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). 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 . 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 .
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.
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:
 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  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  Auber L., (1941) The Colours of Feathers and their Structural Causes in Varieties of the Budgerigar, Melopsittacus undulatus [Shaw] Thesis; p.p.1-137  Dyck J., (1985) The Evolution of Feathers Zool.Scripta Vol.14 no.2; p.p.137-154  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  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  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  Dyck J., (1977) Feather Ultrastructure of Pesquets`s Parrot Psittrichas Fulgidus Ibis Vol.119; p.p.364-366  Dyck J., (1976) Structural Colours Proc.Int.Ornith.Congr.; p.p.426-437 Nissen T., (1958) Elektronenmikroskopische Untersuchungen des Melanotischen Pigments in der Feder des Normalen und Albinotischen Wellensittichs Mikroskopie Vol.13; p.p.1-24 Prum R.O., Torres R.H., Williamson S., Dyck J., (1998) Coherend Light Scattering by Blue Feather Barbs. Nature 396, p.p.28-29 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 Raman C.V., (1935) The Origin of the Colours in the Plumage of Birds Proc. Indian Acad. Sci. Sect.A, p.p.1-7