 105-145. (doi:10.1002/jmor.1052130108) sourceNot checked — no full text of the source

In some taxa, highly asymmetrical primaries separate during the upstroke owing to in-follicle feather rotation, allowing leading vanes that overlap neighbouring feathers in a static spread wing to function as a cutting edge during the dynamic flight stroke.

“In some taxa, highly asymmetrical primaries are known to separate during the upstroke owing to 'in-follicle' feather rotation, allowing leading vanes that overlap neighbouring feathers in a static spread wing to function as a cutting edge during the dynamic flight stroke [7].”

• cites [7] 7. Norberg RA ˚. 1985 Function of vane asymmetry and shaft curvature in bird flight feathers: inferences on flight ability of Archaeopteryx. In The beginnings of birds (eds MK Hecht, JH Ostrom, G Viohl, P Wellnhofer), pp. 303-318. Eichstatt, Germany: Freunde des Jura-Museums.Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

The plesiomorphic Mesozoic flight feather morphology on the hindlimbs of Microraptor was functionally associated with passive gliding, at least in that taxon.

“The presence of the plesiomorphic Mesozoic flight feather morphology (small cutting-edge and small trailling vane barb angles) on the hindlimbs of Microraptor indicates that this feather morphology was functionally associated with passive gliding, at least in that taxon, because it is unlikely that Microraptor 'hindwings 0 functioned with a flapping flight stroke [33].”

• cites [33] 33. Livezey BC. 1992 Morphological corollaries and ecological implications of flightlessness in the kakapo (Psittaciformes: Strigops habroptilus). J. Morphol. 213, 105-145. (doi:10.1002/jmor.1052130108) sourceNot checked — no full text of the source

Barb length and barb angle vary independently of each other.

“indicating that these two barb characters do vary independently of each other”

The capacity of a cutting-edge vane to withstand aerodynamic forces as it cuts into the air is independent of the aerodynamic functions determined by the shape of the feather as a whole.

“In the case of flight feathers, the capacity of a cutting-edge vane to withstand aerodynamic forces as it cuts into the air is independent of the aerodynamic functions determined by the shape of the feather as a whole.”

The gradual, stepwise evolution of modern avian flight feathers parallels a broader, protracted period of flight apparatus refinement along the avian stem.

“Thus, the gradual, stepwise evolution of modern avian flight feathers parallels a broader, protracted period of flight apparatus refinement along the avian stem [1,39].”

• cites [1] 1. Makovicky PJ, Zanno LE. 2011 Theropod diversity and the refinement of avian characteristics. In Living dinosaurs: the evolutionary history of modern birds (eds G Dyke, GW Kaiser), pp. 10 -29. Oxford, UK: John Wiley & Sons, Ltd. sourceNot checked — no full text of the source
• cites [39] 39. Sanz JL, Chiappe LM, PerezMoreno BP, Buscalioni AD, Moratalla JJ, Ortega F, PoyatoAriza FJ. 1996 An Early Cretaceous bird from Spain and its implications for the evolution of avian flight. Nature 382, 442-445. (doi:10.1038/382442a0) sourceNot checked — no full text of the source

These features arose subsequent to the origin of other hypothesized flight-related specializations such as the elongation of the coracoid, the acquisition of a fused pygostyle and the initial acquisition of asymmetrical flight feathers.

“These features arose subsequent to the origin of other hypothesized flight-related specializations such as the elongation of the coracoid, the acquisition of a fused pygostyle and the initial acquisition of asymmetrical flight feathers themselves [4].”

• cites [4] 4. Turner AH, Makovicky PJ, Norell MA. 2012 A review of dromaeosaurid systematics and paravian phylogeny. Bull. Am. Mus. Nat. Hist. 371, 1-206. (doi:10.1206/748.1) sourceNot checked — no full text of the source