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RAIN FOREST TEACHING CURRICULUM
GRADE 9-12 ACTIVITIES
Activity: Structural Versus Pigment Color
This activity is an investigation into color, specifically the color of birds. The tropical rain forests of the world are famous for the numerous species of brightly colored birds. The bird shown below is the red-crested touraco (photo from Aves International on-line catalog) and is native to east Africa.
This bird displays a rare green pigment and a water-soluble red pigment. If the red pigment of this bird is water soluble, how can this species maintain its color in the rain forest without having its red pigment wash away? You will discover this answer shortly.
Procedure:
We will now examine our three samples of feathers (red, blue and white) to learn something about the origin of their colors.
Part 1: Feather Observation
Obtain the red, blue, and white bird feather samples and observe them closely under a dissection microscope or with a good hand lens. Try to observe all three samples at once for the best comparison. (This will require cutting a small piece of each feather so that they can be grouped closely together.) Sketch each as it appears under magnification, using colored pencils.
The blue feather is an example of Tyndall blue found in many different types of animal tissues. The blue is not due to pigment, but to tissue structures that cause constructive interference for the blue rays. Blue feathers are the most abundant and often the most impressive example of Tyndall blue in animals. Other examples include blue eye color and the blue bare-skin patches of certain mammals.
The hard, mid-rib portion of the white feather appears as if it is made of ice crystals or cut glass. The color of the hard matter of the feather results from very much the same principle as ice crystals or cut glass in that it exhibits diffuse reflection. There are many tiny air spaces inside the feather branches that reflect the incoming light in all directions, and the feather looks white.
In the red feather, the hard mid-rib portion is quite colorless and transparent. The hollows of the small branches are filled with red pigment (red, because it absorbs all of the light rays except red and, therefore, reflects red). The interesting point here is that red and yellow pigments (carotinoids) can only be produced by plants (you may recall learning about these pigments from experiments you have done in plant pigment chromatography). These pigments can only be found in birds due to the foods they consume.
Red bird feathers can come about in one of two ways. For some bird species it is a matter of consuming yellow carotinoids and having the physiologic ability to turn the yellow into red color matter. Other species must directly consume the red pigment.
The touracos of Africa (related to cuckoos) mentioned earlier are a good example of unusual pigmentation in birds. They display actual green pigment. Well over ninety percent of green birds are actually blue birds with some amount of yellow pigment or yellow birds with some amount of blue structure. This accounts for the incredible variety of green shades seen in birds throughout the world.
The touraco actually has a green pigment called turacoverdin. This green color is not a mix of blue structure and yellow pigment. The red feathers of these birds are due to a pigment called turacin. This red pigment is a copper salt and it is soluble in water.
Part 2: Color Investigation
- Place a portion of your red feather sample in water. Leave the feather in water overnight and compare to the dry red feather the next day.
- Using the blue feather you can verify that the blue color is due to structure by altering the physical structure of the feather. To do this, snip a portion of the blue feather into small pieces and then crush it with a mortar and pestle. The feather will immediately turn dark because all that is left after the blue producing structure is destroyed is the melanin understructure, which is now visible.
- Dip a portion of the blue feather into water. Observe and note.
- Use an eyedropper to drop some water on several locations of any of your feather samples. Does the water soak in or bead up and stay on the surface?
Questions for Students:
- After the red feather sample was soaked in water, what color did it become?
- Because your investigation has proven that the red pigment in bird feather is water soluble, you might begin to wonder why the red color of bird feathers does not wash out in the tropical rain forests they inhabit. Explain why this does not happen by recalling what occurred when you dropped water on the feather's surface. 95
RAIN FOREST TEACHING CURRICULUM
GRADE 9-12 ACTIVITIES
- What do you suppose is on the surface of a bird feather that might cause the water to act as it does when placed on the feather?
- Knowing what you do about the structure of the blue feather, how do you suppose the water turned the feather dark?
Notes to the Teacher:
The students may figure out that the reason the water soluble red pigment of tropical rain forest birds does not wash out in the rain is due to several factors. First, the color is deeply impregnated into the feather. Second, since the bird gets its red pigment from the foods it eats, it maintains its color through its diet. Third, they should realize that water beads up on feathers-they were able to demonstrate this property of the feather to themselves. They may conclude that this is from natural oils that birds produce. What they may not know is that birds have an oil gland at the base of their tails and use their beaks to collect the oil and then, in their preening, spread it around to all the feathers of the body. This is why all birds are frequently seen reaching around to the base of their tails and then preening - they are spreading natural oils.
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