The appended message from Jack Pettigrew, posted to birding-aus by
Peter Woodall in 1997 - is still the most useful&informed opinion
I've seen on the impact of spotlighting.
Andrew Taylor
Dear Peter,
There are no papers of which I am aware on the effects of spotlights on
bird vision.
Some invertebrate photoreceptors degenerate completely when exposed to
bright light, but this is not described for any vertebrates.
1. Bird Retinas Can Tolerate the Sun: All birds have retinas that are
adapted to work in bright light, with 3 or 4 varieties of cones that permit
colour vision better than our own. Perhaps surprisingly, this is true even
for owls and other nocturnal birds, all of which have much better colour
vision than nocturnal mammals. The sun produces a bright image on the
retina that is orders of magnitude brighter than that produced by a
spotlight. Birds can fixate the sun, putting that image directly on the
retina, with relative impunity compared with mammals.
These observations all support my own view that permanent damage
will not be caused to the retina by using a spotlight on a wild bird.
2.No Empirical Evidence for Worry: In my numerous field trips to study
birds on the Diamantina (letter wing kites, plains wanderers, barn owls,
inland dotterels), I never noticed any special problems using both 50W and
100W spotlights. Captured birds seemed OK after being exposed. Retinas of
birds that were subjected to bright spotlights looked normal in those rare
cases where the spotlight-captured bird provided histological material soon
after exposure.
3.One Hour of Blindness: Retinal rods take 30-40 min to dark adapt (i.e.
achieve their maximal sensitivity so they can detect single photons) after
being exposed to bright light. This is true for both birds and mammals. A
nocturnally-hunting bird would therefore be out of action in dim light for
around 1 hour after being exposed to the spotlight. If that hour were
crucial for survival, or if another team came later and compounded the
problem, then spotlighting would have deleterious effects that night.[I
tried to capture my letter-wings after they had finished hunting (a
relatively easy task with this bird because it hunts with the moon) and
this helped to minimise the impact of spotlighting on birds that were
feeding young. The impact of being effectively blinded for an hour after
being spotlighted would have to be judged carefully for each bird, based
upon knowledge of their natural history and routines.
Another indirect way that the blind period after spotlighting might
compromise the subject is via an effect on susceptibility to predation. I
would not be surprised to learn that predation risk was increased during
this blind period. To keep this risk in perspective, however, note that the
more important senses for evading predators are the mechanical senses, like
vibration sensitivity, touch and hearing. These have much shorter reaction
times than vision, which is sluggish to the extreme in dim light and may
not be missed as much as one might think in the area of predator avoidance.
]
<Peter, I don't might if you disseminate this so long as it is properly
attributed..my address below...and so long as you make sure to let me check
it again if it is modified in any way>
All the best
Jack
JD Pettigrew FRS
Professor of Physiology and Director VTHRC
Vision Touch and Hearing Research Centre
Ritchie Research Labs, Research Rd
University of Queensland 4072
Australia
fax +617 3365 4522
office +617 3365 3842
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