Effects of flash photography on owls.

The effects of night-time flash photography on nocturnal birds, such as owls, is often a matter of debate with nature photographers. Those who frequently practice such photography claim that no harm is done. Others are not so sure. It's hard to find any authoritative references on the subject. Here are the best references that I have found to date. They were publicly posted on bird listservers and so I assume it's OK to reprint them here. If anyone knows of other references, please bring them to my attention via email.

From an an Australian bird listserver:

"There was much comment but little hard data to go on so I asked Prof Jack Pettigrew for his comments. He is both an expert on vision and interested in birds.

I have copied his reply in full below. Please respect his wishes that it shouldn't be modified in any way without his agreement and secondly that it be attributed to him if it is copied elsewhere.

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.

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"

Also posted to a birder list was the following statement:

"Dr. Graham Martin has been described as "the world's leading authority on nocturnal birds and one of the world's foremost vision researchers".

From Dr. Martin

I have dabbled with owls in the past, especially their vision. I have often been asked about the truth of similar statements as regards owls and other nocturnally active bird species such as shorebirds and wildfowl.

In the latter, bright lights are sometimes used in the process of "lamping" to catch birds at night. In this a powerful beam is scanned around a field until a foraging bird is spotted and then the bird held in the beam while it is approached and a net dropped over it. In these birds the bright lamp clearly disrupts their behaviour, presumably because these birds are fully dark adapted and sensitivity is dramatically lost when the bright light is shone into the eyes.

However, this may not be the situation which the photographers are arguing about. Some species of owl will actually exploit artifical light for hunting at night and it is possible for filming purposes to train an owl to hunt for prey in a pool of light at night. Thus it is not the light per se but the disruption of dark adaptation which is the essential problem. Whether flash will disrupt dark adaptation sufficiently to influence on-going behaviour will depend upon flash duration and brightness, and the state of dark adaptation of the retina.

Certainly birds do respond to flash. I have used the single flash from a flash gun to move shorebirds towards mist nets at night, the flash causes sufficient alarm to cause a foraging flock to take off and fly into the nets.

I have flash photographed captive owls and they seem to be undisturbed, but they will even stare straight into a photoflood light, which must erradicate any dark adaptation and leave them functionally blind for a couple of minutes. Therefore this is not a good indication of whether photographic lights will disrupt natural behaviours.

In view of all this, it seems reasonable always to err on the side of caution in these matters. The scotopic spectral sensitivity and rate of dark adaptation of avian retinas are very similar to those of mammals, including ourselves. Thus if our own dark adapted vision is disrupted by the flash used then it is wise to presume that the bird's dark adaptation will be equally affected. If we are temporally "blinded" by the flash then so also will be the bird for a similar length of time. For example, if you knock out a fully dark adapted retina assume it will take up to 30min for full sensitivity to be recovered.

Remember also that disruption of dark adaptation is never likely to occur in nature. Ambient light levels change through dusk and dawn at a relatively slow rate. Even at the equator, where dusk light levels change most rapidly, the rate of change is more or less in step with the rate of dark adaptation, and so under natural conditions a bird will always be well adapted to the ambient. With the exception of entry from a dark roosting site into full day light, there are few, if any, natural situations where marked light level changes are experienced and hence dark adaptation disrupted.

Obviously during day-time when the retina is light adapted a brief photoflash may not even disrupt the state of retinal adaptation.

I hope this furthers the debate.

_________________________________________________
Graham R. Martin BSc, PhD, DSc.
(Reader in Avian Sensory Science)
(Head of School)
Address:
School of Continuing Studies
The University of Birmingham
Edgbaston
Birmingham B15 2TT, UK."

A Usenet Anecdote

Here is a report from Usenet. I personally cannot vouch for its veracity, but I offer it here for consideration.

"I heard from a well known bird photographer (name withheld for the
protection of the guilty) that they set up some flash units to
photograph an owl returning to its roost in a church spire. The
bird tripped the infra-red beam on approach and the photographer
got one of those shots to die for. The owl had its wings fully extended,
exposure and focus were perfect. But the owl crashed into the stonework at
some considerable speed and fell to the ground badly injuring itself.
Obviously the flash dazzled and/or frightened the bird to the point that
it could no longer see well enough and/or was too jittery to land
safely."

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