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Birds of a Feather

Feathers are a defining feature of birds. They provide insulation from cold and heat, repel water, protect sensitive skin and allow for flight. In addition, feather colouration is useful for camouflage as well as to communicate dominance and attract mates. For most birders, it is the primary method used to identify birds.

Feather patterns can create very effective camouflage. These patterns can include the very common streaky brown plumages found in various habitats, but also the black and white patterns of woodpeckers (which spend a lot of time on tree trunks) and olive green patterns of birds that feed in tree canopies.

Birds of the same species in different geographic regions may have lighter or darker colouring. Song sparrows here, for example, are darker than song sparrows in central Canada. The general pattern is that more heavily pigmented forms of a species are found in more humid environments. This pattern is codified as Gloger’s rule, named after the German zoologist C.L. Gloger. A possible explanation for this pattern, is that this trait has been selected for because darker individuals in a more humid environment are less visible to predators compared with dark birds in a drier environment, such as a desert or grassland. A competing theory is that darker feathers contain more of the pigment melanin and are therefore more resistant to degradation by bacteria and mites, both of which prefer humid environments. Whatever the selection pressure, the pattern holds for the vast majority (around 90 per cent) of North American bird species.

Some species, such as the killdeer, have bold markings that break up the normal outline of a bird and thus aid in camouflage. Many shorebirds are darker coloured above and lighter coloured below. This creates a counter-shading effect, which makes the individual look flat, and therefore, predators have a difficult time knowing the distance between them and their quarry.

In some birds, the males and females are similar in colour year round. In others, the differences are stark, especially at this time of year when the mating season for many bird species is close at hand. For these birds, it is the males that are the gaudy showoffs. The discrepancy in colouration between male and female ducks is particularly distinctive and is a good indicator of how household duties are shared. Female ducks incubate the eggs and raise the young. The bright, beautifully coloured plumage of the males is an advertisement to attract mates and a warning to challengers. Mr. Darwin noticed that such characteristics increased a male’s risk of being predated upon, so how was it that this tendency to showiness persisted and lead to bizarre adornments (taken to a bit of an extreme in the male peacock) in some species? The answer he settled on was that sexual selection might lead to a shorter life, but the more adorned or colourful males sire more offspring. Evidence that the cost of showiness is high can be noted in the fact that the bright plumage for many species goes drabber out of the mating season.

Feather colours are produced by the pigments — melanin for yellows, browns, greys and blacks, and caratonoides for oranges and reds. (Iridescence by contrast, for example on a mallard head or a hummingbirds throat, is produced by the refraction of light caused by the structure of the feather barbules. The feathers takes on a different colour in different lights and angles of observation.) A recent news study illustrates the role of some foods in the production of pigments. The northern flicker of eastern North America has yellow under the tail and underwings and has yellow shafts on its primary feathers. The ones in the west are red. There is a zone where hybrids exists, but a lot of flickers in the east, away from the hybrid zone, are now showing red colouration. After a lot of sleuthing, it was discovered that the flickers have taken to feeding on the berries of the introduced and now naturalized and abundant Japanese honeysuckle, which is affecting the tail and wing colouration. Similar colour changes have been noted in cedar waxwings and Baltimore orioles. The question now is how will the presence of this new plant influence territorial behaviour and mate attraction?

The colouration of birds serves purposes that benefit the individual. In regards to feather colouration and its relation to mate attraction, perhaps one could (cautiously) presume that birds can appreciate the beauty of colours. That we find the same colours aesthetically pleasing is perhaps a more challenging thought. That birds and humans can distinguish (and perhaps appreciate) the same colours reflects that we see colour in the same way.

 

Jay Rastogi is a naturalist, horticulturist and educator living in Yellow Point. ecoforestry [at] gmail [dot] com

About the author: Angie

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