Colored reflections from the black-billed magpie feathers
Jean Pol Vignerona and Virginie Loussea,b
a Laboratoire
de Physique du Solide, Facultés Universitaires Notre-Dame de la Paix,
61 rue de Bruxelles, B-5000 Namur Belgium;
b Ginzton Laboratory, Stanford University, Stanford, California 94305 USA
ABSTRACT
The structural origin of the weak iridescence on some of the dark feathers of the black-billed magpie, Pica
pica (Corvidae), is found in the structure of the ribbon-shaped barbules. The cortex of these barbules contains
cylindrical holes distributed as the nodes of an hexagonal lattice in the hard layer cross-section. The cortex
optical properties are described starting from a photonic-crystal film theory. The yellowish-green coloration of
the bird’s tail can be explained by the appearance of a reflection band related to the photonic-crystal lowestlying gap. The bluish reflections from the wings are produced by a more complicated mechanism, involving the
presence of a cortex “second gap”.
Keywords: Reflectance, natural photonic structures
1. INTRODUCTION
The spectral filtering of diffused light by bird feathers is known, and generally explained by Tyndall scattering.1–3
Bird feathers, however, can show much more complex optical properties, and these call for more complex mechanisms and more complex structures than simple isolated Mie scatterers. Light interference and diffraction4
and even more collective mechanisms5 can lead to iridescence. Recent evidences of these complex structures
were found, for example, in the rose-faced lovebird (Agapornis Roseicollis),6, 7 and in the male peacock (Pavo
muticus).8
In this paper, the origin of the iridescence of the feathers of the common magpie (Pica pica) is investigated,
using a combination of reflectance measurements, scanning electron microscopy, theoretical modelling and numerical simulations. As with the peacock feathers, the barbules that appear iridescent under the optical microscope
have a peripheral layer (the “cortex”) structured as a two-dimensional photonic crystal. But contrasting the
case of Pavo sp., this two-dimensional photonic crystal is here hexagonal, rather than rectangular,9 and the blue
wing coloration is not determined by a mere reduction of the lattice parameter found in the greenish feathers
lattice.
2. OPTICAL REFLECTION FACTOR
The specular reflection from specific parts of intact feathers was investigated using an Avaspec 2048/2 fiber optic
spectrometer. Measurements were performed under a normal incidence, collecting the backscattered light by
using bifurcated optical fibers. By normal incidence, we mean along the normal to the feather plane containing
the shaft, the barbs, and the barbules. The results are shown on Fig. 1 and Fig. 2. The reflection factor is
the reflected intensity, expressed in units of the corresponding diffuse reflection obtained from a standard white
diffusor.
A feather has three levels of barbs. The central shaft, which gives the longitudinal rigidity to the feather, is
the first level. The second level gives the barbs which are attached to the shaft and provides the resistance of
the whole feather as an aerodynamic surface. Finally, the barbules, at the third level, are attached to the barbs,
and fill the whole feather surface, providing impermeability to air flow and controlling the feather coloration.
Two kinds of barbules are worth being considered. The first one is part of the tail feathers, and reflects light in
Further author information: (Send correspondence to J.P.V.)
J.P.V. : E-mail: [email protected], Telephone: +32 81 724711
The Nature of Light: Light in Nature, edited by Katherine Creath, Proc. of SPIE
Vol. 6285, 628508, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.682243
Proc. of SPIE Vol. 6285 628508-1
Figure 1. Reflection factor (compared to a white diffuse reflector) of of a yellow-green barbule, on the magpie tail.
the greenish-yellow range (see Fig. 1); the second one is part of some wing feathers and reflects light in the blue
region (see Fig. 2).
The greenish-yellow feather reflects lights around a dominant wavelength close to 550 nm, and the blue feather
reflects light near 450 nm. Both reflection bands are broad, indicating that the reflection is not due to a very
thick dielectric mirror. Electron microscope images of the barbule structure are needed to determine the origin
of the reflection. The next section deals with this investigation.
3. STRUCTURE OF THE BARBULES
We have investigated the structure of barbules from yellowish-green areas of the tail feathers, and bluish areas
of the wings. Figure 3 shows a barbule from a yellowish-green tail feather, broken in order to reveal its interior
Figure 2. Reflection factor (compared to a white diffuse reflector) of of a blue barbule, on the magpie wing.
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Fl pm
Figure 3. Structure of a yellow-green barbule. The cross-section shows the cortex (external part of the barbule, with the
triangular lattice of holes which explains the feather iridescence.
structure. This fracture was simply obtained by cutting the feather in liquid nitrogen (−196o C). A close
examination of Fig. 3 reveals the presence of tiny holes in the cortex of the barbule. These holes are arranged on
a very coherent two-dimensional hexagonal (i.e., triangular) lattice and have a diameter which can be estimated
to be 50 nm. The distance between two neighboring holes, in any directions, is found to be a=180 nm.
A surprise comes with the examination of the cortex structure of feathers taken from the bird wings. These
feathers display bluish reflections and it is expected that the air channels should be closer to each other here,
if the mechanisms for the iridescence were similar and a blue shift should be observed. The cortex structure is
shown on the electron micrograph in Fig. 4. The same cortex structure and longitudinal cylindric holes can
Figure 4. Structure of a blue barbule. The triangular lattice of holes is also present, but the lattice spacing is larger
than for the greenish-yellow feather of Fig. 3. This obliges to consider a “second gap” process to explain the shorter
wavelength iridescence.
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be seen again, but the measurement of the distance between neighboring holes suggests that the mechanism of
production of the iridescence be reconsidered. A careful examination of Fig. 4, and other similar views, indicate
a first-neighbors distance of 270 nm, larger than what is found in the case of the yellowish-green cortexes.
Surprisingly, and contrasting with what was found in the peacock feathers, the shift to shorter wavelengths is
not obtained by a reduction of the photonic crystal lattice parameter.
4. REVERSE ENGINEERING OF THE PHOTONIC STRUCTURE
The normal reflectance of a two-dimensional triangular lattice of cylindrical holes can be easily estimated. Let a
be the distance √
between
two neighboring holes. Perpendicular to the surface, the distance between the reticular
planes is p = a 3 2. If laterally averaged, these reticular planes become equivalent and their interdistance p
become their periodicity. Then, the dominant reflected wavelength can be expressed as
λ=
2pn̄
m
(1)
where n̄ is the average refractive index of the structure (here n̄ 1.9, accounting for the presence of keratin
and melanin), and m is an integer such that the reflected wavelength falls in the visible spectral range. With
a = 180 nm for the greenish-yellow feather, p = 156 nm and the reflected wavelength is predicted at λ = 592 nm,
just a few percent (7%) higher than measured.
For the blue feather, the distance between neighboring holes is significantly larger, a = 270 nm. The
periodicity of the lattice normal to the cortex surface is now p = 234 nm, and we must consider m = 2 to
obtain a contribution in the visible range. We find it at λ = 444 nm, very close, indeed, to the observed value.
Considering m = 2 essentially means that we are considering the “second gap” of the multilayer structure (second
harmonic of the first gap), instead of the first one. The blue coloration of the wing feathers are thus a “second
gap” effect, produced by a triangular lattice with a lattice parameter larger than the one which produces the
greenish-yellow iridescence.
The reflected band cannot be sharp in either cases : due to the very limited thickness of the cortex, the
number of layers is very small, leading to smooth resonance curves. Also, the iridescence is not easily seen except
in a strict specular geometry, close to the feather normal (which coincides with a barbule normal). This may be
related to the presence of melanin, which very effectively absorbs the radiation if the length of its path in the
cortex is larger.
5. CONCLUSION
Though less spectacular than the bright colors of a male peacock, the coloration of some of the feathers of the
common magpie is generated by a similar process : a two-dimensional photonic crystal formed by parallel air
channels in the cortex of the barbules. For some reason, the lattice has evolved to be hexagonal, rather than the
previously known rectangular lattice. Contrasting the mechanism for coloration contrasts of the peacock feather,
different colorations are not obtained by the expected rescaling of the characteristic lengths in the structure.
The yellowish-green reflection seen on the tail of the magpie can easily be explained by a photonic-crystal
“first-gap” effect. This mechanism has been seen on other feathers and, in particular, has been suggested for
explaining the coloration of the peacock tail. The blue reflections from the wings is actually more subtle,
because it involves a photonic-crystal “second-gap” effect, complemented by the avoidance, by confinement, of
the first-gap infrared reflection.
ACKNOWLEDGMENTS
This investigation was conducted with the support of the European BioPhot (NEST) project, under contract
no. 12915. This work was carried out with support from EU5 Centre of Excellence ICAI-CT-2000-70029 and
from the Inter-University Attraction Pole (IUAP P5/1) on “Quantum-size effects in nanostructured materials”
of the Belgian Office for Scientific, Technical, and Cultural Affairs. It was also partly supported by the European
Regional Development Fund (ERDF) and the Walloon Regional Government under the ”PREMIO” INTERREG
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IIIa project. The authors acknowledge the use of Namur Interuniversity Scientific Computing Facility (NamurISCF) for the numerical simulations. V.L. was supported as postdoctoral researcher by the Belgian National
Fund for Scientific Research (FNRS).
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