Scientists have known for a long time that the color of the wings and peripherals of a bird is determined by the food that they eat. This was proven correct for red and yellow feathered birds but when it came to blue a clear relation could not be established. For example, flamingos get their pink hue due to the algae and crustaceans present in their diet. This is because their digestive systems actively extract the pink pigment which later turns the whole body pink.
Test on blue birds
When a similar test was conducted for blue it was noticed that the digestive system usually destroyed the blue pigment in the digestive tract itself. Thus if a bird eats blueberries the chances it has of developing a blue color due to these are slim or next to none. This means pigment does not lead to the colors of the bird and its feathers. If it is not due to pigments what could be the possible reason?
Some scientists theorized that some birds look blue because the sky is blue too. The blue wavelength of the white light of the sun is short and bounces off many things thus giving them a blue appearance. Noted ornithologist of Yale, Richard Prum, has a different theory regarding this matter. He has a large sample of skins and feathers of various contugas present throughout Central and South America. When you look at the collection at first glance they are blue.
On closer inspection, one will notice that these are different shades of blue too. This was a striking discovery as one would have not expected such a variety in a single color. Prum and many of his colleagues put the feathers and skin under the microscope. When observed with such great magnification they discovered that in each cell, keratin molecules are separate from the water. Just like how oil and vinegar do not mix.
When the cell dies the water in most cases dries away and is replaced by air. The structure that remains is very sponge-like in texture and keratin is full of holes and indentations. So now when the light strikes such a feather the yellow and red light cancel each other out. Only the blue light is left which is reflected. This is why the blue light is reinforced and has a such a rich hue when observed in birds. The amount and nature of blue is also dependent on how the keratin protein is scattered and how it has been perforated.
Yale Weighs In
Prum has also said that birds also have a concept of beauty and will most probably mate with what they consider to be beautiful very similar to the behavior observed in humans. This means that birds also have preferences and are not just egg-laying machines. The blue feathers could mean that the male is healthy and devoid of any genetic defects. Prum also acknowledges the fact that we give more importance than necessary to these ornate things like feathers and antlers. He says these might be a signal to the female but also recognizes the fact that this might not be true. Most of these features are merely ornate and do not alter how an animal functions. Thus, the things that are blue on a bird might not have as much significance as we might have been led to believe.
The ornamentation and blue color have long been part of mating rituals and ironically the male birds being more aesthetically pleasing than the female ones. As compared to mammals this is the opposite. What this means is that female birds might prefer things that are blue over some other color like white. The development of such characteristics has led to some significant understanding as to how birds conduct their mating rituals. The blue color has been adorned by birds and other animals alike.
The tiny air pockets theory was also explored by Vinothan Manoharan at Harvard University. He had a different approach to know the birds acquired their colors. He was curious whether the bird can get the red pigment without consuming anything red. This would not only validate Prum’s work but also help in establishing how much the role of ingested pigment is in the color of the bird. Thus things that are blue have another addition to their ever-growing list.
He studied the effect of light scattering due to nanometer plastic beads with inverse versions of air pockets. He noticed that changing the size of the bead changes how the light scattered and how the scattered light from nearby keratin also interfered. One thing that was unique to this was the fact that only certain shades were more pronounced than before rather than anything else.
Manoharan and his team used a few large beads to enhance the red light but what ended up happening was that purple hue was observed. The bluish-purple hue is observed the same way the sky is blue. Red and yellow having longer wavelengths cancel each other put while only blue remains. This not only explains why certain bird feathers are blue but is applicable for other animals too.
For example, beetles are incapable of producing a red color without a pigment. After this experiment, it was noted that they are blue because they can reflect the short length blue light only. The way the light bounces off beads in Manoharan’s experiments is very similar to the macroscopically analogous things like beetle skin and bird feathers. Thus, due to this experiment beetles were also added to the list of things that are blue.
The blue feathers are a part of things that are blue because of how they scatter light. This also leads to the fact that blue is a structural color and not obtained from a pigment. A bird’s bones are pneumatic which means they have enough holes in them to make them light enough flight. Even though they are structurally similar to the perforated keratin structure they are still unable to produce a red color.