Why do peacocks have such unusual feathers?
The standard explanation, first posited by Charles Darwin, is that the peacock is advertising to the peahen what a good mate he will make.
Peahens choose the males with the most impressive tails because it demonstrates that they are healthy and strong enough to drag such an encumbrance through the jungle, and also because the peahen wants to produce sons that will have big tails because then they will be successful in attracting mates themselves.
This theory was recently challenged after a seven-year study in Japan found that large tails didn’t make peacocks more successful in finding a mate. But that research has been criticised for ignoring colour and eyespot density and the consensus is still that the peacock tail is a result of sexual selection.