Is Fluorescence Possible in Twilight?

[FeatherGlow]

TL;DR Summary

Absent moon and light pollution, with the sun around 14 degrees below horizon, could 470nm fluorescent blue light effect exist?

Earlier this year I saw what appeared to be a large glowing bird at twilight. The normal explanation for this is bioluminescent fungus contaminating a bird.

I’m wondering if the effect might actually be a blue light effect where a fluorescent bird only appeared to glow because of the lighting conditions.

Amphibians have recently been discovered to glow under blue (450nm) lights. Many birds are likewise fluorescent.

I’m linking to a study on how the light changes in twilight.

By my reading, a rural area with no moon (conditions of my sighting) will have the sky dominated by 470-ish wavelength blue light.

Is it possible that the Sun can become like a blue light, illuminating fluorescent birds?

Would clouds and humidity have an impact on this effect?

https://www.nature.com/articles/srep26756

 

[hutchphd]

TL;DR Summary: Absent moon and light pollution, with earth around 14 degrees below horizon, could 470nm fluorescent blue light effect exist?

The normal explanation for this is bioluminescent fungus contaminating a bird.

Can you supply a reference for this? Does the bioluminescent fungal stuff also fluoresce?
You mean the sun 14 deg below horizon?

 

[Baluncore]

Welcome to PF.

Is it possible that the Sun can become like a blue light, illuminating fluorescent birds?

The sky looks blue because short wavelength blue light is scattered more than longer wavelength light. I would expect the sky to be more blue after sunset. If a pigment fluoresced under blue light, then that would be the only source of longer wavelength light after dusk.
One question would be if our eyes could detect the colour of florescence when the light level was low, as our human vision is monochrome in low light.

Would clouds and humidity have an impact on this effect?

Yes. Clouds would reduce light levels.
When away from city lights, it is possible to walk around by starlight in light forest.

 

[BillTre]

Fluorescence happens when a photon released from molecules that have been "activated" by a higher energy photons (or combination of them).
Bioluminescence (as I understand it) is based on molecules emitting photons after the molecules have been "Activated" chemically, not by light.

I don't recall fluorescent birds, but lots of bird's feathers can act as structural pigments that produce iridescence which can be quite shiny under the proper conditions.

In my experience iridescence is generally brighter that fluorescence and is more easily seen in non-dark conditions.

A distant bird out shining its surroundings might be easier explained by iridescence.

 

[Baluncore]

A distant bird out shining its surroundings might be easier explained by iridescence.

That might depend on the image sensor employed. The human eye has a different spectral sensitivity than does a sensitive starlight colour camera.

 

[FeatherGlow]

Can you supply a reference for this?

Please read this detailed description of a luminous bird encounter carefully.

It shows ornithologist assumptions to explain the phenomenon.

But it also gives a detailed account of the twilight conditions that I believe might be a "natural blue light" event in which both fluorescent and bioluminescent things were glowing.

https://sora.unm.edu/sites/default/files/journals/auk/v075n02/p0224-p0224.pdf

Does the bioluminescent fungal stuff also fluoresce?

I have seen dead wood fungus fluorescing in by backyard with UV flashlight.

But I don't think anyone knows much about natural fluorescence.

We just discovered that frogs and other amphibians glow under blue lights in 2020.

You mean the sun 14 deg below horizon?

Yes. Fixed that.

 

[FeatherGlow]

Fluorescence happens when a photon released from molecules that have been "activated" by a higher energy photons (or combination of them).
Bioluminescence (as I understand it) is based on molecules emitting photons after the molecules have been "Activated" chemically, not by light.

I don't recall fluorescent birds, but lots of bird's feathers can act as structural pigments that produce iridescence which can be quite shiny under the proper conditions.
View attachment 328244

In my experience iridescence is generally brighter that fluorescence and is more easily seen ini non-dark conditions.

A distant bird out shining its surroundings might be easier explained by iridescence.

Owls definitely glow under black lights

I have carefully reviewed all the old reports on glowing owls, which are often said to be glowing "red" while hunting during twilight on dark moons.

I am not persuaded by the hypothesis that they glow as a result of nesting in bioluminscent wood decay. They don't appear to glow all over. It seems more consistent with people seeing their florescence.

I suspect that the feather-down on herons is actually fluorscent, and that this explains what I saw. This is a hunch that is not documented. I suspect that birds have evolved fluorescnce for use in attracting prey.

Also worth noting, the firebird legends trace back to the Egyptian deity Benu, who was a heron linked to the Sun and named after the Egyptian verb "to shine."

 

[BillTre]

I suspect that the feather-down on herons is actually fluorscent, and that this explains what I saw. This is a hunch that is not documented. I suspect that birds have evolved fluorescnce for use in attracting prey.

Owls are stealthy predators. They do not attract prey they seek them out.
Herons normally don't want to be noticed. However, they do produce shade or deploy lures to attract prey fish.

For example, a lure:
https://www.physicsforums.com/threads/share-animal-pictures-for-animal-lovers.583729/post-6906455

 

[berkeman]

Herons normally don't want to be noticed. However, they do produce shade or deploy lures to attract prey fish.

Or fish with bait, as you posted in the Animal Lovers thread on Monday:

 

[FeatherGlow]

Owls are stealthy predators. They do not attract prey they seek them out.

I wasn't clear in the previous post.

Barn owls are known to intentionally reflect moonlight as part of their hunting strategy.

When I read old legends of barn owls occasionally glowing red while hunting in twilight, I suspect it could be fluorescence rather than secondary bioluminescence contamination.

https://phys.org/news/2019-09-barn-owls-moonlight-stun-prey.html

 

[berkeman]

https://phys.org/news/2019-09-barn-owls-moonlight-stun-prey.html

Interesting article, thanks.

 

[FeatherGlow]

Regarding that night heron...

This is from "Birds of Massachusetts" by E. H. Forbush in 1925.

 

[FeatherGlow]

My hunch is that the powder down on herons has undocumented fluorescence.

My hunch is that people can see this under certain twilight conditions. But it is rarer today because of light pollution.

 

[BillTre]

When I read old legends of barn owls occasionally glowing red while hunting in twilight, I suspect it could be fluorescence rather than secondary bioluminescence contamination.

If it were fluorescence, a higher frequency of light is needed to activate the florescent molecules.

Fireflies are bio-luminescent, not fluorescent.
No stimulating illumination at night, they use chemical energy.

My hunch is that the powder down on herons has undocumented fluorescence.

This would be easily tested.

 

[FeatherGlow]

Herons are well known for spreading this powder down all over themselves.

It is said that they do this to keep from getting fish slime on them.

I suspect that they do it because they have been fishing with lights and bait for millions of years.

 

[BillTre]

Herons are well known for spreading this powder down all over themselves.

It is said that they do this to keep from getting fish slime on them.

I suspect that they do it because they have been fishing with lights and bait for millions of years.

You have a lot of unsourced observations and weakly supported conjectures.

 

[FeatherGlow]

If it were fluorescence, a higher frequency of light is needed to activate the florescent molecules.

Fireflies are bio-luminescent, not fluorescent.
No stimulating illumination at night, they use chemical energy.This would be easily tested.

I don't have a heron to test it, but yes it should be easy.

I also think that time-lapse cameras taking pictures of various fluorescent pigments at twilight would be interesting.

 

[BillTre]

I also think that time-lapse cameras taking pictures of various fluorescent pigments at twilight would be interesting.

Personally, I would not expect this to work, due to:

• the reduction in overall lighting which is needed to activate molecules so they can emit their fluorescent photons and well as
• the light at twilight tends to be more red, which is the opposite end of the spectrum you would want to excite molecules.

But go ahead and try the experiement. Its a good thing to do.

 

[FeatherGlow]

You have a lot of unsourced observations and weakly supported conjectures.

True.

But I've reviewed many alleged observations of this stuff from the 19th and early 20th centuries.

 

[berkeman]

My hunch is that the powder down on herons has undocumented fluorescence.

My hunch is that people can see this under certain twilight conditions. But it is rarer today because of light pollution.

Just a quick reminder that we don't discuss hunches in the technical forums at PF. We discuss science as published in the literature and in mainstream textbooks. Speculation is not allowed (see the rules under INFO at the top of the page). Thanks.

 

[FeatherGlow]

Thanks, berkeman.

I am relinking the Nature article on light wavelengths in twilight to get this back on track.

My reading of this article suggests that early astronomical twilight in a rural area during the dark moon does have wavelengths dominated by the 470-ish nanometer range.

Am I misunderstanding this?

https://www.nature.com/articles/srep26756

I did test fluorescence in twilight under the dark moon this weekend with a negative result. But there was considerable cloud cover and maybe haze from Canada that was enhancing light pollution.

 

[BillTre]

470 nm is bluish I guess.
Shorter wavelengths would often be considered better, but it depends on the molecule.

I may not be understanding your argument correctly.
Is there supposed to be a special time of the day (early astronomical twilight) when this effect works, but not others?
If so, how long does that time last?
Does the bird have to be high in the sky to catch the right rays to be properly illuminated for stimulation?

 

[FeatherGlow]

I apologize for getting too speculative and argumentative.

I don't have the answers and am trying to determine if it is even possible and explore conditions under which it might be.

I hypothesize that a blue light fluorescent effect might occasionally happen in the twilight. But it would require a dark moon and little light pollution.

The Nature article, by my layperson reading, makes this hypothesis more plausible.

 

[hutchphd]

My reading of this article suggests that early astronomical twilight in a rural area during the dark moon does have wavelengths dominated by the 470-ish nanometer range.

It is interesting that the post twilight scans match very well with scans I remember of the sensitivity of the Rod cells (low light eye detectors) vs color. They peak at ~505nm if memory serves.

Also In no particular order:

1. Bioluminescence is common (usually firefly yellow green). I am most familiar with the "red tide" algae blooms in the Atlantic coast which often produce spectacular displays and of course fireflies which can also be pretty good
2. 470 nm is a fairly common blue LED color and so it should not be difficult to use them as a source for experiment . Also there are 405nm violet/UV laser pointers which excite many common fluors. Cheap.
3. Iridescence is an entirely different mechanism and uses wavelength dependence of diffraction (thin films or gratings) or refraction (rainbows) to produce very color-specific geometry of viewing. Probably not salient here
4. The Herons may know.....

 

[FeatherGlow]

I've got the lights, but haven't found a cooperative heron yet!

I'll eventually get a light on one. I see herons out there all the time, but they are wary.

But it could have been any bird.

I estimated it was larger than a turkey vulture.

In case anyone is exploring this someday. My sighting was on 2/19/23 in Blanco County Texas. I have trailcam pics of my truck entering the field at 6:58 PM and leaving at 8:03 PM. I'm not positive about the exact time beyond that interval. I was in a field, and the bird was flying above the trees at the edge of the field to my Northwest flying North. I'd estimate I saw him glowing for 5 seconds, and do not know if he stopped or dipped below the trees. It was shockingly bright, as if it had a lantern on its chest.

Weather reports show a cloudless night.

But the 6:58 trail cam pic is pointing about 10 degrees South of due West. The infrared b&w picture clearly shows visible clouds in the Western sky at 6:58P, about 10 minutes into nautical twilight.

This suggests that there were likely clouds reflecting blue light from the West, while the bird was flying against the dark, moonless, cloudless, rural night sky.