Friday, January 20, 2012

Constructive Interference

“Oh wow!  Look at the size of that turkey!  I never realized how beautiful their feathers are – check out the iridescence.”

The mount of a wild turkey in flight is one of the most-exclaimed about objects in the Museum’s Collections Room.  The 5,000 feathers on a turkey are worth noticing, with their rich browns, shimmery copper, iridescent blues and greens, and elaborate black designs.  Some of the colors are a result of pigments, those molecules that absorb certain wavelengths (colors) of light and reflect others.  Melanin is a common pigment in nature, and does more than make things appear brown.  It can also protect against bacteria and fungus, UV radiation, and high temperatures.  A certain type of fungus appears to be able to use melanin to capture the energy of gamma rays for photosynthesis!

Melanin can’t make a turkey, or a seashell, or a butterfly, or a soap bubble shimmer, though.  The vivid, shifting, enchanting colors of these objects are caused by the structures themselves, not a type of pigment. Remember that light travels in waves, and the color of light is determined by its wavelength.  Red light has longer wavelengths, while purple light has shorter wavelengths.  The rest of the colors fall between in rainbow order. 

When light passes through a thinly layered substance, the light bounces off the back surface toward your eye, and joins light bouncing off the upper surface toward your eye.  Where the waves of light overlap, they interfere with each other. The interference can be either destructive, where the waves cancel each other out (so you see black on an iridescent object), or constructive, where the waves amplify each other.

I experienced constructive interference on a macro scale last fall on a sea kayaking trip in Maine.  As we paddled along the seaward side of an island, waves bouncing off the rocky shore grew bigger and bigger as they overlapped with constructive interference. The color of my face changed from pink, to green, to white, as waves striking me from various angles tossed my kayak gawkily.  Iridescent colors also change when you alter your angle relative to the object, because the wavelength of light that reaches your eye changes.

Turkeys in Wisconsin have experienced destructive and constructive interference before, and not just in their feathers. These large game birds are native to southern Wisconsin, but were hunted almost to extinction by 1881.  Major reductions in their habitat due to logging and farming, plus diseases introduced in domestic fowl were also culprits of their decline.

In 1976, the Wisconsin DNR made a trade with Missouri to bring wild turkeys back to Wisconsin.  We gave them ruffed grouse in return. From the initial release site in Vernon County, turkeys recovered enough to move naturally and with help to other suitable habitats across the state.  Many other states have similar stories. The turkey population in United States was once down to about 30,000 birds.  Now it is over 7 million! 

It’s no surprise, then, that many Museum visitors have reported turkey sightings this winter. Their favorite times to forage are morning and evening, which corresponds nicely to most of our commutes. There are two flocks just in my 10-mile commute on County Hwy M.  After the holidays, I returned home to a whole flock of turkey tracks in my driveway and around our bird feeders.

Although northern Wisconsin isn’t their traditional habitat, we do have plenty of oaks for acorns and large white pines near water for roosting. While you might feel sorry for turkeys during harsh winters, the DNR does not advise feeding them.  To provide a turkey with enough to food to impact its survival would take a lot of food!  Any winter mortality is easily made up for by high breeding success.  Feeding can actually harm turkeys, because concentrated groups spread disease more easily, and become easy targets for predators.  If you want to help these beautiful birds, think about managing your forest for mast trees like oaks, and big pines and hemlocks for roosting.

The wild turkey was once lauded by Benjamin Franklin as a more respectable national symbol than the bald eagle, who eats road kill. Franklin wrote: “He is besides, though a little vain & silly, a Bird of Courage, and would not hesitate to attack a Grenadier of the British Guards who should presume to invade his Farm Yard with a red Coat on.”

For over 44 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 13470 County Highway M. The new exhibit, The Joy of Birds: Feathers in Focus opened in May, 2011. Find us on the web at to learn more about our exhibits and programs. Also discover us on Facebook, or at our blogspot,

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