From: Kevin A.
Date: Wed, 15 Feb 1995 01:49:43 -0500 (EST)
To: Jessica Morton <jmorton>
Subject: Re: humming birds

Dear Lola,

Hummingbirds don't actually change their colors with the weather. It's
the weather, the light, and the angle at which you look at them which
makes them LOOK like their colors change. Most animals, with the
exception of perhaps some underground creatures - like earthworms and
moles - use chemicals called pigments to color themselves. (Pigments
give us our colors, too - people with darker skin have more pigments than
people with lighter skin) Some animals, like squid and lizards ACTUALLY
CHANGE COLORS. This means that sunlight causes a chemical reaction in
the pigments, causing them to turn either darker or lighter depending on
how bright the sun is. Irridescence, as I'll explain, is different than
a chemical change in the pigment. So really, hummingbirds don't change
colors; it's just an illusion.

Hummingbirds demonstrate an interesting development in birds known as
IRRIDESCENCE - it's just a big word for shiny. Irridescence is
interesting for several reasons. One is because the shiny effect it
causes is NOT from the color of the bird! Instead, it is from the
structure of the feather. Irridescent feather are different from other
feathers in that they do not reflect light normally (in a plane) like
normal feathers do. Rather, they reflect light only at narrow viewing
angles, and irridescence can only be seen close-up. (I bet you've never
seen a shiny hummingbird from 3 miles away!) You're probably a little
confused, so I'll try and draw a picture of what I mean:

Normal feathers:
-------
reflected rays
Light rays /
\ /
\ /
\ /
__ __
/o \__/ \
<- \\
\__________\
|| \\
/\
Irridescence:
------
Light rays reflected rays
\ /
\ /
__ \ /
/ o\ \/
<- \---\
\__ \
\_________\
| | \
| |

What the picture is SUPPOSED to be showing is that the second bird has
irridescent feathers which reflect light which looks SHINIER, the angle
at which you can see it is smaller. This explains why, when the light on
the bird changes, it looks like somebody has turned its electricity off.
The bird's appearance can go from bright to dull almost instantly! This
could be helpful to a hummingbird which is being hunted. If you were a
predator, sneaking up on a hummingbird, and suddenly the sunlight shone
at just the right angle and made it irridescent, wouldn't you be
surprised, and perhaps scared off? Another theory is that irridescent
birds tend to live high in the treetops, rather than down low
(Stratification, remember?). Up in the treetops the light is much
different, and brightly colored birds blend in better. With irridescent
birds, they have an added advantage: when they feel they need to stop
shining, they can just come in out of the sun, down to the lower levels
of the forest. Irridescence is probably used by the birds which express
it in courtship and mating displays, since in the majority of irridescent
birds, it is the male which shows off, while the female is less brightly
colored.

Thanks for asking, Lola. Keep on studying those birds!!

Best wishes,
Kevin