An easier-to-read essay in scattering of Blue Bird
Feathers:
http://www.euronet.nl/users/hnl/tyndall.htm
The Myth of the Tyndall Effect in Blue Bird Feathers
By: Inte Onsman, Research coordinator
MUTAVI
Research & Advice Group, The Netherlands
For more than a century, scientists have agreed, saying that feathers look blue
for the same reason that the sky does. The sky is blue because of molecules of
gas and other particles that scatter light waves at the blue end of the colour
spectrum.
If we open an ornithology text or one of our reference books, we will see the
statement that blue feathers are blue because of this scattering [1,2,3,10],
however, this appeared to be wrong.
The first who had a different view on that was Raman (1935)[13]. He postulated
the colour to be due to the interference of light. Obviously his work was
ignored, or stayed unnoticed for it was not before 1971 that Jan Dyck, a Danish
scientist, published two papers with regard to this subject [5,7].
Dyck stated in his first paper that it may be said that the blue and blue-green
colours produced by the spongy structure (FKA cloudy zone or cloudy layer) of
Agapornis roseicollis barbs are due principally, not to Raleigh scattering, but
to the interference of light, and that the interference is probably due to
backscattering from the numerous hollow, randomly oriented keratin cylinders of
which the spongy structure may be considered to consist.
In the second paper he published that year, Dyck studied again rump feathers of
Agapornis roseicollis and also back feathers of Cotinga maynana with the
scanning and the transmission electron microscope.
He found the Agapornis structure to be an irregular three-dimensional network of
connected keratin rods. The air-filled space likewise consists of an irregular
network of connected channels. The Cotinga structure consists of spherical
cavities fairly evenly distributed in a keratin matrix. (Keratin is also a major
constituent of human fingernails and hair)
Dyck stated that he likewise found the explanation by Tyndall scattering (better
termed Raleigh scattering) to be wrong.
He also found it very interesting that the blue colour produced by the spongy
structure of Cotinga visually is indistinguishable from that of Agapornis. It is
therefore reasonable to assume that the ways in which the colour is produced in
the two species are analogous.
In 1998 a paper was published by Prum and coworkers [11]. Rick Prum is curator
of ornithology at the Kansas University Natural History Museum.
They have found that feathers look blue for the same reason that oil slicks do.
The blue in oil slicks and feathers results from differences in the distances
traveled by light waves that are reflected off of each. The same general
principle obviously is at work with feathers. Just as Dyck did in 1971, Prum
used cross sections from feather barbs taken from a cotinga.
Also, in 1999 Prum and his colleagues, including Jan Dyck, published another
paper dealing with this subject [12].
They conducted two-dimensional (2D) discrete Fourier analyses on the spongy
medullary keratin from four different colours of structurally coloured feather
barbs from three species of birds: the rose-faced lovebird, Agapornis
roseicollis, the Budgerigar, Melopsittacus undulatus and the Gouldian finch,
Poephila guttata.
To assist the reader in understanding the rudiments of this analytic method, a
brief tutorial follows.
-Jean Baptiste Fourier, a mathematician, showed that any repetitive waveform can
be broken down into a series of sine waves. A sine wave is a wave of a single
frequency. It has a given frequency, amplitude and phase. The breaking apart of
a complex wave into its component sine waves is called Fourier analyses.-
Using this method Prum and coworkers confirmed that structural colours of avian
feather barbs are produced by constructive interference instead of Raleigh
scattering (Tyndall effect) as previously was suggested by several other
investigators.
Conclusion
There is overwhelming evidence that the blue colour seen in feather barbs of
several bird species including the Budgerigar is produced by interference rather
than Raleigh scattering (Tyndall).
Therefore we should realize that most of our reference books erronously refer to
this phenomenon as the "Tyndall effect" which is wrong.
Consulted and Cited Literature:
[1] Auber L., (1957)
The Structures Producing "Non-Iridescent" Blue Colour in Bird Feathers
Proc.Zool.Soc.London Vol.129 no.4; p.p.455-486
[2] Auber L., (1971)
Formation of 'Polyhedral' Cell Cavities in Cloudy Media of Bird Feathers
Proc.Roy.Soc.Edinb. Vol.74 no.2; p.p.27-41
[3] Auber L., (1941)
The Colours of Feathers and their Structural Causes in Varieties of the
Budgerigar, Melopsittacus undulatus [Shaw]
Thesis; p.p.1-137
[4] Dyck J., (1985)
The Evolution of Feathers
Zool.Scripta Vol.14 no.2; p.p.137-154
[5] Dyck J., (1971)
Structure and Colour-Production of the Blue Barbs of Agapornis roseicollis
and Cotinga maynana
Zeitschr.fur Zellforsch.Vol.115; p.p.17-29
[6] Dyck J., (1966)
Determination of Plumage Colours, Feather Pigments and Structures by Means
of Reflection Spectrophotometry
Dansk Orn. Foren. Tidsskr. 60 ; p.p.50-75
[7] Dyck J., (1971)
Structure and Spectral Reflectance of Green and Blue Feathers of the
Rose-Faced Lovebird (Agapornis Roseicollis)
Biol.Skrifter Vol.18 no.2; p.p.5-65
[8] Dyck J., (1977)
Feather Ultrastructure of Pesquets`s Parrot Psittrichas Fulgidus
Ibis Vol.119; p.p.364-366
[9] Dyck J., (1976)
Structural Colours
Proc.Int.Ornith.Congr.; p.p.426-437
[10]Nissen T., (1958)
Elektronenmikroskopische Untersuchungen des Melanotischen Pigments in der
Feder des Normalen und Albinotischen Wellensittichs
Mikroskopie Vol.13; p.p.1-24
[11]Prum R.O., Torres R.H., Williamson S., Dyck J., (1998)
Coherend Light Scattering by Blue Feather Barbs.
Nature 396, p.p.28-29
[12]Prum R.O., Torres R.H., Williamson S., Dyck J., (1999)
Two-dimensional Fourier Analises of the Spongy Medullary Keratin of Structurally
Coloured Feather Barbs.
Proc. Royal Society London B 266, p.p.13-22
[13]Raman C.V., (1935)
The Origin of the Colours in the Plumage of Birds
Proc. Indian Acad. Sci. Sect.A, p.p.1-7