Subject: Birds in the UV spectrum
Date: Nov 19 21:39:22 2000
From: John Chapman - ragweed at igc.org


Excuse the delay in this post, but. . .

There was a fascinating article in the October 2000 issue of the journal
BioScience (Vol 50, No. 10) about birds and their ability to see in the
near UltraViolet spectrum. The article, written in a news format, covered
the work of several researchers examining the use of UV reflective plumage
as well as other aspects of birds ability to see UV light.

To offer a quick synopsis of the points it covered:

- Most mammals, including humans, have three types of color receptors
(cones) in our eyes - red, green and blue. Birds, as well as most reptiles
and possibly most other vertibrates, have a fourth cone that allows them to
see in the near ultraviolet spectrum - adding a UV dimension to their
vision and thus their perception of the world.
(in fact, it is hyphothesized that UV reception is ancestral to all
tetrapods [four-limbed vertibrates] and that early mammals lost the ability
because they were primarily noctournal. A few rodents have UV vision, but
it seems they re-evolved.)

- Several species of birds have been found to have plumage that is UV
reflective (and many more might also - but so far only a few species have
been studied). In some cases the UV reflective plumage is different from
the "visible" light plumage or is highly significant.
For example - European Starlings, who show no sexual dimorphism in their
"visible" light plumage, have distinct male and female plumage in the UV.
Blue Tits, another European species, have a blue crown on their heads
which is also highly UV reflective - when researchers covered the crown
with sunscreen (that blocked out the UV reflection but left the color
otherwise intact) males were less attractive to females. They also found
younger males had less strong UV reflectiveness, though they looked the
same in visible light.

- However, UV is only one part of birds mating plumage or behavior. In many
cases, UV reflective plumage is identical too or redundant with other
aspects of their plumage. In some species it seems to matter less than others

- Birds ability to see in the UV spectrum also may have a role in food
selection and predator-prey interaction. Some berries, for example, have a
waxy UV-reflective outer coating that may be an indication of ripeness.
Flowers often have UV-reflective patterns that have long been known to help
attract insect pollinators, but they may also work with hummingbirds.
Insect-bird interactions in the UV are also suspected (most insects have a
significant degree of UV vision), though little research has been done.
One of the most fascinating is that certain species of raptors (Eurasian
Kestrels and Rough-Legged Hawks) have been shown to be able to detect UV
reflective chemicals in the urine and feces of voles and can use this to
identify areas of high vole density.

The article is really worth reading.

- John

PS. BioScience is available at the UW library and Seattle Public Library.
It is available online at www.aibs.com, but this article is only available
to subscribers.
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"When my... students ask 'what does this color
look like to a bird?' I have to answer 'You will
never know, you cannot know.' It's like asking
what the music of bats sounds like"

- Prof. Richard Prum, BioScience, October 2000

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John Chapman ====================================================
Seattle, Washington =============================================
ragweed at igc.org =================================================