Thursday, March 01, 2012

What is infrared light, and why can't eagles see it?

We are starting to get a lot of questions about the night-time illumination of the nest, so I thought I would write a little bit about visible light, invisible light, and our IR illuminators. What we see - the world of visible light - is just a tiny fraction of all the types of light that exist. Infrared light, gamma rays, X-rays, ultraviolet light, microwaves and radio waves are all types of invisible light. Like visible light, they are different because the length of their waves is different.

The illustration below shows the electromagnetic spectrum in its entirety.  The IR portion hangs just off the red end of the visible spectrum.  It spans the frequency range from 780 nm to 300,000 nm (.78 to 300 um) and has a longer wavelength and lower frequency than visible light (click the image to embiggen it):
Illustration courtesy Wikipedia: 
http://en.wikipedia.org/wiki/File:EM_spectrum.svg

Birds have very different eyes than we do. They have four color cones instead of three, more cones and rods to see color and light with, a higher proportion of cones to rods (at least in diurnal birds), and different peak sensitivities to light. As the graph below illustrates, we have narrower spectral sensitivity than birds and are much less sensitive to certain wavelengths of light. However, like humans, birds do not see above about 700 nanometers. While some birds range into UV, IR light is as invisible to them as it is to us.
Illustration courtesy of "Color Vision in Birds": http://www.webexhibits.org/causesofcolor/17B.html
So how can we use an IR illuminator to see the birds if we can't see IR light?
Researching and writing about this stuff makes me want to read a trashy romance novel, but I'll put that aside for now and forge ahead. Here is how it works. The IR spectrum starts at about 780 nm. The LED IR illuminators used on our camera emit 'invisible' light at a wavelength of 850 nm:  well above the 700 nm visibility limit we share with birds, but well below the high-energy 'hot' end of the spectrum.  LED illuminators are also smaller and have lower requirements and range than bulb-type illuminators.
The camera and illuminators are located roughly five feet from the eagles. The illuminators shine IR light on the nest. The IR sensor detects the light and focuses it onto a CCD (Charge Coupled Device) imager chip, which has a spectral response of up to roughly  1,000nm. The chip maps IR wavelengths of light down into the visible spectrum via tiny electric charges generated by IR light falling on to an array of tiny sensor cells. Since the photo-sensitive cells don't distinguish between colors, the camera generates a monochrome image. I suspect it is really a LAB color space, but that is far outside the scope of this discussion. If you feel really nerdy, follow this link to learn more about LAB color spaces. There is a lot more to color than RGB, even for us trichromates! http://en.wikipedia.org/wiki/Lab_color_space.

Can the eagles hear or see the PTZ camera?
The cameras use servo motors to drive the camera along a 360 degree path, which minimizes the amount of time and work it takes to arrive at any given destination. These motors are very quiet - the camera was designed for surveillance - and encased in hard plastic. I suspect that the eagles can't hear the servos; certainly, we don't hear the servos and the microphone is much closer to the camera than to the eagles. If the eagles can hear the servos, the sound is not any more alarming to them than the other noises that make up their sonic landscape - traffic on the nearby road, people at the fish hatchery, wind, tree branches, the brook, the horses, and so on.

Although the young eaglets went through a period last year of seeming quite fascinated by the camera - we wondered if they were interested in their reflections - the eagles ignore it generally. Since it rotates around a 360 degree axis, there does not appear to be much movement (think of a spinning top) and the camera is behind a dome, which helps conceal it. If the eagles can detect movement, it isn't bothering them. Again, a lot of things around them move. Since the camera isn't trying to eat or threaten them, it doesn't really concern them.

In short...
Mammals and birds can't see IR light - to the eagles, the nest is dark at night - but our camera can. We use low power IR illumination devices that are roughly five feet from the eagles. They are not hot and do not emit dangerous radiation or noises. A chip inside the camera does the work of converting the IR image into visible light via tiny electric charges. However, because the chip does not detect color, the image is monochrome.

Experimenting with IR light
To see IR light for yourself, take a television remote, which works via IR. Press a button and look at the LED on the end. You won't be able to see anything. Now, take a digital camera or phone camera (I used an Android HTC), point it at the remote to take a photo, and press a button on the TV remote. You should now, through a digital device with a higher spectral sensitivity than either we or the eagles have, be able to see the infrared light. Like our camera, your camera uses a CCD chip that maps IR down into visible light for humans. But neither we nor the eagles can see IR without technological assistance. Again, the nest is dark at night.

What about the cameras?
A lot of people are interested in knowing more about our cameras. Read this blog post for more information: http://raptorresource.blogspot.com/2013/01/what-cameras-do-you-use.html

The following sources helped me understand all this:

10 comments:

Bud Gurl said...

Amy, thank you so much for all you contribute to this blog and all of the time you commit to it. I have learned so much and am so grateful!

Jim said...

This is super information. The technical stuff behind those amazing birds. No slight endevour. Much appreciate your efforts in this arena.

John Corney said...

Great info. Thanks.

Deb said...

Great info! Thanks

Deb said...

Great info! Thanks

KittyD said...

A wonderfully descriptive post. Thanks for keeping us in the know!

christina said...

You might be interested in my Master's thesis from 1998.

HODGSON, J. Christina. European Starlings (Sturnus vulgaris) & American Robins (Turdus migratorius) as Hosts for the Eastern Equine Encephalitis Vector Mosquito, Culiseta Melanura (Diptera: Culicidae) [Master of Science thesis]. Boston: Univ of Mass; 1998.

The second half details how I used IR light from an LED array to illuminate caged robins and starlings on the perch as they roosted overnight at a wooded site where blood-seeking mosquitoes quested. I used a CCD camera to pick up IR and translate it to monochromatic visual then exported the signal via coaxial cable to a video monitor/recorder in a nearby van. The videos showed mosquitoes landing on, probing, and engorging on blood from birds. It showed how the birds responded to blood-seeking, biting mosquitoes.

Robins (the native bird) sat like zombies and docilely allowed mosquitoes to probe and feed to repletion, often upon the face, center of the breast, and leading edges of the wings (areas with lots of blood vessels where breaks in the feather patterns allow the long proboscis of the ornithophilic mosquito to reach through to the skin).

Starlings (an introduced exotic) vigorously defended themselves against mosquitoes, flapping the wings, shaking the head, ruffling body feathers, stamping the feet, whisking the tail, pecking with the beak. Mosquitoes did establish feeding contact with starlings but were quickly dislodged before much blood was obtained. (I also did a chemical bloodmarker study to determine how much blood mosquitoes got from each kind of bird where both were present. The markers showed mosquitoes fed on both kinds of birds but obtained about a tenth as much blood from starlings as from robins.)

A possible evolutionary explanation for why starlings are so irritable and prone to interrupting mosquito blood-feeding suggests itself. It may be that the starlings and the salivary pharmacopeia of the native mosquitoes were not coevolved. Saliva that creates intense itchiness is not in the mosquitoes' interest since it makes it difficult for an insect to obtain a full blood meal with which to nourish egg production.

Itching intensely from the bites of blood-feeding mosquitoes, it turns out, is also not in the best interests of the host bird. Starlings' defensive behavior interrupts blood-feeding mosquitoes, but the mosquitoes persist in blood-seeking and biting until they do obtain the needed blood. Thus mosquitoes must bite more times when feeding upon irritable starlings. Note that robins and starlings roost communally at night, often with hundreds or even thousands of roostmates. Starling roosts might well be hotbeds for mosquito-transmission of viruses, given how the birds interrupt feeding mosquitoes and promote a pattern of feeding upon two or more birds.

In the case of Eastern Equine Encephalitis virus, Nick Komar showed via virological experiments that starlings (an exotic not coevolved with the native virus) generate an extraordinarily potent viremia that is capable of infecting 100% of mosquitoes that feed on them. After four days of illness, the starling dies of the infection, which means there aren't immune birds present thereafter to harmlessly absorb some of the bites.

By means of IR, I was able to look at host/vector interactions. The reaction of starlings to biting mosquitoes in a nocturnal communal roost means the mosquitoes are very apt to transmit pathogens in the course of feeding upon multiple birds. By contrast, robins' pattern of tolerating mosquito bloodfeeding means that sufficient blood can be obtained in one feeding contact, which reduces the number of opportunities for pathogens to be transmitted.

Admin said...

Interesting experiment. Doesn't the IR light draw the mosquitos to the birds? Doesn't seem fair unless they can fly away. I was watching an eaglet being bothered by insects last night to the point where it was disrupting his sleep. Even though the eaglets don't see the light, I guess it draws insects like moths, and blood-sucking mosquitos. The parents do not spend time in the nest at night (unless it is windy or rainy ie no insects) but the poor eaglet is a captive audience because he's unable to leave the nest. Since he is 8 weeks old today and ready to start flying lessons, he needs plenty of sleep. I'm thinking that the IR is having a negative effect even though he doesn't see it.

Amy Ries said...

Christina, I'm sorry I didn't see your post earlier - I'm doing some maintenance and just came across it earlier. I'll pass it on to Bob and maybe we will look at a night zoom once the insects come out to see what is happening with our birds. Thanks very much!

Fred Hastings said...

Power and Data stream ? How is this unit powered and how is this being transmitted - Wifi ? to a local dwelling? ect.
Thank You Bob