In this discussion on Spotlighting (birds and other animals) I think it
should be recognised that there are two very distinct aspects to the topic:
1/ the likelihood or otherwise of causing eye damage (temporary or
permanent) to the 'spotlighted creature;
2/ the likelihood or otherwise of causing stress to the animal/s and thus
interfering with their wellbeing in one way or another.
Claims are regularly made that shining strong lights into the eyes of birds
and mammals will damage, to some degree at least, the creature's eyes.
I don't believe that anything I have read or heard actually proves that
claim. Care should be taken when reading documents such as the one which
mentions Robyn Wilson's observations on the impacts of human disturbance on
possums. (See:
http://www.wettropics.gov.au/res/downloads/info/possumsSpotlight.pdf )
In that document there is no claim that the powerful spotlights actually
caused harm to the possums eyes. There is no claim that the head movements
of the possums were caused by the light being shone directly into their
eyes.
It seems to me that the stress assessed as being caused to the animals
may have been due to a feeling by the possums that they were being exposed
to possible predators rather than from disruption to their vision. It was
claimed that powerful spotlights penetrated further into the bush thus
giving the animals earlier warning that the humans were coming. Fear of
being exposed may have been the reason why the animals moved away from the
light rather than fear of having their vision disrupted. It may even be the
case that animals turn away to prevent their eyes shining and thus revealing
where they are. In my experience possums which turn their heads away from
the spotlight often simply 'freeze' and rely on their fur colour or pattern
to hide them from the potential predator.
It should also be noted that the spotlights used in the experiment were
so-called 30 watt units. Robyn's conclusions as quoted in the document
classify unfiltered 30 watt spotlights as having "higher intensity light".
The significant claim in the document, to my way of thinking, is that using
"lower intensity" (than that produced by an unfiltered 30 watt spotlight)
will benefit the animals by causing them less stress (without saying what
the actually cause of the stress is) and will benefit the observers by
allowing them to see more animals. There is no claim, one way or the other
that high-intensity spotlights cause or don't cause eye damage (to any
degree) to the animals. There is no claim that low-intensity spotlights
cause less disruption to the vision of the animals. There is a claim that
often the possums in these 'tests' stayed in view longer when a
lower-intensity spotlight was used in comparison to when a higher-intensity
spotlight was used. This does not prove (or disprove) that the animals
eventually moved away because the spotlight was causing problems with their
eyes or even simply because a spotlight was being trained on them. Any
conclusions from the 'tests' as described in the document can only be
considered as being subjective. Of course, the complete notes of Robyn's
observations might show more conclusive evidence one way or the other.
As others have indicated, the term "30 watt", "50 watt", etc, is meaningless
as far as light intensity is concerned. The wattage value attributed to a
spotlight is only an indication of the power being consumed from the battery
and not an indication of the power of the light-beam being produced. The
intensity (and colour) of the light-beam will depend on the technology of
the 'bulb' fitted to the spotlight. Cheap tungsten filament bulbs will
produce less light from 30 watts than will dearer 30 watt Halogen bulbs.
Ultra-bright LEDs which consume 1 to 5 watts from the battery are capable of
producing light as powerful as a 'conventional' bulb which consumes many
times that power. In fact, 2 or 3 watt Super-bright LED equipped spotlights
would probably be ideal for use by the 'paying customers' involved in
night-time spotlighting safaris.
I am interested in the concept of a variable 5 to 100 watt globe for a
spotlight.
It is interesting reading the comments from the 'professional' members of
BirdingAus who regularly lead groups on spotlighting safaris. At this stage
their observations seem to be the closest we are likely to get to the facts
for some time to come.
I don't think I have any more to add to this discussion.
Cheers
Bob Inglis
Sandstone Point
Qld
===============================
www.birding-aus.org
birding-aus.blogspot.com
To unsubscribe from this mailing list,
send the message:
unsubscribe
(in the body of the message, with no Subject line)
to:
===============================
|
