Morphological properties of the last primaries, the tail feathers, and the alulae of Accipiter nisus, Columba livia, Falco peregrinus, and Falco tinnunculus
Abstract
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.
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Written from the paper’s full text.
TL;DR
This study compares morphological and mechanical properties of the tenth primary, outermost alula, and middle tail feathers in peregrine falcons, kestrels, sparrowhawks, and pigeons. It finds broadly similar mass-specific mechanics across species, with notable exceptions: peregrine tail feathers are especially stiff at the base, kestrel primary 10 is stiffest when normalized to body mass, and sparrowhawk alulae are relatively stiff.
Key findings
- Across all four species, feather-rachis E-moduli ranged from 5.9 to 8.4 GPa in the abstract; results text reports section means within 6.5–8.7 GPa and states values were highest for *C. livia* in all feather types.
- *F. peregrinus* feather shafts had the largest cross-sections and highest specific bending stiffness overall; its tail feathers were much stiffer at the base than those of the other three species (abstract range: 3.3–12.6 Nmm g⁻¹; results report section one, dorso-ventral, mean 16.12 Nmm g⁻¹).
- When normalized to body mass, primary 10 specific bending stiffness was highest in *F. tinnunculus* (16 Nmm g⁻¹), while the alula was highest in *A. nisus*; *F. peregrinus* tail feathers remained especially stiff, particularly at the base and in dorso-ventral bending.
- Barb separation forces were generally larger in *F. peregrinus* than in the other species, except in the alula, where separation forces were highest in *A. nisus* at 25% and 50% of the outer vane.
Methods
- Feathers from adult males and females of four species were studied: *F. peregrinus* (N=4), *F. tinnunculus* (N=4), *A. nisus* (N=4), and *C. livia* (N=4), obtained as frozen carcasses. The tenth primary, outermost alula, and middle tail feather were removed; primary 1 and 5 were also investigated in *F. peregrinus*.
- Morphology included barb counts per cm rachis and barb–rachis angles. Mechanical measurements used a force gauge (Sauter FK10, resolution 0.001 N) for barb separation, nanoindentation (Hysitron Triboscope D3100; Berkovich tip; 2500 µN load; ~500 nm contact depth; 24 indents per feather cortex) for Young’s modulus, cross-sections and point counting for second moment of area, and a balance (Mettler Toledo XA 105) plus calculated E×I/w for specific bending stiffness.
- Statistics used means and standard deviations, t-tests, and one-way ANOVA with Bonferroni post-hoc tests in SPSS v22, with significance at p<0.01.
Limitations
- The paper states that barb separation forces were measured perpendicular to the feather surface, whereas forces during flight are probably rarely perpendicular; the method was still considered useful for species comparisons.
- The paper states that balance-measured specific bending stiffness was always higher than calculated values, likely because the medulla was neglected even though it can increase bending stiffness by 7–16%, and because the equation was derived for paperboard. Nanoindentation E-values were higher than most literature values, possibly due to keratin anisotropy and water content; samples were air-dried, though the authors argue this is justified because fully developed feathers are no longer hydrated by blood vessels.
- The paper states that some measurements were impossible: alulae and primary 10 of *A. nisus* were too short to attach for bending-stiffness measurements, and outer vanes of primary 10 were too narrow for force measurements except near the rachis. It also notes that tail-feather homology is uncertain because tail-feather number differs between birds, and that aerodynamic measurements are needed to verify functional interpretations. Evident from the text: specific bending stiffness was determined from only one specimen per species, and the functional links to flight behavior are largely inferential.
Model-written summary. Check it against the paper before relying on it.
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Claims the paper makes
0 of 34 cited sources checked · 0 supported · 0 flagged
- Findingown data§ City St George's, University of London · p. 3
The feather shafts of F. peregrinus had the largest cross-sections.
“The feather shafts of F. peregrinus had the largest cross-sections and the highest specific bending stiffness.”
Quote in the paperSubstantial claim - Findingown data§ City St George's, University of London · p. 3
The feather shafts of F. peregrinus had the highest specific bending stiffness.
“The feather shafts of F. peregrinus had the largest cross-sections and the highest specific bending stiffness.”
Quote in the paperSubstantial claim - Findingown data§ City St George's, University of London · p. 3
In all four species, the E-moduli of the feathers ranged from 5.9 to 8.4 GPa.
“In all four species the E-moduli of the feathers ranged from 5.9 to 8.4 GPa.”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 13
In primaries 1, 5 and 10 of F. peregrinus, the calculated specific bending stiffness was similar for the transverse bending direction in all sections.
“In primaries 1, 5 and 10 of F. peregrinus the calculated specific bending stiffness was similar for the transverse bending direction in all sections.”
Quote in the paperNot a substantial claim - Findingown data§ Vane morphology · p. 12
Primary 1 and 5 of F. pereginus contained 17-22 barbs per cm.
“Primary 1 and 5 of F. pereginus contained 17-22 barbs per cm.”
Quote in the paperNot a substantial claim
- Findingown data§ Nanoindentation · p. 13
The E-values of primaries 1, 5 and 10 of F. peregrinus were comparable.
“The E-values of primaries 1, 5 and 10 of F. peregrinus were comparable.”
Quote in the paperNot a substantial claim - Findingown data§ Nanoindentation · p. 13
Evaluation of the mean E-values of the single sections of primaries 10, the tail feathers and the Alulae revealed differences for about half of the spots.
“Evaluation of the mean E-values of the single sections of primaries 10, the tail feathers and the Alulae revealed differences for about half of the spots (inserted tables in Fig.7).”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 15
The cross-sections of the investigated feathers revealed almost inhomogeneous profiles.
“The cross-sections of the investigated feathers (Figs. 4-6) revealed allmost inhomogeneous profiles.”
Quote in the paperNot a substantial claim - Findingown data§ City St George's, University of London · p. 3
When normalized with respect to body mass, the specific bending stiffness of primary 10 was highest in F. tinnunculus (16 Nmm g-1).
“When normalized with respect to body mass, the specific bending stiffness of primary 10 was highest in F. tinnunculus (16 Nmm g -1 ) , while that of the the Alula was highest in A. nisus.”
Quote in the paperSubstantial claim - Findingown data§ Specific bending stiffness · p. 14
For the second moment of area divided by sample width and body mass, tail feather values were higher in F. peregrinus at most spots.
“Tail feather values were higher in the peregrine at most spots.”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 14
Primary 10 of F. tinnunculus, when bent in a transverse or dorso-ventral direction, had a higher specific bending stiffness than the respective feathers of all other species.
“Primary 10 of F. tinnunculus, if bend in a transverse or dorso-ventral direction, and the Alula of A. nisus had a higher specific bending stiffness than the respective feathers of all other species (Fig. 13).”
Quote in the paperSubstantial claim - Findingown data§ Nanoindentation · p. 13
Values of mean E-values of single sections ranged within 6.5 - 8.7 GPa.
“Values ranged within 6.5 - 8.7 GPa.”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 14
For the second moment of area divided by sample width and body mass, in the Alula, feathers of F. tinnunculus and A. nisus most often had higher values than the peregrines.
“In the Alula, however, feathers of F. tinnunculus and A. nisus most often had higher values than the peregrines.”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 14
The specific bending stiffness per body mass of section one of the tail feathers of F. peregrinus in the dorso-ventral bending direction was much higher (mean 16.12 Nmm g -1 ) than the values obtained from all other sections.
“Calculating the specific bending stiffness per body mass (Fig. 13) revealed that the values obtained from section one of the tail feathers of F. peregrinus (bending direction dorso-ventral) were much higher (mean 16.12 Nmm g -1 ) than the values obtained from all other sections.”
Quote in the paperNot a substantial claim - Findingown data§ Second moment of area · p. 11
Feather lengths did not correlate with body mass.
“Note that feather lengths did not correlate with body mass (Table 1).”
Quote in the paperNot a substantial claim - Findingown data§ Vane structural integrity · p. 12
For most measuring points, the forces required to separate the barbs of primaries 10, the tail feathers and the Alulae were significantly larger in F. peregrinus than in the other species.
“For most measuring points these forces were significantly larger in F. peregrinus than in the other species.”
Quote in the paperSubstantial claim - Findingown data§ Specific bending stiffness · p. 13
If the bending direction was dorso-ventral, the specific bending stiffness of primary 5 was higher than the specific bending stiffness of primary 1 and 10.
“If the bending direction was dorso-ventral, the specific bending stiffness of primary 5 was higher than the specific bending stiffness of primary 1 and 10 (Fig. 9).”
Quote in the paperNot a substantial claim - Findingown data§ Nanoindentation · p. 13
In all species, the E-values of the primaries 10, the Alulae and the tail feathers slightly decreased from base to tip.
“In all species the E-values of the primaries 10, the Alulae and the tail feathers slightly decreased from base to tip (regression analysis).”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 15
The inhomogeneous cross-section profiles were most conspicuous in primary 10 of F. peregrinus and C. livia.
“This was most conspicuous in primary 10 of F. peregrinus and C. livia (Fig. 4).”
Quote in the paperNot a substantial claim - Findingown data§ City St George's, University of London · p. 3
When normalized with respect to body mass, the specific bending stiffness of the Alula was highest in A. nisus.
“When normalized with respect to body mass, the specific bending stiffness of primary 10 was highest in F. tinnunculus (16 Nmm g -1 ) , while that of the the Alula was highest in A. nisus.”
Quote in the paperNot a substantial claim - Findingown data§ Vane morphology · p. 12
The feathers of C. livia had the highest number of barbs per cm rachis length.
“The feathers of C. livia had the highest number of barbs per cm rachis length (Fig. 2).”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 14
For the second moment of area divided by sample width and body mass, the rachises of primary 10 of F. peregrinus had higher or lower values than the rachises of primary 10 of all other species.
“The rachises of primary 10 of F. peregrinus had higher or lower values than the rachisis of primary 10 of all other species.”
Quote in the paperNot a substantial claim - Findingown data§ Vane morphology · p. 12
Angles between barbs and rachis differed least between species in the tail feathers.
“Angles were larger for the inner vanes and differed least between species in the tail feathers.”
Quote in the paperNot a substantial claim - Findingown data§ Nanoindentation · p. 13
If averaged across all sections, the E-values of primary 10 (6.85-7.2 GPa) of the four species investigated were not significantly different.
“If averaged across all sections, the E-values of primary 10 (6.85-7.2 GPa) of the four species investigated were not significantly different (t-test, p=0.06 and 0.07) (Fig. 7).”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 14
When plotting specific bending stiffness with respect to average body mass, the specific bending stiffness of the peregrine feathers, with the exception of the tail feathers, no longer surpassed the specific bending stiffness of the feathers of the other species.
“However, plotting the specific bending stiffness with respect to average body mass (see also Worcester 1996) revealed, that the specific bending stiffness of the peregrine feathers, with the exception of the tail feathers (Fig. 11), no longer surpassed the specific bending stiffness of the feathers of the other species.”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Worcester 1996
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- Findingown data§ Vane structural integrity · p. 12
In the Alula, the separation forces were highest in A. nisus at the measuring points 25% and 50% of the outer vane.
“An exception is the Alula: in these feathers the separation forces were highest in A. nisus at the measuring points 25% and 50% of the outer vane.”
Quote in the paperNot a substantial claim - Findingown data§ Vane morphology · p. 12
Angles between barbs and rachis were larger for the inner vanes.
“Angles were larger for the inner vanes and differed least between species in the tail feathers.”
Quote in the paperNot a substantial claim - Findingown data§ Nanoindentation · p. 13
In all feather types, E-values were highest for C. livia.
“In all feather types values were highest for C. livia .”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 13
If the bending direction was lateral, the specific bending stiffness of primary 10 exceeded that of primary 1 and 5.
“If the bending direction was lateral, however, the specific bending stiffness of primary 10 exceeded that of primary 1 and 5.”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 13
In all feathers, the specific bending stiffness decreased from base to tip.
“Furthermore, in all feathers the specific bending stiffness decreased from base to tip (Fig. 8).”
Quote in the paperNot a substantial claim - Findingown data§ Vane morphology · p. 12
In all species, the angles between barbs and rachis decreased from base (30° up to 88°) towards tip (15-30°).
“In all species the angles between barbs and rachis decreased from base (30° up to 88°) towards tip (15-30°).”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 14
The Alula of A. nisus had a higher specific bending stiffness than the respective feathers of all other species.
“Primary 10 of F. tinnunculus, if bend in a transverse or dorso-ventral direction, and the Alula of A. nisus had a higher specific bending stiffness than the respective feathers of all other species (Fig. 13).”
- Also stated in: § Discussion · p. 15 · plain
“In A.nisus the Alulae were more stable than in the other species.”
Quote in the paperSubstantial claim - Also stated in: § Discussion · p. 15 · plain
- Findingown data§ City St George's, University of London · p. 3
The specific bending stiffness measured at the base of the tail feathers was much higher in F. peregrinus than in the other three species (3.3-12.6 Nmm g-1).
“In comparison, , the specific bending stiffness, measured at the base of the tail feathers, was much higher in F. peregrinus than in the other three species (3.3-12.6 Nmm g -1 ).”
Quote in the paperSubstantial claim - Findingown data§ Specific bending stiffness · p. 14
When comparing the specific bending stiffness of primaries 10, the tail feathers and the Alulae, F. peregrinus had the most stiff feathers.
“By comparing the specific bending stiffness of primaries 10, the tail feathers and the Alulae (Fig. 10) it became apparent that F. peregrinus had the most stiff feathers.”
Quote in the paperSubstantial claim - Findingnone given§ Discussion · p. 16
The two vanes of each feather had different mechanical properties.
“Moreover, the two vanes of each feather had different mechanical properties; most likely, this is an adaptation to specific aerodynamic loads.”
Quote in the paperSubstantial claim - Findingnone given§ Discussion · p. 16
In terms of barb numbers, the vanes of the feathers of F. peregrinus did not differ from the other species investigated.
“In terms of barb numbers the vanes of the feathers of F. peregrinus did not differ from the other species investigated.”
Quote in the paperSubstantial claim - Findingown data§ Discussion · p. 17
In F. peregrinus, the inner vanes had a higher mechanical robustness than the inner vanes of F. tinnunculus and A. nisus.
“In F.peregrinus the inner vanes had a higher mechanical robustness than the inner vanes of F.tinnunculus and A.nisus .”
Quote in the paperSubstantial claim - Findingreasoning§ Discussion · p. 15
The specific bending stiffness of primaries was highest in F. tinnunculus.
“The specific bending stiffness of primaries was highest in F. tinnunculus.”
Quote in the paperSubstantial claim - Findingown data§ Specific bending stiffness · p. 15
Sections 2 to 5 of the investigated feathers showed pronounced protruding processes.
“Sections 2 to 5 showed pronounced protruding processes.”
Quote in the paperNot a substantial claim - Findingown data§ Discussion · p. 15
At their base, the tail feathers of F. peregrinus were significantly more stiff than the tail feathers of the other three species investigated.
“For instance, at their base the tail feathers of F. peregrinus were significantly more stiff than the tail feathers of the other three species investigated.”
Quote in the paperSubstantial claim - Findingnone given§ Discussion · p. 18
The E-value of C. livia was higher than the E-values of the other three species.
“E.g. since the E-value of C. livia was higher than the E-values of the other three species.”
Quote in the paperNot a substantial claim - Findingnone given§ Discussion · p. 16
At the most distal part of the feathers, differences in their mechanical properties decreased.
“In general, at the most distal part of the feathers differences in their mechanical properties decreased.”
Quote in the paperNot a substantial claim - Findingnone given§ Discussion · p. 17
The vanes of the tail feathers of F. peregrinus were the most robust among the four species.
“However, the vanes of the tail feathers of F. peregrinus were the most robust among the four species.”
Quote in the paperSubstantial claim - Findingnone given§ Discussion · p. 17
The structural integrity of the outer vane of primary 10 of F. peregrinus was comparable to that of C. livia.
“The structural integrity of the outer vane of primary 10 of F. peregrinus was comparable to that of C. livia .”
Quote in the paperNot a substantial claim - Findingreasoning§ Discussion · p. 16
The measured Young's modulus based on nanoindentation experiments for the different species was higher than most of the E-values documented in literature so far.
“The measured Young's modulus based on nanoindentation experiments for the different species was higher than most of the E-values documented in literature so far (see below).”
Quote in the paperNot a substantial claim - Findingown data§ Specific bending stiffness · p. 15
The alulae of F. peregrinus and A. nisus were inhomogeneous, particularly in the middle of the feather shafts.
“The alulae of F. peregrinus and A. nisus were also inhomogeneous and this was particularly evident in the middle of the feather shafts (Fig. 6).”
Quote in the paperNot a substantial claim - Findingreasoning§ Discussion · p. 17
Differences in robustness between F. peregrinus and the other species were especially pronounced close to the rachis.
“Differences in robustness between F.peregrinus and the other species were especially pronounced close to the rachis.”
Quote in the paperSubstantial claim - Findingreasoning§ Discussion · p. 16
The specific bending stiffness obtained with the balance was always higher than the calculated bending stiffness.
“The specific bending stiffness obtained with the balance was always higher than the calculated bending stiffness.”
Quote in the paperNot a substantial claim - Findingnone given§ Discussion · p. 18
For all feathers, the measured E-values of the feather rachis were higher than the values documented in literature so far.
“For all feathers the measured E-values of the feather rachis was higher than the values documented in literature so far.”
Quote in the paperNot a substantial claim - Findingown data§ Discussion · p. 19
The tail feathers of F. peregrinus had a higher mass-specific bending stiffness than the tail feathers of the other three species, especially at the base and in the dorso-ventral bending direction.
“The tail feathers of F. peregrinus had a higher massspecific bending stiffness than the tail feathers of the other three species, especially at the base and in the dorso-ventral bending direction (natural bending direction in flight!).”
Quote in the paperSubstantial claim - Findingcitation§ Discussion · p. 18
The second moment of area values for C. livia in this study (0.00004-2.33 mm 4, 480 g) are comparable to the published values for a 300 g C. livia.
“This is comparable to our values for C. livia (0.00004-2.33 mm 4 , 480 g).”
Quote in the paperNot a substantial claim - Premisecitation§ City St George's, University of London · p. 3
During the stoop, peregrine falcons (Falco peregrinus) can reach velocities of up to 320 km h-1.
“During the stoop of peregrine falcons ( Falco peregrinus ) they can reach velocities of up to 320 km h -1 .”
- Also stated in: § City St George's, University of London · p. 4 · plain
“A nose-diving peregrine falcon ( Falco peregrinus ) can reach in its stoop velocities of up to 320 km/h (Tucker and Parrott, 1970; Orton, 1975; Tucker, 1990; Savage, 1992; Clark, 1995; Tucker 1998; Franklin, 1999).”
Quote in the paperSubstantial claim0 of 3 sources checked- cites Tucker and Parrott, 1970
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- cites Orton, 1975
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- cites Tucker, 1990 source
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- Also stated in: § City St George's, University of London · p. 4 · plain
- Interpretationcitation§ Discussion · p. 16
The difference between the two vanes of each feather is most likely an adaptation to specific aerodynamic loads.
“Moreover, the two vanes of each feather had different mechanical properties; most likely, this is an adaptation to specific aerodynamic loads.”
Quote in the paperSubstantial claim - Premisecitation§ Discussion · p. 17
In F. peregrinus, the Alulae are not particularly exposed to the high velocities at the end of a dive.
“In F. peregrinus , the Alulae are not particularly exposed to the high velocities at the end of a dive (Ponitz et al., 2014).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Ponitz et al., 2014
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- Interpretationreasoning§ Discussion · p. 15
While hovering, the primaries of F. tinnunculus are probably exposed to higher mechanical loads than in steady flight conditions.
“This falcon uses the primaries for hovering, i.e. while hovering the primaries are probably exposed to higher mechanical loads than in steady flight conditions.”
Quote in the paperNot a substantial claim- Rests on an assumption: F. tinnunculus uses its primaries for hovering.
- Interpretationcitation§ Discussion · p. 17
The lack of exposure of F. peregrinus Alulae to high velocities at the end of a dive could explain why the Alulae of F. peregrinus are not more robust than the Alulae of the other species.
“This could explain why the Alulae of F. peregrinus are not more robust than the Alulae of the other species.”
Quote in the paperSubstantial claim - Interpretationcitation§ Discussion · p. 16
The higher measured Young's modulus could be due to the anisotropy of the feather keratin, which is more aligned in the middle of the rachis and less aligned at the calamus and at the tip of the rachis.
“This could be due to the anisotropy of the feather keratin since it is more aligned in the middle of the rachis and less aligned at the calamus and at the tip of the rachis (Cameron et al., 2003).”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Cameron et al., 2003
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- Premisecitation§ Discussion · p. 16
The middle and distal part of each feather resist out-of-plane forces better than the proximal part, which will be covered during flight by neighbouring feathers.
“In addition the middle and distal part of each feather resist out-of-plane forces better than the proximal part, which will be covered during flight by neighbouring feathers (Ennos et al. 1995).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Ennos et al. 1995 source
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- Interpretationreasoning§ Discussion · p. 16
The higher integrity of the peregrine feathers (with the exception of the Alulae) is most likely due to the anchoring of hooks and bows of the vanes.
“Therefore, the higher integrity of the peregrine feathers (with the exception of the Alulae) is most likely due to the anchoring of hooks and bows of the vanes.”
Quote in the paperSubstantial claim - Interpretationcitation§ Discussion · p. 16
Differences in keratin alignment may result in variations in stiffness along the rachis.
“These differences in alignment may result in variations in the stiffness along the rachis.”
Quote in the paperNot a substantial claim - Interpretationcitation§ Discussion · p. 16
Neglecting the medulla in the calculations most likely caused the balance-derived specific bending stiffness to be higher than the calculated values.
“This was most likely due to the fact that we neglected the medulla in our calculations, even though the medulla is known to increase the bending stiffness of vanes by 7-16% (Purslow and Vincent, 1978; Bonser, 1996).”
Quote in the paperSubstantial claim0 of 2 sources checked- cites Purslow and Vincent, 1978
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- cites Bonser, 1996
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- Interpretationown data§ Discussion · p. 15
The mechanical properties of the primaries 10, the tail feathers and the Alulae of the four species investigated were strikingly similar when related to individual body mass.
“According to the present study the mechanical properties of the primaries 10, the tail feathers and the Alulae of the four species investigated were strikingly similar if related to the individual body mass.”
Quote in the paperSubstantial claim - Interpretationcitation§ Discussion · p. 18
The shape of the cross-section is likely to have a larger influence on feather stability than the material properties of the keratin.
“Since the flexural stiffness of the rachis depends on its cross-sectional geometry to a much greater degree than the E-value (Bonser, 1996), the shape of the cross-section is likely to have a larger influence on feather stability than the material properties of the keratin (Purslow and Vincent, 1978; Corning and Biewener, 1998; Bonser and Purslow, 1995; Bachmann et al., 2012).”
Quote in the paperSubstantial claim0 of 3 sources checked- cites Bonser, 1996
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- cites Purslow and Vincent, 1978
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- cites Corning and Biewener, 1998
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- Premisecitation§ Discussion · p. 18
Fully developed feathers are no longer hydrated by blood vessels.
“Fully developed feathers are, however, no longer hydrated by blood vessels (Busching, 1997).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Busching, 1997
Not checked in the free run.
- Interpretationreasoning§ Discussion · p. 18
The use of air-dried feathers seems justified.
“Therefore the use of air-dried feathers seems justified.”
Quote in the paperSubstantial claim - Interpretationown data§ Discussion · p. 19
The higher mass-specific bending stiffness of F. peregrinus tail feathers helps peregrine falcons to cope with the large aerodynamic forces that develop while pulling out of a dive.
“We suggest that this helps peregrine falcons to cope with the large aerodynamic forces that develop while pulling out of a dive.”
Quote in the paperSubstantial claim - Interpretationreasoning§ Discussion · p. 19
The resistance of the rachis to bending should be greater in heavier birds.
“This in turn implies that the resistance of the rachis to bending should be greater in heavier birds (Purslow and Vincent, 1978).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Purslow and Vincent, 1978
Not checked in the free run.
- Rests on an assumption: Heavier birds must generate larger forces than smaller birds to keep airborne.
- Interpretationreasoning§ Conclusions · p. 19
The tested feathers show differences that can be correlated to the life style of a bird.
“The tested feathers show differences that can be correlated to the life style of a bird.”
Quote in the paperSubstantial claim - Premisecitation§ Cross-sections · p. 19
The size and shape of the cross-sections most likely determine the specific bending stiffness of a feather.
“The size and shape of the cross-sections most likely determine the specific bending stiffness of a feather (Bachmann et al. 2012; Purslow and Vincent 1978).”
Quote in the paperSubstantial claim0 of 2 sources checked- cites Bachmann et al. 2012
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- cites Purslow and Vincent 1978
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- Interpretationcitation§ Cross-sections · p. 19
Differences in flexural stiffness originate most likely from differences in the shape of the cross-sections rather than differences in the material properties.
“Differences in flexural stiffness originate most likely from differences in the shape of the cross-sections rather than differences in the material properties (e.g. varying the cross-link density of the keratin) (Purslow and Vincent, 1978).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Purslow and Vincent, 1978
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- May combine two claims. 13
- Classified as probably not a substantial claim. 15
- The quote only partly states this claim. 1
May combine two claims.
Findingown data§ Nanoindentation · p. 13Declination of E-values was 0.112-0.26 in primaries 10, 0.079-0.21 in Alulae and 0.035-0.08 in the tail feathers.
“Declination was 0.112-0.26 in primaries 10, 0.079-0.21 in Alulae and 0.035-0.08 in the tail feathers (Fig. 7).”
Quote in the paperNot a substantial claimProbably not a substantial claim: may not pass S3 (load-bearing).
Findingown data§ Specific bending stiffness · p. 13Measured values of specific bending stiffness were always slightly higher than the calculated values.
“Measured values were always slightly higher than the calculated values (Fig. 8).”
Quote in the paperNot a substantial claimMay combine two claims.
Findingown data§ Specific bending stiffness · p. 14The primary 10 of F. tinnunculus and the Alula of A. nisus were especially stable in the first 3 to 5 sections.
“These feathers were especially stable in the first 3 to 5 sections.”
Quote in the paperNot a substantial claimMay combine two claims.
Findingown data§ Vane morphology · p. 12In all feathers and species, the number of barbs decreased from the base of the rachis towards the tip and increased again slightly for the most proximal 20% of the rachis.
“In all feathers and species, respectively, the number of barbs decreased from the base of the rachis towards the tip and increased again slightly for the most proximal 20% of the rachis.”
Quote in the paperNot a substantial claimMay combine two claims.
Findingown data§ Specific bending stiffness · p. 15The cross sections of the tail feathers were more homogeneous in all species, but the basic section in F. peregrinus differed from the other species in its oval structure.
“In contrast, the cross sections of the tail feathers were more homogeneous in all species but the basic section in F. peregrinus still differed from the other species in its oval structure (Fig. 5).”
Quote in the paperNot a substantial claimMay combine two claims.
Findingown data§ Discussion · p. 18The Young's modulus itself has an influence on the specific bending stiffness, as values lay between 5.8 and 8.4 GPa.
“As values lay between 5.8 and 8.4 GPa the Young's modulus itself has an influence on the specific bending stiffness.”
Quote in the paperNot a substantial claimMay combine two claims.
Findingown data§ Discussion · p. 18For the four species investigated, E-values differ between species and between different areas of the feathers.
“For the four species investigated it was shown that E-values differ between species and between different areas of the feathers.”
Quote in the paperSubstantial claimThe quote only partly states this claim.
Findingown data§ Cross-sections · p. 19The larger cross-sections and more protrusions of F. peregrinus feathers were especially striking at the base of the rachises.
“This was especially striking at the base of the rachises.”
Quote in the paperNot a substantial claimMay combine two claims.
Findingown data§ Cross-sections · p. 19The feathers of F. peregrinus had larger cross-sections and more protrusions than the feathers of the other three species.
“The feathers of F. peregrinus had larger cross-sections and more protrusions than the feathers of the other three species.”
Quote in the paperSubstantial claimProbably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.
Findingown data§ Cross-sections · p. 19These findings are in line with our results, as in Fig. 12 and 13 the curves run in very similar manners.
“These finding are in line with our results (Figs. 4-6, 13), as in Fig.12 and 13 the curves run in very similar manners.”
Quote in the paperNot a substantial claimProbably not a substantial claim: may not pass S4 (based).
Interpretationreasoning§ City St George's, University of London · p. 3While pulling out of the stoop from such high velocities, the wing and tail feathers are likely to be exposed for a certain time-span to large mechanical forces.
“While pulling out of the stoop from such high velocities the wing and tail feathers are likely to be exposed for a certain time-span to large mechanical forces.”
Quote in the paperNot a substantial claimClassified as probably not a substantial claim.
Premisecitation§ City St George's, University of London · p. 4During flight manoeuvres, birds experience large aerodynamic forces.
“During flight manoeuvres birds experience large aerodynamic forces (Corning and Biewener, 1998; Tucker et al 1998; Usherwood et al., 2005).”
Quote in the paperSubstantial claim0 of 3 sources checked- cites Corning and Biewener, 1998
Not checked in the free run.
- cites Tucker et al 1998
Not checked in the free run.
- cites Usherwood et al., 2005
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
Premisecitation§ City St George's, University of London · p. 5The presence of foam-like material within the rachis enhances the bending stiffness of a feather by 7-16% compared to the situation without the inner foam.
“It has already been shown that the presence of the foam-like material within the rachis enhances the bending stiffness of a feather by 7-16% compared to the situation without the inner foam (Purslow and Vincent, 1978; Bonser, 1996).”
Quote in the paperNot a substantial claim0 of 2 sources checked- cites Purslow and Vincent, 1978
Not checked in the free run.
- cites Bonser, 1996
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
Premisecitation§ City St George's, University of London · p. 4The wing and tail feathers of birds are designed to provide aerodynamic lift under conditions of minimum weight and maximum structural stability.
“The wing and tail feathers of birds are designed to provide aerodynamic lift under conditions of minimum weight and maximum structural stability (Hertel, 1966; Crenshaw, 1980; Gibson and Ashby, 1988).”
Quote in the paperNot a substantial claim0 of 3 sources checked- cites Hertel, 1966
Not checked in the free run.
- cites Crenshaw, 1980
Not checked in the free run.
- cites Gibson and Ashby, 1988
Not checked in the free run.
Classified as probably not a substantial claim. May combine two claims.
Premisecitation§ City St George's, University of London · p. 4The mechanical loads the wing and tail feathers are exposed to depend on the flying speed and the particular flight manoeuvre of a bird, and also on its body weight.
“The mechanical loads the wing and tail feathers are exposed to not only depend on the flying speed and the particular flight manoeuvre of a bird, but also on its body weight (e.g. Lentink et al. 2007).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Lentink et al. 2007
Not checked in the free run.
May combine two claims.
Interpretationreasoning§ Discussion · p. 17The higher mechanical robustness of the inner vanes of F. peregrinus may be an adaptation to its higher mass and/or to the higher aerodynamic forces peregrines most likely experience during fast flight manoeuvres.
“This may be an adaptation to the higher mass of F.peregrinus and/or to the higher aerodynamic forces peregrines most likely experience during fast flight manoeuvres.”
Quote in the paperSubstantial claimMay combine two claims.
Study design and conductreasoning§ Discussion · p. 16The equation used in this study was derived for samples made from paperboard, so the balance results could be higher than expected when using rachises.
“Moreover the equation used herein was was derived for samples made from paperboard. Therefore the results for the balance could be higher than expected when using the rachises.”
Quote in the paperNot a substantial claim- Rests on an assumption: The equation used in this study was derived for samples made from paperboard.
Probably not a substantial claim: may not pass S3 (load-bearing).
Premisecitation§ Discussion · p. 15Peregrines probably use their tail feathers to reduce the flight velocity at the end of a dive.
“Peregrines probably use their tail feathers to reduce the flight velocity at the end of a dive (National Geographic Channel 2007, High velocity falcon, you tube).”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites National Geographic Channel 2007, High velocity falcon, you tube
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
Interpretationcitation§ Discussion · p. 18A possible explanation for the discrepancy between measured and literature E-values is different levels of water content in the samples.
“A possible for this discrepancy could be the different levels in water content of the samples.”
Quote in the paperNot a substantial claimProbably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based).
Interpretationnone given§ Discussion · p. 17In the region close to the rachis, stability may be of special importance.
“In this region stability may be of special importance.”
Quote in the paperNot a substantial claimMay combine two claims.
Premisecitation§ Discussion · p. 16The vanes of the outer primaries of C. livia are exposed to high aerodynamic loads during a down stroke in flapping flight and can resist larger out-of-plane forces than the inner primaries and the secondaries.
“The vanes of the outer primaries of C.livia are exposed to high aerodynamic loads during a down stroke in flapping flight and can resist larger out-of-plane forces than the inner primaries and the secondaries (Ennos et al. 1995).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Ennos et al. 1995 source
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
Premisecitation§ Discussion · p. 17The E-values of the cortices of the feathers of various bird species range between 1.8 and 10 GPa.
“The E-values of the cortices of the feathers of various bird species range between 1.8 10 GPa (Hertel, 1966; Rutschke, 1976; Purslow and Vincent, 1978; Crenshaw, 1980; Fraser and Macrae, 1980; MacLeod, 1980; Bonser and Purslow, 1995, Bonser, 2001; Cameron et al., 2003; Pannkuk et al., 2010; Bachmann et al., 2012; Vincent, 2012).”
Quote in the paperNot a substantial claim0 of 3 sources checked- cites Hertel, 1966
Not checked in the free run.
- cites Rutschke, 1976
Not checked in the free run.
- cites Purslow and Vincent, 1978
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).
Interpretationnone given§ Discussion · p. 17Overlapping will especially influence the forces acting on the vanes.
“Overlapping will especially influence the forces acting on the vanes.”
Quote in the paperNot a substantial claim- Rests on an assumption: The overlap of the feathers while the wings are moved plays a role in force distribution, at least in the proximal portions of most feathers.
Probably not a substantial claim: may not pass S3 (load-bearing).
Premisecitation§ Discussion · p. 18Bachmann et al. (2012) found 0.017-1.7 mm 4 for primary 5 in C. livia.
“Bachmann et al. (2012) found 0.017-1.7 mm 4 for primary 5 in the same species.”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Bachmann et al. (2012)
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
Premisecitation§ Discussion · p. 18The second moment of area of primary 10 of a 300 g C. livia is 0.00008-1.6 mm 4.
“The second moment of area of primary 10 of a 300 g C. livia is 0.00008-1.6 mm 4 (Purslow and Vincent 1978).”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Purslow and Vincent 1978
Not checked in the free run.
May combine two claims.
Interpretationcitation§ Discussion · p. 18Our samples were air-dried, but in live birds the water content of the feathers is higher, so the actual E-values may be lower.
“Our samples were air-dried, but in alive birds the water content of the feathers is higher and thus the actual E-values may be lower.”
Quote in the paperNot a substantial claimClassified as probably not a substantial claim.
Premisecitation§ Discussion · p. 18In ostrich contour feathers, the E-value decreased with increasing water content (from 3.66 GPa at 0% water content to 1.47 GPa at 100% water content).
“This was shown in ostrich contour feathers in which the E-value decreased with increasing water content (0% water content 3.66 GPa to 100% 1.47 GPa; Taylor et al., 2004).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Taylor et al., 2004
Not checked in the free run.
The paper relies on this without support.
The forces acting on a feather during flight are rarely perpendicular to the surface of a feather.
“Although the forces acting on a feather during flight are probably rarely perpendicular to the surface of a feather our method already provided some valuable information on the feather stability by comparing the measured values between different species.”
The paper relies on this without support.
The number of tail feathers differs between birds, and their homology is uncertain.
“As the number of tail feathers differs between birds, their homology is uncertain.”
The paper relies on this without support.
F. tinnunculus uses its primaries for hovering.
“This falcon uses the primaries for hovering, i.e. while hovering the primaries are probably exposed to higher mechanical loads than in steady flight conditions.”
The paper relies on this without support.
The overlap of the feathers while the wings are moved plays a role in force distribution, at least in the proximal portions of most feathers.
“Moreover, the overlap of the feathers while the wings are moved plays a role in force distribution, at least in the proximal portions of most feathers.”
The paper relies on this without support.
Heavier birds must generate larger forces than smaller birds to keep airborne.
“Heavier birds must generate larger forces than smaller birds to keep airborne.”
The paper relies on this without support.
The equation used in this study was derived for samples made from paperboard.
“Moreover the equation used herein was was derived for samples made from paperboard.”
Claims other papers make about this one
Each is checked against this paper’s own text: does it say what the citing paper says it does?
The position dependence of bending stiffness in the lateral direction for the eyespot feathers is similar to that found for flight and tail feathers in other bird species.
“The position dependence of bending stiffness in the lateral direction for the eyespot feathers (as shown by I in Fig 8D) is similar to that found for flight and tail feathers in other bird species [38, 43].”
From Biomechanics of the Peacock’s Display: How Feather Structure and Resonance Influence Multimodal Signaling (2016) · cites this paper as [43]
Self-check
Percentages, sums and reported differences re-computed from the paper’s own numbers.
Each contradiction between two passages listed here was confirmed by an independent check.
Figure caption and results disagree on where barb counts differ
MajorThe results say barb counts differ between peregrines and other species in every vane except the inner tail vane. The Figure 2 caption says differences occur only in the outer vanes of primary 10 and the alula.
“Differences between F. pereginus and individuals of the other species occurred in all vanes with the exception of the inner vane of the tail feather (post-hoc Bonferroni, p<0.01) (Fig. 2).”
“Differences were only found in the outer vane of primary 10 and of the alula (p<0.01).”
Reported E-modulus ranges have conflicting upper limits
MajorThe abstract gives 8.4 GPa as the upper end of the E-modulus range for the four species, while the results report values up to 8.7 GPa.
“In all four species the E-moduli of the feathers ranged from 5.9 to 8.4 GPa.”
“Values ranged within 6.5 - 8.7 GPa.”
Discussion denies reported differences in barb counts
MinorThe discussion says peregrine barb numbers did not differ from those of the other species, but the Figure 2 caption reports significant differences in two vanes.
“In terms of barb numbers the vanes of the feathers of F. peregrinus did not differ from the other species investigated.”
“Differences were only found in the outer vane of primary 10 and of the alula (p<0.01).”
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“The position dependence of bending stiffness in the lateral direction for the eyespot feathers (as shown by I in Fig 8D) is similar to that found for flight and tail feathers in other bird species [38, 43].”
ResultAerodynamics of the Cupped Wings during Peregrine Falcon’s Diving Flight
Ponitz, B., Triep, M., & Brücker, C. (2014) · Open Journal of Fluid Dynamics · doi:10.4236/ojfd.2014.44027 · Cited by 27 · see citing works
“In nature the body is covered with feathers which may also play a role [22].”
Self-citationVortices enable the complex aerobatics of peregrine falcons
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“This sudden alteration of morphology to achieve such a complex manoeuvre is enabled by the robust musculo-skeletal structure and the superior mechanical strength of the feathers 9 .”
Self-citationHow localized force spreads on elastic contour feathers
Bhar, K., Chang, B., Virot, E., Straker, L. C., Kang, H., Paris, R., Clanet, C., & Jung, S. (2019) · Journal of The Royal Society Interface · doi:10.1098/rsif.2019.0267 · Cited by 17 · see citing works
The feather’s multi-functional structure across nano to macro scales inspires hierarchical design
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“These values, and the orientation of these molecules, affects the elastic modulus, strength and anisotropy of the cortex [70,72–74, 155, 156], medulla [ 76 ,78] and barbules [ bird orders?”
Feather Vibration as a Stimulus for Sensing Incipient Separation in Falcon Diving Flight
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“The artificial feathers have approximately the same bending stiffness as the feathers measured in [14] and are arranged in a similar way as the secondary feathers appear in life recordings.”
BackgroundInfluentialSelf-citationThe peregrine falcon’s rapid dive: on the adaptedness of the arm skeleton and shoulder girdle
Schmitz, A., Ondreka, N., Poleschinski, J. M., et al. (2018) · Journal of Comparative Physiology A · doi:10.1007/s00359-018-1276-y · Cited by 10 · see citing works
Self-citationFeathers for escape: the transition from juvenile to adult in red-legged partridges (Alectoris rufa)
Nadal, J., Ponz, C., & Margalida, A. (2017) · Biological Journal of the Linnean Society · doi:10.1093/biolinnean/blx130 · Cited by 9 · see citing works
“These characteristics generate greater lift/drag ratios than in juvenile wings, which have flexible rachis and less cohesive barbules (Schmitz et al., 2015).”
BackgroundSievwright, H., & Higuchi, H. (2016) · ZOOLOGICAL SCIENCE · doi:10.2108/zs150175 · Cited by 8 · see citing works
“…distal pennaceous barbules; the hardness of the OHB feathers may be related to better anchoring of the hooks at the base of the vane, or increased thickness of the rachilla, as remarked by Schmitz et al. (2015) on examination of the flight feathers of the peregrine falcon, compared to the kestrel.”
BackgroundVortex dynamics in the wake of bio-inspired flexible, slotted winglets
Midmer, A. S., & Brücker, C. (2024) · Journal of Fluids and Structures · doi:10.1016/j.jfluidstructs.2024.104138 · Cited by 5 · see citing works
Self-citation3D printed feathers with embedded aerodynamic sensing
Tu, R., Delplanche, R. A., Tobalske, B. W., et al. (2024) · Bioinspiration & Biomimetics · doi:10.1088/1748-3190/ad87a0 · Cited by 3 · see citing works
“…of both P9Transducer01 and P9Feather01 decreases as the X position moves closer to the tip, mainly due to the distribution of the rachis second moment of area ( I ) if assuming a constant modulus ( E ), which agrees with literature results on the feather flexural rigidity distribution [41– 43].”
Bleckmann, H. (2023) · Journal of Comparative Physiology A · doi:10.1007/s00359-023-01681-3 · Cited by 2 · see citing works
“Together with Benjamin Ponitz (a PhD student of Christoph), Anke Schmitz and Dominik Fischer, who had access to live falcons, we investigated the aerodynamics of a diving peregrine (Ponitz et al. 2014).”
BackgroundSelf-citationWang, H., Ju, M., Huang, Y., et al. (2026) · Bioinspiration & Biomimetics · doi:10.1088/1748-3190/ae4af2 · Cited by 1 · see citing works
Avian Locomotion: Flying, Running, Walking, Climbing, Swimming, and Diving
Ritchison, G. (2023) · Fascinating life sciences · doi:10.1007/978-3-031-14852-1_11 · Cited by 2 · see citing works
Leonardi, G. (2020) · Springer eBooks · doi:10.1007/978-3-030-60541-4_1
Wen, Z., Yang, W., Liu, Y., et al. (2026) · Bioinspiration & Biomimetics · doi:10.1088/1748-3190/ae7787
“…11] Natural keratin ∼ 817 2 Neural (Phasic displacement) Fur (rat) [5, 12] Natural keratin ∼ 147 1.5 Neural (Phasic displacement) Avian filoplumes [5, 14–16] Natural keratin > 10 (Dynamic) flow regimes, the fluid velocity is insufficient to overcome the high modulus of these rigid materials.”
Influential
- Authors
- Schmitz, A., Ponitz, B., Brücker, C., Schmitz, H., Herweg, J., & Bleckmann, H.
- Affiliations
- University of Bonn (DE)
- TU Bergakademie Freiberg (DE)
- Corresponding author
- Schmitz, Anke
- Venue
- Journal of Morphology
- Publisher
- Wiley
- Published
- 2014-08-04
- Volume
- 276
- Issue
- 1
- Pages
- 33-46
- ISSN
- 0362-2525 (ISSN-L)
- 1097-4687
- OpenAlex
- W2172200817
- Semantic Scholar
- 98ae05e00a81fb2a154c101a67ec1b1d643ceb24
- Cited by
- 18
- References
- 39
- Citations by year
- 2026: 3 · 2025: 1 · 2024: 2 · 2023: 2 · 2020: 1 · 2019: 1 · 2018: 3 · 2017: 1 · 2016: 3 · 2014: 1
- FWCI
- 2.12
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Journal of Morphology (publishedVersion)
- PubMed (publishedVersion)
- City Research Online (City University London) (acceptedVersion)
- Funders
- Deutsche Forschungsgemeinschaft (BL 242/19-1)
- Topics
- Health Sciences › Medicine › Urology › Comparative Animal Anatomy Studies
- Physical Sciences › Engineering › Aerospace Engineering › Biomimetic flight and propulsion mechanisms
- Physical Sciences › Environmental Science › Ecology › Avian ecology and behavior
- Keywords
- primary feathers, tail feathers, Young's modulus, bending stiffness, Eurasian sparrowhawk, Columba livia, Falco peregrinus, common kestrel, flight style, hover, avian flight biomechanics
- MeSH terms
- Animals, Feathers, Columbidae, Falconiformes, Elastic Modulus
About these checks
Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.
Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-09.
Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-09.
Statistics and arithmetic are recomputed, not judged.
Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.
What counts as a claim ›How we measure confidence ›How we check this ›
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