Re: feather color

[Leigh Palmer <palmer@SFU.CA>]

At 8:38 PM -0600 1/9/01, cliff parker wrote:
> I was showing off some unique holiday ribbon today to several
> colleagues.  This ribbon looks green when seen from one angle and
> red when the angle is reversed.  Very Cool!  On the ribbon this
> is caused by the weave showing one thread when viewed from some
> particular perspective and another thread when viewed form the
> opposite direction.  One of the Biology teachers said that this
> was like the feathers on some birds, in particular a Mallard
> Duck.  He indicated that the color was not due to a particular
> pigment but rather some other physical characteristic causes the
> light reflected back by the feather to be different based on the
> angle at which it is viewed.  I have to admit that I know
> virtually nothing about birds especially ducks so this has left
> me rather stumped.  Can anyone explain to me what would cause
> this kind of effect in the feathers of a bird?

The mallard colors (and those of hummingbirds, morpho butterflies,
etc.) which are angle dependent are interference colors. They do
not resemble in mechanism the ribbon you mentioned. Here in Canada
we have a patch on most of our bank notes which exhibits the same
phenomenon, but over a wider range of colors. Depending on the
illumination and viewing angles, the patch can go from yellow to
blue violet. The special quality of these colors derives from the
fact that they are nearly spectral, and also the they often look
to be different to each of the two eyes. The former property is
called "iridescence", and the latter "chatoyance". The etymologies
of these words are interesting, too!

Leigh