Why are Birds of Paradise so colourful?

Birds of Paradise are widely considered by many to be some of the most beautiful creatures on the Planet. With their iridescent and erratically structured plumage of rich colour, a heart of stone is required not to be moved by the sight of them.
But why would such characteristics be of evolutionary benefit to these creatures? If anything they are a hindrance as they can get in the way during movement through the treetops and could make flight more difficult, while also making them more visible to predators. 

Well the answer all comes down to an evolutionary mechanism called Sexual Selection.


Sexual Selection (first described by the great Charles Darwin) is defined as the competition for mates causing traits that improve breeding success to be passed to the next generation.
So in simpler terms, if an animal evolves a characteristic that makes it more attractive to the opposite sex, it will be more likely to breed than others without it and pass this characteristic to its offspring.
This mechanism explains the colours of Birds of Paradise. In the past before the colours of this avian lineage had evolved, a Bird of Paradise ancestor may have developed slightly brighter plumage, or larger and more noticeable feathers.
This would have made it more attractive as evolutionarily speaking, if the bird was able to produce such colour and/or patterning while being able to survive the downsides of such characteristics (predation, flight disturbances etc) it was fit and healthy with good genes which would allow the offspring of such a bird to inherit these genes as well. This would, due to continuous evolutionary pressure for these traits lead to greater colours and greater patterning.

Sexual selection explains why Birds of Paradise are so colourful, but the beautiful ones we all know and love are all males. The female Birds of Paradise are dramatically less eye catching, most of which are quite brown and none descript. Why is this?
Well the females are the ones choosing the males that are most aesthetically pleasing to them, and therefore have the best genes.

 The reason evolution has created these characteristic differences between the two sexes (sexual dimorphisms), is all due to differences in offspring investment between the two genders. Females are the ones that produce the eggs, and will have to carry and look after the offspring, taking up their own time and energy to enable the offspring to survive and her genes to be passed on.
Therefore picking a mating partner with good genes is a must to make her huge investment worthwhile.
Whereas male Birds of Paradise produce sperm, do not look after their own offspring, and practice Polygamy (mate with a number of partners). Males produce millions of easy to make sperm, so can afford to spread them far and wide with any partners who will accept them, not really needing to care about the genetic fitness of the females. As females are not sexually selected to be aesthetically attractive, they will remain less extravagant to allow greater camouflage to surrounding vegetation, so as not to attract predators towards them, as well as to their offspring.

To sum up, females choose males with good genes that are shown by their ability to survive with such dramatic feathers patterns, so the females (as the egg carriers) do not waste time and energy on offspring carrying her genes that are unlikely to survive.

Males have easily-produced sperm that they try and spread to as many females as possible by attracting them with their colour and mating rituals, these traits becoming more extravagant over evolutionary time as fitness and health are selected for.

-Thomas Glen

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Images: Not my own