Birds And Colour
Pigments are chemical compounds found in the feathers and skin of birds and the main source of birds’ different colours. Pigments absorb some wavelengths of light and reflect others, with the reflected light reaching our eyes and stimulating the receptors of our retinas. The longest wavelengths are seen as red and the shortest are seen as violet.
Robins, for example, have pigments in their feathers that absorb all wavelengths of light except the ones that we visualize as red. If the pigments reflect no light back, then we would see black and if they reflect back the entire spectrum of light then we would see white.
However, blue and iridescent colours in birds are not produced by pigments but are ‘structural colours’, produced by tiny particles in the feathers that are smaller in diameter than the wavelength of red light. These particles only have an effect on shorter wavelengths and are ‘scattered’ or reflected in all directions. Structural blue colours therefore remain the same if viewed at different angles in reflected light. If they are viewed in transmitted light where the feather is between the light source and the observer, the blue disappears.
Iridescent colours such as the dark blue and green of a magpie are produced by differential reflection of wavelengths from modified barbules (a minute filament projecting from the barb of a feather), that are rotated so that a flat surface faces the incoming light. The structure of the barbule reflects some wavelengths and absorbs others, and the reflected wavelength changes with the angle of reflection. Structural colour is therefore registered by our eyes in response to the reflected wavelengths and changes with the angle formed by the light, the reflecting surface and the eye.
The brightest colours in the plumage of birds are generally to impress the opposite sex and in most species of birds where there is sexual dimorphism it is the male that is more brightly coloured than the female. A couple of obvious examples from around the world are the birds-of-paradise or the tail of a peacock, but in the UK there are also some examples of plumage dimorphism.
In most species of duck found in Britain the male’s breeding plumage contains at least flashes of brighter colour, such as the yellow crown of wigeons, the yellow undertail of teal and the bottle-green heads of mallards and shovelers. And in some species of smaller birds, such as redstarts, greenfinches and chaffinches the females are duller relative to the males.
Ducks also demonstrate another use for colour. Female ducks are typically dull brown and cryptically patterned so they are camouflaged when they are at their most vulnerable during breeding season when sitting on the nest or caring for their young.
In other species of birds such as woodcocks, wrynecks and nightjars, both male and female are highly camouflaged to allow the birds to blend into the background. In all three species, the individual feathers contain many different colour tones making them incredibly hard to spot. Camouflage can also be fairly simple. Ptarmigans, for example, turn pure white in the winter to allow them to hide on snowy mountainsides, and many seabirds have grey or black upperwings and upperparts and pale undersides so they are less visible against the sea and sky.
Sometimes relatively bright colours and strong contrasts or ‘disruptive colouration’ can form part of camouflage. Good examples of this are found on ringed plovers, little ringed plovers and Kentish plovers where their striking and obvious patters break up the outline of the bird against their typical habitats of seashores and gravel pits. Disruptive colouration makes the birds difficult to pick out from their background and the same principle has been used to camouflage.ships, particularly in World War I.
Colour is also used to identify birds to their fellow species, especially in species that form large flocks with other birds such as finches or thrushes as well as for survival in young birds. Passerine nestlings usually have bright yellow gapes, designed to be highly visible to their parents in dark, cramped nests, and to signal to the adults that they want to be fed. The red spot on the bill of an adult herring gull serves the same purpose but in reverse by encouraging the young birds to stick their heads in the adult’s mouth in order to be fed.