A Review of accounts of luminosity in Barn Owls Tyto alba

Paper by Fred Silcock 2004-06-04, last updated 2006-06-07

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Barn Owl plumage and site of luminosity

The large amount of white in the Barn Owl is a puzzle to some experts (Bunn et al. 1982). White would seem not an ideal colour in a bird that hunts at night. So much white does the Barn Owl have that the species is sometimes referred to as the White Owl (as by Shakespeare and Tennyson; Purdy 1908; Hollands 1991). But there is no colour better than white for the reflecting of light, nor a better colour (one that would not hinder the passage of light) if the light source were intrinsic and coming from the bird's skin, then having to pass through a baffle of feathers. If half a dozen layers of white linen, for example, are placed over the head of a torch a definite glow will result.

'A Country Teacher' (Anon. 1907), who saw luminescing Barn Owls at close range, observed that 'The beautiful gleam appeared to come from the breast and undersides of the wings and body'. These parts are the whitest to be found on the Barn Owl and are the most conspicuous when the bird is in flight. If fungi were responsible for luminous Barn Owls, why not also on the owls' backs or uppersides of the wings, which presumably would come into contact with the insides of hollows?

The paleness of the owl's breast and underwings might suggest that it is simply better reflectance of these parts that makes them seem to be luminous, in contrast with the darker back and surroundings. This possibility would be rendered plausible or not by comparing the times and weather conditions when luminous owls and Min Mins were reported. If reported on dark, moonless or overcast nights well distant from artificial lighting, and several hours after sunset, the case for luminosity is rendered more plausible.
If they tend to be seen at dusk, in inhabited (and hence illuminated) regions, or at night with a full or bright moon, the case for luminescence being reflection of ambient light is more likely. Regrettably, pertinent details are unavailable, other than that some Min Mins were seen on very dark nights in remote areas. Again, reflectance of ambient light does not explain the owls' apparent ability to control the light.

The circumstances of Barn Owl luminescence

If we are dealing with true and perhaps intrinsic bioluminescence in Barn Owls, might a clue of some kind lie in the observation of 'A Country Teacher' (Anon. 1907) that the glow
'always appeared when the birds were in poor condition'? Rolfe (1908, cited by Pigott 1908) said that after he shot down the light he found a 'poor old, half-starved barn owl'. In one Australian case a light in a barn was investigated and nothing found, but next morning a dead Barn Owl in emaciated condition was found on the floor. There may have been no connection between the light seen the previous night and the dead bird, but in this context the incident is noteworthy. In another case reported to me, after a display of luminosity by a bird, thought to be an owl, the bird had trouble rising from the ground in attempting to become airborne. Could its difficulty have been caused by physical weakness or other handicap? Minutes before, it had been observed feeding on insects attracted to its light. Insects are not the food preferred by Barn Owls, but might insects be acceptable if the birds' hunting skills have been impaired? The bird seen shut off the light quickly in effecting retreat. Observation occurred from about 5 metres. This bird's control of the light suggests true bioluminosity.

It seems that most Min Min sightings occur in autumn and winter, and notably in severe drought. This is significant if the Barn Owl hypothesis as the cause of the Light has credibility.

Details of winter hardships suffered by Barn Owls in Australia are given by Fleay (1968) and Hollands (1991), and in England and Scotland respectively by Bunn et al. (1982) and Taylor (1994). Infection by a luminous bacterium, in weak or moribund owls, might account for luminosity, but no such terrestrial species has been described and there remains the problem of the owls' apparent ability to control the light. Infection from bill to preen gland to feathers is inconsistent with the lack of luminescence on the dorsal plumage.

Mention has been made of a luminous bird seen feeding on insects apparently attracted to the light (Wharton, above). Could this luminosity be of value in the hunting of ground prey (rats and mice)? A luminescing Barn Owl was seen to catch a rodent (Gurney 1908), and many Min Mins have been observed behaving in a manner very similar to that of a Barn Owl in hunting mode. Perhaps prey are blinded by the light, which could conceal the approaching owl.

If true bioluminescence is involved, that is, luminescence coming from organs or substances designed to produce light, could there commonly exist the situation in which the species, or some members of it, rarely have any need to put this ability to use? Thus, generally, not many luminous Barn Owls are seen. The hunting skills of the Barn Owl are considerable and if the bird uses luminosity as a hunting device, as I suspect it does, one can understand the bird not often resorting to its use under ordinary circumstances. There might be a parallel to this among fishes. The Midshipman Fish Porichthys notatus in some areas is not luminous because it needs a specific dietary source of its luciferin, the chemical required for bioluminescence (Warner & Case 1980). Where the right prey species occur, the fish is luminous; where they do not, the fish is not luminous yet is still capable of thriving. So why not the Barn Owl too?

Discussion

Although circumstantial and anecdotal, there is strong evidence that Barn Owls do luminesce and it is possible that they are intrinsically bioluminescent. There is a precedent concerning a parallel biochemical. The 'contact' chemical defence, homobatrachotoxin, is known in a vertebrate class (the amphibians) and otherwise only in a single bird genus, Pitohui (Dumbacher et al. 1992). It is therefore possible that luciferin, present in one vertebrate class (the fishes), could occur otherwise in a single genus of another vertebrate class (Tyto among the birds), as luciferin is widely distributed among invertebrate classes and phyla. One possibility worth examining, regarding the means by which the bird might bring about luminescence, is that of a skin secretion released onto the feathers (as happens with homobatrachotoxin). Luminescing earthworms use a similar system (Jamieson & Wampler 1979).

When I began this research, approximately four years ago, it was in the study of the Min Min Light. The study led me to a bird and, in time, the Barn Owl, and that was before I came by any knowledge of that species' history of luminosity in other countries. I have not yet come across a report on luminosity in any owl species but the Barn Owl. The information available so far gives ample reason to persist with investigations, in order to prove whether the Barn Owl is capable of luminosity and to determine the source of the fabled Min Min Light.

To return to the four premises at the start of this paper: the fourth cannot be correct if the second is not true. At this stage it is a matter of opinion whether the first premise is true, a situation which can only be addressed by scientific research.
As the reports exist and there must be an explanation for them, the goal of this paper is to stimulate that research.

If intrinsic luminosity in the Barn Owl is proven, one must wonder what stage it is at: is the phenomenon coming or going, is it a characteristic still in development, or is evolution taking it in the opposite direction? Where was it at, say, a thousand years ago? Where might it have been at any time during the 12 million years that genus Tyto is thought to have existed (Bunn et al. 1982)? Might other species of owl have possessed luminosity at one time?

Future research

Opportunities for field work have been limited to date. Thus far, scientific assistance,
particularly in the branch of bioluminescence, has been almost impossible to obtain. Studying live birds under controlled conditions, probably involving simulation of natural habitat, seems an enormous task. The opportunity to observe Min Min Lights may be much reduced in modern times. The man who once walked or rode or drove a horse to where he wanted to go now usually travels by motor vehicle and frequently at speeds that allow little opportunity to observe anything but the road ahead. For much information on Min Min sightings I am indebted to retired drovers, men who spent weeks or months at a time living out of doors. Droving, to the extent that it was once known, is a thing of the past, road trains having largely taken over the drover's work.

The next step is to test fresh Barn Owl tissue, in the laboratory, for the presence of luciferin and to search Barn Owl skin microscopically for photophores, the organs of bioluminescence. Feathers should also be tested, for unusual structural features and for remains of fungi and bacteria. The first place to look would be the skin of the breast. In order to do this, a supply of fresh Barn Owl carcasses is needed, as is a scientific institution willing to conduct the tests. Barn Owl bodies are readily available as road kills and through the wildlife rehabilitation network if birds die or are euthanatised.

Poultry experts canvassed have reported no cases of luminosity in poultry. Perhaps keepers of raptors, in zoos and similar institutions, might look for luminosity in Barn Owls and other species. If the phenomenon can be demonstrated in captivity, then further research and support might be forthcoming. Similarly, contemporary Min Min sightings require much better documentation in terms of precise dates, times, weather conditions, moon phase and visibility, ambient lighting conditions, and proximity to artificial light. If reflectance of ambient light can be ruled out, then the luminous owl hypothesis is more plausible.
In closing, the following quotes are worthy of consideration:

It may be worthwhile, however, to point out briefly what is known concerning the use of light to the animal. The light may be of no use whatever, purely fortuitous, an accompaniment of some necessary or even unnecessary chemical reaction. This appears to be the case in the bacteria and fungi and perhaps the great majority of forms which make up the marine plankton, Noctiluca, dinoflagellates, jelly-fish, ctenophores and even the sessile sea-pens. We know that luminous bacteria occasionally lose the power of lighting and that in certain culture media they develop as non-luminous forms. Luminescence is not indispensable to them. (E. Newton Harvey)

The purpose or the necessity of luminosity to fungi? None that we know of. (Dr Tom May, Royal Botanic Gardens, Melbourne)

Unnecessary though luminosity might be to certain organisms, I see no reason why some might not be able to find a use for it. It is certainly functional in deep-sea organisms and luminous insects. Perhaps the final word should go to Purdy (1908):

I have recorded these observations merely in the hope that some naturalist may be found whose scientific attainments will enable him to elucidate the cause of a highly interesting phenomenon, and thereby refute the contemptible assertions of those who pour ridicule on everything they have not seen themselves, to the great detriment of scientific research.

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