Digest Number 57

Richard Beban <beban@...>

Dan: Try http://nmnhwww.si.edu/BIRDNET/OC/OCinfo/OCBv1n8.html, which
is more about tower lighting, but talks about bird mortality,
particularly in fog, when birds fly toward lights.

There's also http://www.fws.gov/~r9mbmo/issues/towers/beason.html, which
is also more about tower lighting, and is inconclusive, but contains
anecdotal evidence. It says:

...avian mortality at communication towers occurs when the birds
hit a tower or its guy wires. The rate of collision increases as birds
are attracted to the tower or become disoriented near
the tower and fly in circles around it, getting repeated chances at
hitting the guy wires. Two aspects of the tower that
might potentially affect its attractiveness are its illumination and the
FR signal that is transmitted by the antenna itself.
Light can have behavioral effects on the birds through two sensory
systems: the visual system and the magnetic
perception system - the magnetic compass. Color perception in birds is
much more complex than it is in humans. Birds
have 4-6 different types of color receptors, or cones, where as humans
have only 3. The avian photoreceptor itself is
more complex than in humans and other mammals. In addition to the visual
pigments, birds also have an oil droplet in
their inner eye segment that acts as a filter determining which light
reaches the photo pigments themselves. Each
photoreceptor has one oil droplet and one photo pigment or visual
pigment. So far, of all the avian species that have
been examined, all of them have a very narrow, very sensitive channel in
the red spectrum. This is of interest because
most of the illumination that is put on towers is in the red region.
This red cone has a peak sensitivity of about 600 nm,
which is what we call a reddish orange. By comparison, the human red
cone has a peak sensitivity of about 560 nm.
Depending upon the species of the bird, they either have an ultraviolet
sensitive cone, or a violet sensitive cone that is
totally missing in humans and most mammals. In fact, humans have oil
droplets in the lens that filter out the ultraviolet.
So birds can see ultraviolet and apparently have specialized receptors
for detecting it. It varies from species to species,
but there are 2 or 3 additional receptors that might be analogous to
what we call blue, the green and the yellow
wavelengths. In the Bobolink, one of the species I work with, these
peaks are at 460, 535 and 570 nm. Humans, for
comparison, in addition to the red cones have the blue and green cones
that are at 430 and 530 nm.

Of the 10,000 species or so of birds, depending on whose taxonomy you
want to deal with, we know the photo
pigments or the visual pigments and associated oil droplets for exactly
11. Only two of these are nocturnal migrants in
the Western Hemisphere: the Bobolink, again the species I work with, and
the Mallard. Another is considered to be a
diurnal migrant: the European Starling. Partial information is available
for a few other species, but for very, very few,
and most of this is simply limited to oil droplets information.

We don't know the spectral sensitivities of the visual pigments with
which they are associated. Consequently, we know
very little about what colors birds can actually detect and how well
they can differentiate between colors. The visual
pigment of the rod for comparison is very similar to the human rod
pigment with a peak of around 500 to 510 nm, in
the green range. Birds have very large rods, at least the species that I
have looked at so far, which means that they have
very good night vision � they have very good sensitivity to moving
around at night. The rods lack the oil droplets; they
have only the visual pigments, which makes sense if you want to have
something that is very sensitive to light.
Illumination at specific wavelengths of light might affect a taxis-like
response, whereby the bird is attracted to the light
or the communication tower. There are anecdotal reports that the
attraction of birds to lights is strongest in adverse
weather especially in fog, as Todd pointed out previously. The
attraction of birds to these lights might simply be an
escape response, whereby the bird flies towards the brightest part of
the night sky, which under natural conditions would
represent the moon. Flying towards the moon would simply get the bird
above any fog or low-lying clouds and out of
any potential problems.

Two aspects of tower lighting that can attract birds are its color
(white lights, ultraviolet, or specific wavelengths) and
the duration of light (strobes, flashing lights, or steady lights) as
pointed out previously. Both these aspects remain
unresearched. Unfortunately, there have been no controlled experiments
as to which colors birds find most or least
attractive. Anecdotal reports, again as Al has pointed out earlier, are
that white lights seem less attractive that red lights,
and strobes might even be less attractive, but we really don't know.

A second avenue of the influence of light is disorientation that is
caused by the disruption of the magnetic compass.
Long wavelengths of light in the red and orange part of the spectrum
have been shown to produce disorientation, or a
change in the direction of orientation, in the 5 species of migratory
birds that have been tested. This long wavelength
illumination interferes with the magnetic compass of the species, but it
isn't known what the birds might do if other
sources of information, such as stars, were available at the same time.
The mechanism by which the wavelengths of
light influence magnetic orientation is not known either. There are a
couple models put out, but no one has been able to
validate or invalidate any of them. All experiments that have been
tested with migratory birds have been done with very
narrow band filters or LEDs and researchers have only looked at the
particular wavelengths that were of interest. These
might resemble the conditions that a bird would encounter during fog or
inclement weather when it was flying very near
to a communication tower that was illuminated by say, red lights. Under
normal conditions, in addition to this red light
from the tower, the birds would also have starlight and perhaps even
moonlight. Whether this additional illumination
would simply cancel out or negate the effects of the red illumination on
the magnetic compass isn't known. No one has
looked at it. Disruption of the bird�s navigation system and the
magnetic navigation system might occur with either red
lights or the RF signal if it were to interfere with the bird�s ability
to detect the magnetic field. If this resulting
disorientation causes the birds to circle, to be unable to establish its
directional cues, it would increase the probability
of striking either the tower or the guy wires.

From: Dan Murphy <murphsf@att.net>
Subject: Help! They're turning on the lights.


Is anybody out there familiar with impacts of night lighting on birds,
plants or other organisms?