Birds are the only Vertebrates with Feathers. The whole body of a bird is covered with feathers except its beak and feet. The feathers are essential for regulating the body temperature and flying. They also have another important function, to make the bird colorful so that it can attract mates, provide camouflage, and indicate the life stage of the bird – Juvenile/adult.
The colors in a feather occur in two ways Chemical colour and Mechanical Colour
Chemical Colour: These are cause due to presence or absence of pigments in the feather. The pigments can be divided into two categories, the Carotenoids and Melanin. The Carotenoids are responsible for making feather colourful in the longer wavelengths of the spectrum like RED, ORANGE,YELLOW . These colours can also mix with each other to produce some colours in shades of Green and Blue. Melanin are pigments that are responsible for producing dark colours. Absence of Melanin makes the bird paler. Melanin and Carotenoids mix together to create brighter or darker colours in a feather.
The chemical colours are not very SHINY and they are not permanent colours. Once the bird dies the chemical colours also fade away.
The chemical colours do not need sunlight to be seen. They remain same in Sunlight and Shade.
Mechanical colours: They are formed by reflection or refraction of certain wavelengths of light. The Keratin layer on the feather reflects or refracts light, this is called Iridescence. The Iridescence is caused by the cellular structure of Keratin (angle/orientation of the crystals) and atomic nature. The colors here are in Shorter wavelength ( Blue, Indigo, Purple etc). The Keratin is a clear with a complex crystallized layer structure. This gives rise to many possible combinations with respect to how the bird is positioned to the Sunlight. As the angle of reflection/Refraction changes so does the color.
In most birds the Iridescences is enhanced by all other wavelength being absorbed by a Black background because of another pigment layer called Eumelanin. This is present in the Barbs of the feathers below the Keratin layer.
The Mechanical colours are Permanent colours. They do not fade away after the bird dies. But they require Sunlight. When the bird is in the Sunlight the colors are displayed but once the bird moves to the shade it loses the colors and usually becomes dark.
Scattering: This is a combination of the above two methods. Carotenoid pigments can be present in the Keratin layer. This causes the colour of carotenoid pigment to be enhanced. Example would be non-Iridescent Green colour. If the Keratin Layer is porous with lots of airspace that causes light to deviate wildly – SCATTER, then this causes Scattering. This is further enhanced by the dark layer of Eumelanin. Example: Vibrant Blues.
Birds undergo Plumage changes when they move from one life stage to the other ( Juvenile to Adult). This happens because of the production of Carotenoids in the body and/or Keratins in the feather. These changes are permanent
Some birds change the plumage for their Breeding period. This is to attract the Mates. This occurs as the hormonal changes in the bird trigger a release of the colour pigment cells called Chromotoblasts. Once the breeding season is over the birds will lose the plumage and will regain it in the next breeding cycle.
Source : Wikipedia, Beat About the bush -Birds by Trevor Carnaby
Photographs : Saravanan Janakarajan