J. exp. Biol. (1980), 87, 65^71
3 figures
Great Britain
65
MECHANICAL PROPERTIES OF CONTOUR FEATHERS
BY G. D. MACLEOD
University of Nottingham, School of Agriculture,
Sutton Bonington, Loughborough, LE12 $RD
(Received 5 October 1979)
SUMMARY
Young's moduli (E) in bending and extension were measured for selected
contour feather shafts from the chicken (Gallus domesticus), turkey (Meleagris
gallopavo), ring-necked pheasant (Phasianus colchicus) and herring gull
(Larus argentatus). These were treated mechanically as tapering beams. In
extension, E ranged from 45 to 181 MN m~8 and from 210 to 682 MN m~2
for the proximal and distal regions, respectively. Values obtained for the same
regions in bending were from 5 to 24 MN m~2 and 457 to 1850 MN m~i.
These results suggested that the cortex of the proximal region generally has a
lower Young's modulus than the medulla, while for the distal region this is
reversed. This was confirmed by further measurements on treated shafts.
The observed systematic differences in the mechanical properties of the
proximal and distal parts of contour feather shafts are consistent with their
probable mechanical functions.
INTRODUCTION
Feathers are probably the most complex derivatives of the integument to be found
in any vertebrate animal. However, though their structural characteristics have been
extensively investigated (e.g. see the review by Lucas & Stettenheim, 1972), knowledge of the relationship between the functions of feathers and their morphology is still
incomplete.
The major parts of the feather are the shaft and the vanes which it supports on
either side. For flight feathers, the vanes provide an aerofoil surface; for contour
feathers they cover and insulate the body. The shaft of contour feathers consists of two
layers: a porous, inner medulla, and a solid, outer cortex. The cross-section gradually
changes shape, from approximately rectangular near the base, to elliptical as the shaft
tapers towards the tip (Lucas & Stettenheim, 1972).
Purslow & Vincent (1978) have studied the relationship between the structure of a
flight feather shaft and its bending behaviour. Although contour feathers are not
necessarily subjected to such large forces as are flight feathers, they must still contend
with a great variety of loads. This has been observed recently for domestic hens
(Hughes, 1978).
This paper reports measurements of the mechanical properties of contour feathers,
dealing a gradient along the shaft which has not previously been described.
66
G . D . MACLEOD
Table i. Characteristic transverse sectional areas in mm*, of proximal and dis^
samples of pelvic contour feather shafts, as used in the measurement of the Young's
moduli.
(Standard errors refer to the number of feathers measured.)
Proximal
S.E. (rt = IO)
Distal
S.E. (n =
10)
Brown
chicken
Turkey
Pheasant
0-50
O-O2
rao
o-io
°'45
Herring
gull
o-6o
0-05
0-04
0-03
0'0O4
013
O-O2
0-03
0'02
0-003
o-ooi
Neutral axis
Fig. i. Static cantilever method for determination of Young's modulus in bending.
MATERIALS AND METHODS
In the present study, values of Young's modulus were determined for contour
feathers taken from the pelvic tract of a variety of avian species. All the birds were in
basic plumage at the time of sampling. Feathers from this tract are relatively straight,
allowing effects due to twisting to be ignored. All samples were washed with detergent
and then alcohol, and were then rinsed in several changes of distilled water before use.
They were equilibrated at 15 °C in an atmosphere of 60 % relative humidity and all the
experiments were carried out under these conditions. The feathers were prepared by
removing the vanes and dividing the shaft into proximal and distal regions of equal
length. Samples of 15 mm in length were then cut from the middle of these regions
and with less than a 10% variation in cross-sectional area along them. For measurements of Young's modulus in extension both ends of each sample were embedded in
AraJdite to give a test length of at least 10 mm. All samples then had an aspect ratio
of more than 10. The samples were clamped in an Instron testing machine (model
1026) and the shaft was extended at a constant rate (10 mm min"1). The load-extension
curve was automatically traced on a chart which had previously been calibrated to give
a full-scale deflexion of 20 N, chosen in preliminary tests. The Young's modulus
was calculated only for the linear region of the trace, where extension was proportional
to load. For measurements of Young's modulus in extension the shape of the croaa«.
section will be of no consequence; however, stiffness in extension has to take
Mechanical properties of contour feathers
67
Fig. 2. Second moments of area (I) for bending in a doreo-ventral plane. Neutral axes are indicated by broken lines.
of cross-sectional area. Young's modulus (E) is therefore determined by measuring the
ratio between stress and strain:
force per unit area
fractional change in length"
The area of the cross-section of the shaft at the breaking point was estimated, to enable
calculation of the tensile stress. (This method is considered satisfactory since the
deformation before breakage was small.) A magnification of x 100, in conjunction with
an eyepiece scale, was employed for the measurement of dimensions (Table 1).
Young's modulus was then determined in the dorso-ventral plane of bending for a
similar set of samples using the static cantilever method (Fig. 1).
In bending, the shape of the cross-section has a great bearing on stiffness. The
resistance to bending is determined both by the amount of material in the crosssection and by its distribution about the neutral axis of the section, as well as by the
modulus of elasticity. The first two factors are measured by the second moment of area
of the section /, which gives more significance to material further away from the neutral
axis than to that close to it (see Fig. 1). In the present study established formulae for the
calculation of / were used (Alexander, 1968). Equations for a rectangle and an ellipse
were used for the proximal and distal regions of the feather shaft, respectively (Fig. 2).
Since the semi-axis is raised to the fourth power in both cases, the dimensions of the
structure must be measured with great accuracy. Also, a short length must be used in
the case of a tapering structure such as the feather shaft. If / is measured, the Young's
modulus in bending can be determined from
_
where d is the deflexion produced by a load, F, applied at a distance L from the
fixed end of a simple cantilever (see Fig. 1).
One end of each sample was embedded in Araldite which was then rigidly clamped.
The point of emergence from the mount was made abrupt and clear, to reduce error in
the measurement of the bent length. The shaft was bent by a load applied 10 mm from
the fixed end by means of a delicate spring. The deflexion at the point of application
of the load was measured to ± 0-02 mm with a travelling microscope. The dimensions
of the shaft (Table 1), as well as the bent length, were measured at ten points along the
th by using a second microscope (x 100 magnification, as previously). Initially,
creasing loads were applied to one sample for each species. For all the subsequent
68
G. D . MACLEOD
Table 2. Young's modulus in extension (MN m~ s ), measured for the proximal and
distal regions of pelvic contour feathers from a selection of birds
Proximal
S.E. (n = 10)
Distal
s.E (n = 10)
Brown
chicken
Turkey
Pheasant
Herring
gull
73
5
45
4
57
4
180
15
210
30
260
40
330
40
680
90
Table 3. Young's modulus in bending (MN m~2), measured for samples
as in Table 2
Brown
chicken
Proximal
S.E. (n •= 20)
Distal
S.E. (n = 20)
24
ro
1850
70
Turkey
2
o-i
620
40
Pheasant
5
0-3
1100
50
Herring
gull
5
0-3
460
30
samples a load was used which fell in the linear region of the load-deflexion
curve.
Finally, two complementary procedures were adopted to confirm the discrepancies
between the Young's moduli in extension and bending. Part of the cortex was removed
from one set of samples by abrasion with emery paper, while in a second set most of the
medulla was bored out with a hypodermic needle. The Young's modulus in bending
was determined as previously, with values for the second moment of area corrected for
the quantity of material removed.
Since the length of shaft chosen for the measurement of Young's modulus in
bending was short, shear deformation could be significant. For short beams Ugural
& Fenster (1975) suggest a correction factor equal to 0-75 (i + n/2) (h 2 /L s ), where
n is the Poisson ratio for the material, and h and L are the thickness and length of the
sample respectively. The Poisson ratio for the feather is likely to be similar to the
published value of 0-3 determined for wool fibres (W.I.R.A., 1955), while the largest
value of h/L for the present samples was o-i. Therefore the maximum correction
applicable to these measurements is less than 1 %. This is insignificant in relation to
the other sources of error in the measurements and has therefore been neglected.
RESULTS
The measurements of Young's modulus (E) in extension are presented in Table 2.
The interspecific variance is large and the values for the distal region are systematically
higher than those for the proximal region. E ranged from 45 to 181 MN m~2 and from
210 to 682 MN m~2 for the proximal and distal regions, respectively. In both cases the
highest values were for gull feathers, and the lowest values were for the chicken,
turkey and pheasant feathers.
In the proximal region, the modulus for extension is higher than Young's modulufl
for bending (Table 3); in the distal region the converse is true. This suggests that, H
Mechanical properties of contour feathers
69
Table 4. Young's modulus in bending (MN m~2), (1) after partial removal
of the cortex - Em and (») after partial removal of medulla - Ec
Brown
chicken
(i) Proximal Em
S.E. (n = 20)
65
6
Distal Em
s.B. (n = 20)
1580
150
(ii) Proximal EB
S.E. (n = so)
Distal Ec
s.B. (n = 20)
Turkey
Pheasant
Herring
gull
5
06
300
IS
1
22
2
490
50
850
9O
3°
8
08
2190
270
1
o-i
1140
100
2
O-2
1460
170
4
O'5
420
40
the birds studied, the cortex of the proximal region has a lower modulus than the
medulla, while for the distal region this is reversed. Both the scatter between species
and the variation with shaft position are much greater in the case of the Young's
modulus in bending than in extension. Values obtained for the proximal and distal
regions in bending were from 5 to 24 MN m~2 and 457 to 1849 MN m~2, respectively.
The treated shafts showed a similar qualitative difference in elastic properties between proximal and distal regions. The results of the treatments confirm the above
suggestion of different relative contributions by medulla and cortex in the two regions;
for removal of that part of the feather which either has a comparatively low or high E
value should respectively increase or decrease the modulus measured for the remaining
structure.
Standard errors in all the experiments were of the order of 10%. The major part of
this was associated with the measurement of shaft dimensions. In extension the area of
the shaft was measured at the point of fracture, but the strain before fracture varied to
some extent between shafts. To account for this, samples whose strain at breakage was
more than 2 standard deviations from the mean strain of 10% were discarded.
DISCUSSION
Mechanically, a feather shaft can be treated as a tapering beam. Its elastic properties
may therefore be determined by its deformation under load, as treated mathematically
in engineering texts (see Ugural & Fenster, 1975). The equations usually assume that
the material concerned is perfectly elastic, homogeneous and isotropic. When this
assumption is true, then the stiffness of the material will be the same in bending as in
extension. In a perfectly elastic system the relationship between stress and strain is
linear and reproducible, shows no hysteresis and is independent of the direction in
which the test piece is cut from the sample. Most animal fibres, including feathers,
cannot be expected to fulfil these criteria since they are heterogeneous, anisotropic and
viscoelastic. For the contour feathers studied here the initial part of the stress-strain
relationship was linear and reproducible and all measurements were taken in this
region.
Many determinations of Young's modulus have been made on mammalian hair
•Bres. For example, Khayatt & Chamberlain (1948) obtained mean values for the
G . D . MACLEOD
Fig. 3. Typical pelvic contour feather, scale line 10 mm. Generalized cross-sections through the
shaft at two points (not to scale) show regions of high (hatched) and low (darkened) stiffness.
Young's modulus of human hair of 3-6 GN m~2 and 1-9 GN m~a in stretching and
bending, respectively. The values of E measured in the present work are lower than
those generally reported for wool and hair, perhaps because wool and hair are both of
an alpha-keratin type structure, whereas feather is of a beta-keratin type (Woods,
1955). Purslow & Vincent (1978) quote E values estimated from cantilever beam tests
of 7-75 and 10 GN m~2 for the shaft cortex of two types of pigeon flight feathers. These
are about two orders of magnitude higher than the values reported here for intact
feathers in bending. However, in measurements made on intact shafts, the air-filled
cavities within the medulla (Lucas & Stettenheim, 1972), which could not be accounted
for in cross-sectional area measurements, would tend to reduce the stiffness of the
shaft as compared to that of the shaft cortex. This suggests that in both bending and
extension the effective stiffness depends on the feather structure as much as on the
material.
Purslow and Vincent (1978) reported measurements of the second moment of area
of the shaft of pigeon flight feathers. Taking a mean value for / of 2-3 mm4 for a 400 g
bird from their results, this is three orders of magnitude greater than the equivalent
value for a contour feather from a 1600 g bird in the present study. Purslow and
Vincent also found that the cortex provided most of the resistance to bending.
The variation in the elastic properties of the shaft layers with position (Fig. 3) may
be related to the bending forces to which they are subjected. A flexible thin-walled
beam, such as the proximal region of a feather shaft, would tend to fail by elastic
instability if it were not supported internally. Struts have been noted in certain bone
cavities which have been presumed to serve this purpose (Alexander, 1968). The substantial medulla in the basal region of a feather shaft could play the same role.
At the outer end of a contour feather the vane is generally narrower and any bending
moment about the tip will be small. These factors allow tapering of the feather shaft,
thus contributing to lightness. However, the shaft in this region must maintain th&
vanes as part of a wind resistant coat cover, so maximum rigidity for a given c r o ^
Mechanical properties of contour feathers
7l
Section would be an advantage. The distal region is relatively more stiff than the base of
the feather, as shown by the present results. In the distal region, the cortex typically
provided 60% of the area. Strength in bending for this region is therefore more likely
to be limited by the tensile strength of keratin rather than by elastic instability. Thus
a wide-bore tube can be lighter than a narrow tube of the same rigidity, for the wall
can be thinner. The results reported here suggest that the outer end of a contour
feather can be considered as a wide-bore tube with the medulla being of minor significance. The lowest Young's modulus for the distal region was observed for the
gull (at P > 0-05). In contrast to the feathers of the other birds the medulla also had a
higher modulus than the cortex in this region, as confirmed by the experimental treatments. Such a structure may be advantageous in maintaining the aerodynamics of the
coat during flight.
Purslow & Vincent (1978) suggest that the differences inflexuralstiffness which they
found among pigeon flight feathers are more likely to be due to changes in morphology
rather than changes in the material of the shaft. In the present study, however, the
cortex and medulla of contour feather rachis have both been shown to have mechanical
properties which vary with position along the shaft. This may be due to variations in
the keratin but further study is required to resolve the matter.
The work was supported by a British Egg Marketing Board Research and Education
Trust studentship, supplemented by the University of Nottingham. The author also
wishes to thank J. A. Clark for his interest and guidance.
REFERENCES
ALEXANDER, R. M C N . (1968). Animal Mechanics. London: Sidgwick and Jackson.
HUGHES, B. (1978). The frequency of neck movements in laying hens and the improbability of cage
abrasion causing feather wear. Br. Poult. Set. 19, 289-294.
KHAYATT, R. M. & CHAMBERLAIN, N. H. (1948). The bending modulus of animal fibres. J. Text. Intt.
39, T I 8 S - T I 9 7 .
LUCAS, A. M. & STETTENHEIM, P. R. (197*). Avam Anatomy, Integument, part I. U.S.D.A. Agriculture
Handbook, 362. Washington, D.C.
PURSLOW, P. P. & VINCENT, J. F. V. (1978). Mechanical properties of primary feathers from the
pigeon. J. exp. Biol. 72, 251-260.
UGURAL, A. C. & FENSTER, S. K. (197s)- Advanced Strength and Applied Elasticity. New York:
American Elsevier.
W.I.R.A. (1955). Wool Research, vol. 2. Leeds: Wool Industries Research Association.
WOODS, H. J. (1955). Physics of Fibres. London: The Institute of Physics.